Radiological Safety Aspects of the Operation of … Safety Standards/Safety...Nuclear Safety Information Centre, B0655 O B S O L E T E S A F E T Y S E R IE S N o. 42 Radiological Safety

\Nuclear Safety Information Centre, B0655

O B S O L E T E S A F E T Y S E R I E S N o . 42

Radiological Safety Aspects of the Operation

of Neutron Generators

INTERNAT IONAL A T O M IC ENERGY AGENCY, VIENNA, 1 976

This publication is no longer valid Please see http://www-ns.iaea.org/standards/


RADIOLOGICAL SAFETY ASPECTS OF THE OPERATION

OF NEUTRON GENERATORS


The following States are Members of the International Atomic Energy Agency:

A F G H A N IS T A N H O L Y S E E P H IL IP P IN E SA L B A N IA H U N G A R Y P O L A N DA L G E R IA IC E L A N D P O R T U G A LA R G E N T IN A IN D IA Q A T A RA U S T R A L IA IN D O N E S IA R E P U B L IC O F S O U T H V IE T N A MA U S T R IA IR A N R O M A N IAB A N G L A D E S H IR A Q S A U D I A R A B IAB E L G IU M IR E L A N D S E N E G A LB O L IV IA IS R A E L S IE R R A L E O N EB R A Z IL IT A L Y S IN G A P O R EB U L G A R IA IV O R Y C O A S T S O U T H A F R IC AB U R M A JA M A IC A S P A INB Y E L O R U S S IA N S O V IE T JA P A N S R I L A N K A

S O C IA L IS T R E P U B L IC JO R D A N S U D A NC A M B O D IA K E N Y A SW E D E NC A N A D A K O R E A , R E P U B L IC O F S W IT Z E R L A N DC H IL E K U W A IT S Y R IA N A R A B R E P U B L ICC O L O M B IA L E B A N O N T H A IL A N DC O S T A R IC A L IB E R IA T U N IS IAC U B A L IB Y A N A R A B R E P U B L IC T U R K E YC Y P R U S L IE C H T E N S T E IN U G A N D AC Z E C H O S L O V A K IA L U X E M B O U R G U K R A IN IA N S O V IE T S O C IA L IS TD E M O C R A T IC P E O P L E ’S M A D A G A S C A R R E P U B L IC

R E P U B L IC O F K O R E A M A L A Y S IA U N IO N O F S O V IE T S O C IA L IS TD E N M A R K M A LI R E P U B L IC SD O M IN IC A N R E P U B L IC M A U R IT IU S U N IT E D A R A B E M IR A T E SE C U A D O R M E X IC O U N IT E D K IN G D O M O F G R E A TE G Y P T M O N A C O B R IT A IN A N D N O R T H E R NE L S A L V A D O R M O N G O L IA IR E L A N DE T H IO P IA M O R O C C O U N IT E D R E P U B L IC O FF IN L A N D N E T H E R L A N D S C A M E R O O NF R A N C E N E W Z E A L A N D U N IT E D R E P U B L IC O FG A B O N N IG E R T A N Z A N IAG E R M A N D E M O C R A T IC R E P U B L IC N IG E R IA U N IT E D S T A T E S O F A M E R IC AG E R M A N Y , F E D E R A L R E P U B L IC O F N O R W A Y U R U G U A YG H A N A P A K IS T A N V E N E Z U E L AG R E E C E PA N A M A Y U G O S L A V IAG U A T E M A L A P A R A G U A Y Z A IR EH A IT I P E R U Z A M B IA

T h e A g e n c y ’s S t a tu t e w as a p p ro v e d o n 2 3 O c to b e r 1 9 5 6 b y t h e C o n fe re n c e o n th e S t a tu te o f th e IA E A h e ld a t U n i te d N a tio n s H e a d q u a r te r s , N ew Y o rk ; i t e n te r e d in to fo rc e o n 2 9 J u ly 1 9 5 7 . T h e H e a d q u a r te r s o f th e A g e n c y a re s i tu a te d in V ie n n a . I ts p r in c ip a l o b je c tiv e is “ to a c c e le ra te a n d e n la rg e t h e c o n t r ib u t io n o f a to m ic e n e rg y to p e a c e , h e a l th a n d p ro s p e r i ty th ro u g h o u t th e w o r ld ” .

© IA E A , 1 9 7 6

P e rm iss io n to r e p ro d u c e o r t ra n s la te th e in fo rm a tio n c o n ta in e d in th is p u b lic a t io n m a y b e o b ta in e d by w rit in g to th e I n te rn a t io n a l A to m ic E n e rg y A g e n c y , K S rn tn e r R in g 11, P .O . B o x 5 9 0 , A -1 0 1 1 V ie n n a , A u s tria .

P r in te d b y th e IA E A in A u stria A p ril 1 9 7 6


S A F E T Y S E R I E S N o . 4 2

RADIOLOGICAL SAFETY ASPECTS OF THE OPERATION

OF NEUTRON GENERATORS

A manual w ritten by R .F . BOGGS

D epartm ent of Health, Education and W elfare,Public Health S erv ice ,R ockville, M aryland,

United S tates of A m erica

INTERNATIONAL ATOMIC EN ERG Y AGENCY VIENNA, 1976


RADIOLOGICAL S A FE T Y ASPECTS OF THE OPERATION OF NEUTRON GENERATORS

IA EA , VIENNA, 1976 S T I/P U B /4 2 7

ISBN 9 2 - 0 - 1 2 3 0 7 6 - 1


FOREWORD

This manual is a contribution to the International Atomic Energy Agency’s programme on the protection of man against possible damage resulting from the use of radiation. Dr. Richard F. Boggs, of the Department of Health, Education and Welfare, Maryland, USA, was engaged as a consultant to write it, and the work was undertaken with the co-operation of the staff of the Agency.

A draft of the manual was sent to certain experts in various countries and the Agency gratefully acknowledges the helpful comments received from Messrs D.H. Sykes (Canada), W. Eyrich (Federal Republic of Germany), H. Kawai, R. Miki (Japan), R. Burkhart, W.E. Gundaker, E. Moss, A.C. Tapert and W.E. Thompson (United States of America), which have been taken into account in the final text.

The publication is aimed at advising and assisting those who, with little or no experience of the subject, wish to gain knowledge of the radiological safety aspects of neutron generators. Comments from readers for possible inclusion in a later edition of the manual would be welcome; they should be addressed to the Director, Division of Nuclear Safety and Environmental Protection, International Atomic Energy Agency, Karntner Ring 11, P.O. Box 590, A-1011 Vienna, Austria.



C O N TEN TS

IN TRODUCTION................................................................................................................................ 1

PU R PO SE............................................................................................................................................... 1

SC O PE...................................................................................................................................................... 2

1. Characteristics and Use of Neutron Generators....................................................... 2

1.1. Sealed-tube type neutron generators ..................................................................... 21.2. Cockcroft-Walton type neutron generators......................................................... 3

1.2.1. High voltage power supply1.2.2. Ion source1.2.3. Acceleration tube1.2.4. Drift tube and target assembly1.2.5. Vacuum system

1.3. Fields o f application o f neutron generators......................................................... 8

2. Radiation Hazards and Safety Considerations forNeutron Generators..................................................................................... .............. 8

2.1. Tritium hazards............................................................................................................... 92.2. Neutron hazards ............................................................................................................. 112.3. Considerations o f neutron activation..................................................................... 122.4. X-ray production........................................................................................................... 1 52.5. Shielding requirements................................................................................................. 152.6. Safety interlocks ........................................................................................................... 182.7. Warning d evices.............................................................................................................. 21

3. Radiation Monitoring and Interpretation of Measurements.................................. 22

3.1. Tritium monitoring instrum ents.............................................................................. 233.2. Neutron monitoring instruments ............................................................................. 243.3. X-ray and gamma-ray monitoring instrum ents................................................... 253.4. Personnel monitoring ................................................................................................... 26


4. Requirements for an Effective Safety Programme................................................ 26

4.1. Safety organization ....................................................................................................... 284.2. Radiation safety .............................................................................................................. 284.3. Neutron generator sa fe ty ............................................................................................. 294.4. General safety .................................. .............................................................................. 29

APPENDIX I: Non-radiation hazards and safety considerations..................................... 30

APPENDIX II: Considerations for a neutron generator laboratory................................ 33

REFEREN CES ................................................................................................................................... 40BIBLIO G RA PH Y................................................................................................................................. 42


INTRODUCTION

The num ber of neutron gen erato rs has in creased at a rapid ra te during the p ast 15 y e a rs . The u ses fo r neutron g en era to rs in the many fields of education, re s e a rc h , industry and m edicine a re s till expanding [ 1 ].

Two a sp ects a re of p articu lar im portance for the evaluation of the potential h azards of newly installed neutron g en era to rs :

(i) neutron g en era to rs a re relatively inexpensive m achines and it could be overlooked that an additional amount of money is n e ce ssa ry for their safe operation, including shielding m a te ria ls and radiation su rveillan ce s e rv ic e s .

(ii) neutron g en era to rs a re easy to op erate and highly train ed personnel a re not req u ired . They could be operated in a re a s where little or no health physics capability is available.

M ost of the m achines now in use use the C ockcroft-W alton type of voltage multiplying c ircu itry . The neutrons re su lt from the d eu tero n -tritiu m in teractio n , that is deuterons a re a cce le ra te d onto a tritiu m ta rg e t with the subsequent re le a s e of 14 MeV neutrons. Until recen tly , very little inform ation concerning the. asso cia ted health h azard s has been available. T his m anual attem pts to p resen t additional and cu rre n t inform ation re la ted to the production an d /o r re le a s e of tritiu m , X -r a y s , neutrons, and the resu ltan t need for adequate shielding and radiation m onitoring.

PURPO SE

The purpose of th is manual is to provide som e b asic guidelines to p erson s with a minimum of training in rad iological health or health p hysics, on som e safety asp ects of the operation of sealed - tube and C ock croft-W alton type neutron g e n e ra to rs . The manual does not state ru les and regulations, but p resen ts a d escrip tion of h azard s which a re m ost likely to exist around such d evices.

1


B ecau se of the re la tiv ely low co st of th ese sm all neutron g e n e ra to rs , they a re frequently obtained by organizations with little o r no com petency in the a re a of radiation safety . The purpose is to p resen t in b asic te rm s som e of the problem s which m ust be anticipated when operating th ese neutron g en era to rs .

SCOPE

The concepts and ideas presented are intended to co v er rad io lo g ical health asp ects fo r those relatively com pact neutron g en era to rs which usually op erate at less than 150 - 200 kV for the purpose of producing 14 MeV neutrons. The scope is lim ited to b asic d iscu ssion s of h azard s and m easu rem en t techniques and the re a d e r m u st re f e r to m ore detailed tech n ical lite ra tu re to obtain sp ecific inform ation on the concepts p resen ted ,

1. CHARACTERISTICS AND USE OF NEUTRON GENERATORS

The neutron g en erato r design c h a ra c te r is tic s a re such that high cu rre n t beam s of protons an d /or deuterons a re used for the production of fast neutrons by the following reactio n s:

^H + JH — 2 H e + Jn + 17 -6 MeV T(d,n)

2H + jH — gH e+Jn + 3 .25 MeV D(d,n)

In the T (d,n) reactio n , the energy re le a se d is about 17.6 MeV. By applying the b asic energy and momentum th eory, which req u ires that the ligh ter p artic le acq u ires the g re a te r amount of en ergy, it can be determ ined that the resu ltan t neutron will have an energy of ap proxim ately 14.6 MeV.

By applying the sam e calculations to the D(d,n) reactio n , the neutron energy can be determ ined to be about 2 .6 MeV.

The neutron yield from the T(d,n) reactio n at the typ ical ta rg e t bombarding energy of 100 keV is about 100 tim es g re a te r than that from the D(d,n) reactio n and is th erefo re of g re a te r in te re st.

1 .1 . Sealed-tube type neutron gen erato rs

The sealed -tub e type of neutron g en erato r provides a so u rce of 14 MeV neutrons with typ ical yields of up to around 109 n -s '1,

2


with m axim um outputs of about 1011 m s"1. The sy stem s a re r e l a tively com p act to allow for a degree of portability and can be operated in either a continuous or pulsed m ode. B ecau se of th eir degree of sim plicity , they req u ire le ss m onitoring and supervision than the la rg e r C ock croft-W alton type neutron g en erato r.

The tubes contain an ion so u rce , an a c c e le ra to r system (usually 100 - 2 0 0 kV), a rep len ish er to m aintain a constant p re s su re within the tube and a self-rep len ish in g tritiu m gaseous or m etallic ta rg e t. The tubes a re frequently 30 to 65 cm in length and interconn ected to the req u ired power supplies and con trol con soles.

The m etallic ta rg e ts a re usually tritiu m loaded titanium ta rg e ts containing 10 to 20 Ci (370 to 740 GBq) of tritiu m . The ta rg e t a re a s a re usually w ater or oil cooled.

Tube life for continuous operation for neutron outputs of 1010 n -s"1 can be exp ected to exceed 500 h ours, while under pulsed conditions the lifetim e can be reduced by as much as 50%.

1 .2 . C ock croft-W alton type neutron g en erato rs

Of m o re im portan ce to the health p h ysicist a re the extrem ely high yield m achines (up to 1013 n -s"1) that have been developed from the early experim entation of the two English p h ysicists C ockcroft and Walton in 1932. T hese m achines a re suitable as electron a c c e le r a to rs o r as ion a c c e le r a to rs and usually operate at voltages of about 150 keV and cu rre n ts of up to 2 .5 mA. Most of the C ock croft-W alton type neutron g en erato rs now in use a c c e le ra te deuterium ions onto a m etallic-b ack ed tritiu m ta rg e t to produce 14 MeV neutrons by the T(d,n) re a ctio n . D etailed inform ation on the operation of th ese m achines is available in the lite ra tu re [2 -4 ] .

The operation of C ock croft-W alton type neutron g en erato rs involves the form ation of positive ions (deuterons), accelera tio n , flow through a d rift tube and finally collision with the ta rg e t.

The m ajor component section s in a neutron gen erato r a re :(i) High voltage power supply; (ii) Ion so u rce ; (iii) A cceleratio n tube; (iv) D rift tube, ta rg e t assem b ly and ta rg e t; and (v) Vacuum sy stem .

An outline drawing of a C ock croft-W alton type neutron g en era to r is shown in F i g . l .

1 .2 .1 . High voltage power supply

The high voltage power supply is usually contained in a sep arate oil or g a s-filled tank that m ay or m ay not be physically

3


F IG .l. Outline drawing of a Crockcroft-Walton type neutron generator equipped with an oil diffusion pumping system and post acceleration pulsing [2 ] .


rem oved from the im m ediate a re a of the neutron g en era to r .A good extern al ground m ust be p resen t during operation of the power supply.

1 .2 .2 . Ion so u rce

The ion so u rce is usually one of two different typ es. The f irs t u ses a rad io-freq u en cy (R F) field operating between 30 to 80 MHz to produce an intense ionization of deuterium g as. The R F field p asses through the q uartz envelope into the g as, causing electro n s to m ove back and forth rapidly. As they m ove, co llision s with atom s in the gas cau se additional e lectro n s to be produced, thus generating a p lasm a consisting of many fre e electro n s and free deuterium ions. A d .c . e le c tr ic field applied by m eans of the extractio n e lectro d e will fo rce the ions tow ard the exit can al and those ions which leave the. exit can al a re then focused by gap len ses and a cce le ra te d in the m ain beam [4].

The other ion so u rce is the Penning ion gauge. This ion sou rce op erates on d irect cu rren t (thus elim inating any R F in terferen ce with health physics m onitoring equipment). E le c tr ic and m agnetic fields a re used to focus and align the ion beam tow ard the a c c e le r ating tube.

With each of th ese sy stem s a palladium or nickel leak (a m em brane of palladium between the deuterium and the vacuum in the ion so u rce ) is usually employed to introduce the deuterium or hydrogen gas into the ion so u rce (at tim es a m ech an ical leak is used). Palladium b ecom es m o re porous to hydrogen atom s when heated. T h erefo re , the flow of gas is regu lated by controlling the tem p eratu re of the palladium . Since palladium is not porous to atom s other than hydrogen, it a c ts as a f i lte r , preventing in g ress of other ions or foreign m a te ria ls .

1 .2 .3 . A cce lera tio n tube

A fter leaving the ion so u rce and gap len s, the beam en ters the acce le ra tio n tube. This tube usually co n sists of a s e r ie s of e le c tro d es used to a cce le ra te the beam . E ach e lectrod e is at an in creasin g potential so that when passing through the e lectro d es , the ions of the beam acq uire a potential of 150 kV or m o re .

1 .2 .4 . D rift tube and ta rg e t assem b ly

A fter being a cce le ra te d to the d esired velocity , the ion beam en ters a p o ten tial-free d rift tube. The purpose of this drift tube is

5


to provide a vacuum link between the ta rg e t and acce lera tin g section w here a c c e s s o ry equipment can be located . Some item s that may be located in o r on the drift tube include:

(a) vacuum pumps,(b) ta rg e t isolation valves,(c) beam d eflecto rs ,(d) beam dumps,(e) beam v iew ers, and(f) e lectro n su p p resso rs .

The ta rg e t isolation valves a re located so that the vacuum in the drift tube can be m aintained during ta rg e t changes, thus p erm itting sh o rte r 'pump-down' tim es and m inim izing contam ination of the vacuum system components o r the a ir .

Some C ock croft-W alton neutron g en era to rs a re capable of providing a pulsed beam in which the ion beam can be interrupted b efore or after acce le ra tio n by diverting it out of its norm al path.The beam may be diverted after acce le ra tio n by e le c tro s ta tic d eflecto rs . In such c a s e s , beam ca tch e rs a re installed on the drift tube to absorb and d issip ate the energy of the deflected beam .

B efo re allowing the beam to re a ch the ta rg e t, a beam view er is frequently used to determ ine the size and location of the beam spot on the beam ca tch e r .

An electro n beam su p p ressor may be installed in the drift tube to red u ce the re v e rs e flow of secondary e lectro n s back through the tube and into the a cce lera tin g section.

V arious other a c c e s s o r ie s may be added to the d rift tube section depending on the p articu lar type of neutron gen erator being used.

The ta rg e t assem bly co n sists of a ta rg e t holder and a cooling sy stem (w ater, oil or freon is usually employed as the coolant). The ta rg e t and holder have a vacuum - or g as-tig h t sea l at the end of the drift tube.

The ta rg e ts a re usually about 3 cm d iam eter disks consisting of tritiu m gas occluded in a thin lay er of titanium (about 1 m g-cm *2). The titanium is then evaporated onto a thin copper backing m a te ria l. Usually about 1 C i-cm -2 (37 G B q-cm -2) of tritiu m is used; however, this can be in creased to about 10 C i-cm "2. Replenishing sy stem s a re som etim es available where up to 100 Ci (3.7 TBq) can be deposited over a period of tim e. The 'h alf-life ' for such ta rg e ts in continuous operation v a rie s depending on the beam cu rre n ts used. The av erag e lifetim e of a standard ta rg e t is approxim ately 4 mA-h. T h erefo re ta rg e t life can be as low as one hour for high cu rren t a c c e le r a to rs .

6


In o rd er to in cre a se the length of operation before the ta rg e t m ust be d isassem b led , neutron g en era to rs have been m anufactured with rotatin g ta rg e ts (the tritiu m being plated out on the side of a cy lin d rical ta rg e t) and m ultiple ta rg e t assem b lies (se v e ra l ta rg e ts placed on a la r g e r disk). This enables the o p erator to m ove the ta rg e t such that a fresh a re a of tritiu m is in the beam .

1 .2 .5 . Vacuum system

A high vacuum m ust be produced within the a c c e le r a to r for the p a rtic le beam to be effectively a cce le ra te d . The m o st com m on arran gem en t of pumping apparatus co n sists of a forepump to rough out the sy stem , and an additional pump which is turned on a fte r the roughing out, thus producing the high vacuum req u ired . This additional pump may be a tu rb om olecu lar pump, a diffusion pump, o r an ion pump. If an ion pump is used on the sy stem , the forepum p (or backing pump) is isolated from the sy stem by a valve afte r the ion pump is s ta rte d . The other pumps d isch arge d irectly into the forepump.

Forep u m p s used with m ost sy stem s a re of the m ech an ical oil- filled ro tary type. The a ir p assin g through the pump com es in d irect con tact with the oil during the pumping cy cle and bubbles through a lay er of oil before reach in g the exhaust p ort.

Diffusion pumps containing eith er oil or m ercu ry a re used between the a c c e le ra to r and the forepump to obtain vacuum s in the region below about 10"3 mm Hg. The diffusion pump is s tarted after the p re ssu re in the sy stem has been low ered to approxim ately 10‘ 2 mmHg by the forepump.

In place of oil or m e rcu ry diffusion pumps, ion pumps (often called sp utter ion pumps o r g e tte r ion pumps) a re frequently used to produce the high vacuum req u ired for a c c e le ra to r operation.Ion pumps trap gas m olecu les in the pump elem en ts, and once the pump has s ta rted its pumping action the valve between it and the forepump is closed . B a sica lly , ion pumps con sist of a s ta in less s te e l con tain er housing one or m ore elem en ts, often re fe rre d to as Penning c e lls . T hese ce lls co n sist of two titanium cathodes with a honey bom b-like sta in less s te e l anode between the cath od es. The design of the Penning ce ll is such that a la rg e num ber of e lectro n s m ove back and forth through the anode where they ionize gas m o lecu les that a re p resen t. The gas ions thus produced a re a cce le ra te d tow ard the cathode, striking with se v e ra l thousands of volts of energy, and sputtering fresh titanium over the su rface of the cathode.G ases like nitrogen and oxygen form stable low vapour p re ssu re

7


compounds with the titanium cathode, while the light g a se s , such as hydrogen, diffuse into the cathode form ing what is con sid ered to be a solid solution of hydrogen in titanium .

The use of ion pumps on a c c e le ra to rs containing tritiu m can c re a te a health h azard since the tritiu m is trapped within the pump elem ents th em selv es. S everal hundred cu rie s of tritiu m can accum ulate during se v e ra l y e a rs of operation.

1 .3 . F ie ld s of application of neutron g en erato rs

The high yields and rela tiv e low co st of neutron gen erato rs m ake it p ossible to use these devices on a wide sca le in education, r e s e a r c h , industry and m edicine [ 1 ].

In education, the neutron g en erato r is being used to dem onstrate the b asic prin cip les of atom ic and n u clear p h ysics, to dem onstrate and develop methods for neutron production and m easu rem en t, and to dem on strate the prin cip les and applications of activation an alysis.

E xten siv e use of neutron g en era to rs is being made in b asic and applied re s e a r c h . Included are the study of n u clear reactio n s , new developm ental techniques in activation an aly sis, determ ination of biological dam age and effects from fa s t, slow and th erm al neutrons, evaluation of shielding m a te ria ls and adsorption and s c a tte r p attern s.

Industrial applications include an extensive use of many te ch niques in activation an aly sis , neutron radiography, radioisotope production rad iation effects on m a te ria ls , m etallu rg ical an alysis, geological exploration , and ion im plantation for sem iconductor developm ent.

Now that neutron yields from g en era to rs a re approaching 1012 n -s '1, it appears that such devices m ay find in creased use in biology and m edicine. In addition to studies on b iological effects to ce lls and tissu e s , the use of high yield g en era to rs for rad io therapy is being actively investigated.

It is beyond the scope of this section to d escrib e in detail o r provide sp ecifics on the many applications fo r which neutron g e n e ra to rs a re being utilized. The re a d e r m ust consult the l i te r a tu re for additional applications for fast neutrons.

2 . RADIATION HAZARDS AND S A FE T Y CONSIDERATIONS FORNEUTRON GENERATORS

The potential rad io log ical health con sid erations fo r neutron g en era to rs can be put into the following m ain ca teg o ries :


(i) T ritiu m h azard s; (ii) Neutron h azard s; (iii) Neutron activation; (iv) X - r a y production; (v) Shielding req u irem en ts; (vi) Safety in terlo ck s; (vii) W arning d evices.

2 .1 . T ritiu m h azard s

The inherent design c h a ra c te r is tic s of the sealed -tub e type neutron g en erato r offers a degree of protection from exposure to tritiu m . A m ajor con cern is asso cia ted with the accid en tal breakage of such tubes, although the amount of tritiu m re le a se d in such a situation has been shown to be sm all [5]. In addition to any re le a se of gaseous tritiu m , the occluded tritiu m rem aining in the broken tube p resen ts no m ore than the n orm al h azard involved in handling conventional tritiu m ta rg e ts . The tubes should be kept away from e x ce ss iv e heat to prevent re le a s e of additional tritiu m and should be disposed of as rad ioactive w aste. Unless the lab oratory is equipped to handle tritiu m contam ination p rob lem s, sealed tubes should not be rem oved from th eir m etal heads. The en tire head should be returned to the m an u factu rer.

A much m ore significant tritiu m h azard exists with the use of the pumped type of C ockcroft-W alton neutron g en erato r. During bom bardm ent of the ta rg e t by the deuterium , tritiu m is re le a se d . M ost of the tritiu m is in gaseous form (HT orT 2 ) when it leaves the ta rg e t; how ever, a sm all fractio n of the tritiu m rem ain s in the sy stem in the form of an oxide (HTO or TzO).

T ritiu m is the only rad io active isotope of hydrogen. It em its an 18 keV beta p a rtic le , and has a h alf-life of about tw elve y e a rs . The range of this radiation is about 0.6 m g -cm -2, which is le ss than the thickness of the outer lay er of skin on the body [6]. Thus, it is of m inim al extern al radiation h azard . How ever, within the body, th ere is no p rotection to living tissu e s . Although the h alf-life of tritiu m is approxim ately tw elve y e a r s , tritiu m is elim inated by the body with an effective h alf-life of approxim ately twelve days [6].

When a p erson is exposed to possible flaking of tr itia ted titanium chips or to dust from ta rg e ts or to an atm osphere containing HTO, the HTO entering the body through the to tal skin a re a is ap proxim ately equal to that entering through the lungs [7], Once tritiu m is absorbed by the body, the isotope becom es uniform ly distributed throughout the body fluids within approxim ately 90 m inutes [8]. T ritiu m g as , in the elem ental form , does not p resen t as significant an in tern al hazard as tritiu m oxide, sin ce elem ental tritiu m is absorbed into the body at a much slow er ra te .

A uthorities have gen erally agreed that the p erm issib le soluble and insoluble tritiu m breathing con centration for a 40-h ou r week

9


for the in d ustrial w orker should be 5 .0 X 10"6 |UCi‘cirf3 (185 m Bq-cm "3). The m axim um p erm issib le a ir con centration for tritiu m re c o m mended by the IC R P for continuous occupational exposure (168-h week) is 2 .0 X 1 0 '6 fiC i'cm "3 (74 m B q x m -3) [9], One m illicu rie (1 m C i =37 MBq) has been gen erally established as the m axim um p erm issib le body burden for tritiu m , and is applicable when the body tissu e s a re con sid ered as the c r i t ic a l organ.

M ost re fe re n ce s also lis t a m axim um p erm issib le body burden of 2 .0 m C i (74 MBq) which is to be used when the to tal body is con sid ered as the c r i t ic a l organ [3, 7], This value is obtained in essen tially the sam e m anner as the 1.0 m Ci m axim um p erm issib le body burden. How ever, in the ca se of the 2 .0 m Ci m axim um p e rm issib le body burden, the tritiu m (HTO o r Ts O) is con sid ered to be evenly distributed throughout 70 kg of body m a ss , ra th e r than the 43 kg of body w ater.

U rin alysis cu rren tly con stitutes one of the m ost reliab le m ethods for determ ining the p resen ce and magnitude of human exposure to tritiu m . B ecau se of the p ecu liar rad io logical c h a r a c te r is t ic s of tritiu m , including the sh o rt effective h alf-life for gaseous tritiu m , a preplanned bioassay capability should be readily available for use during any tritiu m handling and at any tim e that a possible overexp osu re is suspected. With the sh ort biological h alf-life , a routine monthly or sem i-an nu al b ioassay will not be adequate. Follow ing a re som e sp ecific tritiu m h azard s which e x ist around the neutron g en erato r and con sid erations which m ust be evaluated when using such m onitoring equipment.

(a) L a rg e quantities of tritiu m build-up in the e lectro n ic ion vacuum pumps. T hese pumps should not be serv iced unless proper facilities and train ed p ersonnel a re available. P otentially lethal amounts of tritiu m m ay be p resen t on the inner su rfaces of the pumps.

(b) Many cu rie s of tritiu m may be re leased during any r e s ta r t of ion pumps and should not be perform ed without serio u s con sid eration being given to the potential problem s and hazard s which will o ccu r.

(c) T ritiu m accu m u lates in various m ech an ical and diffusion vacuum pumps. During vacuum system operation significant quantities of tritiu m m ay p ass through the diffusion pump. Some tritiu m will be trapped in the oil. No attem pt to se rv ice these pumps should be m ade unless p rop er facilities and train ed personnel a re available.

(d) C ertain components of the a c c e le ra to r system m ay be contam inated by tritiu m . The tritiu m can becom e a h azard when the vacuum sy stem is opened to the atm osph ere, for exam ple for ta rg e t

10


changes, m aintenance, o r through leakage or b reak age. T arg et rep lacem en t p ro ced u res and other m an u factu rer recom m endations should be closely observed. The vacuum system should be opened by p ersonnel knowledgeable about the h azard s and well v ersed in the p rop er techniques to be used.

(e) M ost tritiu m will be re le a se d to the environm ent through the pump exhaust. M echanical pumps, used in conjunction with both diffusion pumps and io n -g e tte r pumps, should be vented outside the building. The exhaust should be m onitored to ensure that tritiu m d isch arge lim its fo r the sp ecific a re a a re not exceed ed.

(f) Used oil and ta rg e ts should be contained in m a te ria l through which tritiu m will not read ily diffuse (glass is not re c o m mended). Ventilation and tem p eratu re co n tro l should be provided in the a re a where tritiu m w aste is held pending disposal as ra d io active w aste.

(g) The con sid erations regard in g unused tritiu m ta rg e ts a re very m uch the sam e as for other rad ioactive w aste. A slightly g re a te r potential h azard e x ists in th is situation, how ever, due to la r g e r quantities of tr itiu m . C ontrol of ventilation and tem p eratu re in the sto rag e a re a m u st be con sid ered . Unused ta rg e ts should be kept in the con tain er in which they w ere supplied. E a ch container should be placed in a ventilated a re a . Unused ta rg e ts should not be combined in one la rg e con tain er, as the build-up over se v e ra l weeks m ay exceed the m axim um p erm issib le con centration.

(h) N uclear instrum entation capable of a ccu ra te tritiu m d etection is co m m ercia lly available; how ever, sen sitiv ity , d iscrim ination between tritiu m and other g a se s , and calib ration of th ese d evices, p resen t problem s which m ust be c le a rly understood by the u se r .

Neutron g en erato r in struction m anuals should be re fe rre d to for sp ecific operating in stru ctio n s. Additional inform ation and re fe re n ce s on the subject of tritiu m h azard s should be consulted. Useful inform ation on a new pumping system for a 150-kV neutron g en erator to reduce the p resen t tritiu m h azard will be found in R ef. [10].

2 .2 . Neutron h azards

N eutrons a re c lassed accord in g to th eir kinetic en ergy. F o r convenience neutron energy ran ges have been gen erally defined as follows:

T h erm al 0 .025 eV or le s s in term ed iate 1 keV - 1 MeVep ith erm al 0 .025 eV - 1 eV fast 1 MeV - 100 MeVslow 1 eV - 1 keV u ltra -fa s t above 100 MeV

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As was previously stated , the p rim ary purpose of a neutron g en erato r is to produce neutrons by either the T(d,n) or D{d,n) reactio n . The T(d,n) reactio n produces neutrons with an average energy of about 14 MeV at yields of up to 5 X 1011 m s"1, w hereas the D(d,n) reactio n produces neutrons with an average energy of about 2 .5 MeV at yields of 1010 to 1011 n -s '1. B ecau se of the higher fluxes possible and higher energy attainable, the T(d,n) is of m ore im p ortan ce.

A neutron g en era to r producing 14 MeV neutrons at 1 0 10n*s_:L is approxim ately equivalent to a 700 Ci (26 TBq) rad iu m -beryIliu m neutron so u rce . C alculations indicate that such an unshielded m achine would produce a m axim um p erm issib le dose (assum ing a 4 0 -h week) at a distance of about 100 m from the ta rg e t [2],

The biological dam age caused by neutrons in cre a se s as the energy in c re a s e s . The m axim um p erm issib le flux density (40-hw eek) for 14 MeV neutrons is 12 n’cm ’ ^ s ' 1 and for th erm al (0 .025 eV or le s s ) neutrons is 680 n x m '^ s '1. This is su m m arized in Table I, which a lso lis ts variou s quality facto rs (QF ; a facto r which p rovides the m odification re la ted to radiation quality of the absorbed dose value [12]) for se lected neutron en erg ies .

Other a sp ects of the neutron h azard s a re d escrib ed in the sectio n s on neutron activation and shielding.

2 .3 . C onsiderations of neutron activation

Neutron induced rad ioactivity from the p rim ary beam (including during beam alignm ent) and from secondary radiation can becom e a significant problem . Many te x ts have been w ritten concerning neutron activation of m a te ria ls and it is fa r beyond the scope of this m anual to d escrib e in any depth the reactio n s involved, radioisotopes produced or lev els of rad ioactivity developed. While additional inform ation is presen ted in the section on shielding, a descrip tion of the con sid erations which m ust be evaluated is given.

Any m a te ria l which the beam strik es or which is exposed to intense secon dary rad iation could becom e rad io activ e . Additionally, the p rim ary beam m ay strik e objects such as vacuum cham ber walls and e lectrod e su pp orts. Radiation from th ese so u rce s m ay not be a p erson nel h azard until the m achine is turned off and p erson s en ter irrad iatio n room s and the a c c e le ra to r room fo r m aintenance, ta rg e t changes, routine adjustm ents, o r manual placem ent of sam p les for irrad iatio n . A survey of th ese a re a s should be m ade to evaluate the radiation h azard before or while entering.

A ctivation of cooling w ater around the ta rg e ts m ay be a problem . If a re circu la tin g sy stem is used, a portion of the system may

12


TABLE I. NEUTRON FLUX DOSE EQUIVALENTS [11]

, Neutron energy (M eV)

Quality factor

Average flux density to deliver 100 mrem

(1 mj*kg dose equivalent) in 40 h (n -c m "2 ‘ s*1)

2 .5 x 10“8(thermal) 2 680

1 X 10"7 2 680

1 X 1 0 'G 2 560

1 x 10"5 2 560

1 x 10"4 * 2 580

1 x 10-3 2 680

1 X 10-2 2 .5 700

1 x 1 0"1 7 .5 115

5 X 1 0 '1 11 27

1 11 19

2 .5 9 20

5 8 16

7 7 17

10 6 .5 17

14 7 .5 12

20 8 11

40 7 10

60 5 ,5 11

1 x 102 4 14

2 X 102 3 .5 13

3 X 102 3 .5 11

4 X 102 3 .5 10

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T A B L E II. E X P E C T E D L E V E L S OF INDUCED RADIOACTIVITY AT 10 cm A F T E R 1 h OF OPERATION WITH A NEUTRON GENERATOR Y IELD OF 2 .5 X 10 11 n -s '1 [2]

Exposure rate at 10 cmReaction ----------------------------------------------------------------- Half-life

(m R -h -1) (MC-kg-1 -h-1 )

27 Al(n, p )27Mg 200 52 9 .5 min

27A1 ( n, a )24Na 30 7 .7 1 4 .9 h

63Cu(n, 2n)62Cu 60 15 9 .8 min

“ Cufn, 2n)MCu 60 15 1 2 .8 h

expose personnel in a re a s which could be occupied during operation of the neutron g en era to r . Any re circu la tin g system should rem ain within the shielded a re a and be rem oved from a re a s where personnel exp osures could o ccu r. Even though the activated w ater is no h azard during norm al operation, the resid u al activity p resen t during m aintenance work on the w ater system m ay be hazardous for a tim e a fte r shutting down the g en era to r . The activated cooling m edia should be m onitored and dealt with accord in g to the levels of activity involved. Shielding m ay be req u ired around circu latin g pumps, heat exch angers and holding tanks.

Induced rad ioactiv ity m ay co v er a wide range in intensity and in h alf-life and com e from many rad ioactive nuclides. Many different m a te ria ls used in and around the neutron g en erato r which can be bombarded by the neutron beam can lead to a spread in h alf-liv es n early as g re a t as in the fission -p rod u ct sp ectru m from a n u clear re a c to r , with a sim ila r com posite decay cu rve .

Table II lis ts som e expected exposure ra te s at 10 cm following one hour of operation at 2 .5 X 1011n -s '1 produced by the ta rg e t backing m a te ria l and assem b ly from the neutron activation of aluminium , sta in less s tee l and copper [2].

The combined gam m a exposure dose ra te for a one-hour bom bardm ent at an output of 4 X 1010 n -c m '^ s '1 is estim ated to be 200 to 300 mR*h_1((50to 80 nC •kg"1-h"1) at 10 cm . T hese estim ated exposure ra te values show that con sid erab le activity will be induced in the neutron g en erato r com ponents.

14


2.4. X -ra y production

Unwanted e lectro n s a re produced in a neutron g en erato r when the beam strik es gas m olecules o r atom s or when a sm all p art of the positive beam str ik e s the w alls o r com ponents in the a c c e le r ating and drift tube a re a s . T hese e lectro n s then produce b re m sstrahlung (150 keV m ax .) when drifting back into the acce lera tin g section and a re a cce le ra te d to the upper e lectro d e . The intensity of the X - r a y s produced will depend on the vacuum conditions and the amount of positive beam being a cce le ra te d . X -r a y production gen erally is higher for high beam cu rre n ts and poor vacuum . M easured X - r a y intensity n ear the a cce lera tin g a re a is usually about 2 5 to 100 m R-h"1 (6 .5 to 2 6 tiC-kg"1^ ' 1), but levels as high as 750 mR-h_1(194 MC’kg-1^ " 1) have been noted during neutron production and as high as 450 mR-h_1(116 A*C*kg"1-h_1) when the m achine is operating but not generating neutrons [3]. It is im portant to re a liz e that X -r a y s will be produced during beam alignm ents and at other operating tim es even though no neutrons a re being gen erated .During such operations, it is advisable to shield the op erator console with about 0 .5 cm of lead o r its equivalent. The shielding req u ired during neutron production should be sufficient to stop all X -r a y s .

It is worth noting that while the standard neutron gen erato r is p rim arily designed to a cce le ra te ionized beam s of hydrogen and deuterium , it can be converted to an electro n a c c e le ra to r fo r the production of X - r a y s . An electro n beam cu rren t of g re a te r than 0.5 mA can be obtained when operating at 150 kV. It should be em phasized that X - r a y production during the e lectro n beam mode can be as much as ten tim es higher than during a cce lera tio n of p ositive ions. This is esp ecially c r i t ic a l during alignm ent of the electro n beam .

2 .5 . Shielding req u irem en ts

E xp erien ce has shown that the co st of providing adequate shielding for a neutron g en erato r can often exceed the co st of the m achine itse lf. Shielding.for fast neutrons is a com plex topic encom passing the en tire field of neutron in teractio n s. D etailed inform ation on p rotection against neutrons m ay be of lim ited value in designing sp ecific p ro tectiv e e n clo su res. Gam ma ray s and induced rad io activ ities produced by neutron g en erato rs should not p resen t an extern al h azard , sin ce the shielding req u irem en ts for m ajor fluxes of neutrons and X -r a y s a re sufficient to attenuate such re la tiv ely low intensity rad iation . H ow ever, p ersonnel should carefu lly m onitor the a re a when entering the en closu re following a c c e le ra to r operation to be assu red of safe conditions.

15


The higher the neutron energy, the m o re difficult it is to shield. F o r exam ple, 30 cm of w ater will attenuate 1 MeV incident neutrons by a fa c to r of ap proxim ately 10s w hereas the sam e thickness of w ater only attenuates 14 M eVneutrons by a factor of approxim ately 10 [2],

Shielding fast neutrons is usually accom plished by slowing down en ergetic neutrons to th erm al or n ear th erm al en ergies and final cap tu re . The m ost effective way to d e cre a se the energy of the neutron is to req u ire that it m akes multiple e lastic collision s with light nuclei. A hydrogenous m a te ria l is the lo gical choice for shielding, sin ce the neutron will lose m o re energy p er collision with hydrogen than with any other atom . A fter neutrons have been slowed down by collision with light nuclei, they will usually be captured through a n u clear reactio n . If the shielding m a te ria l is w ater, the m ost probable reactio n is 1H(n, y)2H. The energy of the gam m a ray s is 2 .3 MeV, and they a re em itted prom ptly. Gam ma ray s of this energy a re v ery penetrating, and difficult to shield against.

M ateria ls containing hydrogen, such as w ater, paraffin wax, polyethylene or co n cre te a re com m only used for shielding fast neutrons. Heavy elem ents such as iron o r tungsten provide very effective fast neutron sh ield s, but a re not often used b ecause of th e ir weight and high co st. W ater and polyethylene have a higher density of hydrogen atom s than paraffin wax o r co n cre te . P o ly ethylene is som ewhat b e tte r than w ater in this re s p e c t , but has the disadvantage of high co s t. F o r 14 MeV neutrons, co n crete and paraffin wax (paraffin wax p resen ts a fire hazard) have ap proxim ately the sam e th erm alizin g efficien cy, but co n crete is less expensive. Sealed-tube neutron g en erato rs can som etim es be installed so that the head is placed below ground, thus utilizing the earth as shielding m a te ria l for the neutron radiation .

C on crete is a n atu ral choice for shielding m a te ria l; it is inexpensive, re liab le , s tru ctu ra lly useful and easily installed . If co n cre te blocks a re used, the shield can be easily rem oved or re a rra n g e d . The blocks should be stacked in such a way as to m inim ize the num ber of through cra ck s in the wall. The c r i te r ia to use in selectin g co n cre te a re high density and high m oistu re content.

The 14 MeV neutrons p resen t a shielding problem that seem s to be disproportionately g re a t in com p arison to the size of the a c c e le r a to r and the energy of its deuteron beam [13].

The attenuation ra tio , R , that is req u ired to ensure a given m axim um p erm issib le flux density can be determ ined from the relation :

16


actual neutron flux density per unit surface area

m ax. perm issible flux density per unit surface areaactual neutron flux density per unit surface area at unit distance from source_______( 1 )

m ax. permissible flux density per unit surface a re ax (actu al distance from source)2

using the in verse square law. At 1 cm distance from a point so u rce , a flux density (per unit solid angle) of N n -s"1-s r "1 is , to within 0.1%, equivalent to a flux density of N n -s_1-cm "2. F u rth e r , at d istan ces g re a te r than 3 cm from a point so u rce , the 'cu rved ' a re a on the sphere su rface can be con sid ered to be flat (with an e r r o r of < 1%), while for non-point so u rce this holds fo r d istan ces g re a te r than ten tim es the m axim um dim ension of the so u rce , i .e . with a 3 cm X 2 cm so u rce it holds fo r d istan ces g re a te r than 30 cm .

Exam ple

If the o b serv er ta rg e t distance is 300 cm ( » 1 OX m ax. so u rce dim ension), the flux density is 1010 n‘s"1 sr"1(= 1010 n*s'1-cm ‘ 2 at1 cm ), and the m axim um p erm issib le flux density is 10 n 'c m ^ s " 1, then, from E q .( l ) , the req u ired attenuation ra tio is :

R - Y _ l c m j a l a l X i 04 , 104lO n -c m '^ s -1 \3 00 c m /

If the tenth-value la y e r of an av erag e , n on-arm ou red co n crete m ix, which is the thickness allowing only 1 /1 0 (i .e . 10%) of the neutrons to p ass through, is t, then for an attenuation facto r of 104, a thickness of 4t would be req u ired . F o r 14 MeV neutrons and an average co n cre te , t is 34 cm — that is:

Thickness of co n cre te for attenuation of 104 = 4 X 34 = 136 cm

Many neutron g en era to rs a re utilized for activation an alysis w here sm all irrad iatio n volum es a re needed, hence com p act design.If the a c c e le ra to r is used in a m o re open configuration, e .g . for gen eral re s e a r c h p urposes, the wall th ick n esses could be somewhat reduced b ecause of the g re a te r d istan ces involved.

Shielding design will vary considerably between in stallations and for this reaso n any new installation will req u ire very carefu l an alysis by an exp ert in this a re a . As a v ery gen eral guideline, data indicate that a minimum of 150 cm of w ater an d /o r co n crete or paraffin wax is req u ired to properly shield a neutron g en erato r [2 ,14 ].

17


C onsideration m ust be given to the effect of sky shine through the roof. This can provide a significant contribution to the to tal radiation levels observed outside the shielding w alls.

Ventilation ducts and penetrations through shielding w alls could c re a te a potential radiation hazard. Surveys should be conducted to determ ine the levels of radiation that e x is t outside of the shielding in the a re a of penetration.

If ventilation ducts a re la rg e enough for a man to craw l through, an additional hazard may ex ist. Inspection co v e rs and d oors, outside the shielding, should be interlocked to avoid inadvertent opening of the duct during operation. This p recau tion may prevent the accid en tal exp osure of a p erson doing a routine inspection of the sy stem or the escap e of toxic gases into the building.

W here labyrinths a re used, the neutron dose outside the shielding w alls m ay be dominated by sca tte rin g through the labyrinth. E ach neutron s c a tte r through a sm all opening such as a doorway will reduce the flux density by a facto r of 100, so that provisions should be m ade for at least two s c a tte rs .

The shielding, which may have been adequate at the tim e of installation , may at a la te r date be inadequate for one of se v e ra l reaso n s:

(a) c ra c k s or other openings may o ccu r,(b) the m achine m ay be modified to produce higher beam

en ergies or in ten sities,(c) the shielding configuration may be changed,(d) the shield m a te ria l, or s tru ctu re , may d eterio ra te .

Surveys of the a re a around the shielding should be made p eriod ica lly . If a defect in the shielding does exist, the background ra d iation in the im m ediate a re a m ay in cre a se . Steps should be taken to lo cate the defect and make th e .n ecessary c o rre c tio n s . If the defect is difficult to lo cate , a map of the distribution of radiation intensity, showing contours of equal intensity, m ay be useful.

In conclusion, and bearing in mind the problem s d iscu ssed , it will be n e cce sa ry to consult the li te ra tu re , of which a few exam p les a re re feren ced h ere [1 5 -2 4 ], It should be pointed out, how ever, that it is alw ays well worth the effort to obtain the se rv ice s of a qualified exp ert on shielding before the design of a facility has reach ed too advanced a stage .

2 .6 . Safety in terlock s

In gen eral in terlock s and warning devices (Section 2 .7 ) a re no b ette r than the individual responsible fo r th e ir operation. T h ere is

18


no e le c tr ic a l o r m ech an ical safety device which is totally 'f a il -s a f e '. E ach in terlock is intended to furnish protection to personnel or equipment in the event of a sp ecific m alfunction or im proper operating condition.

E a ch in terlock functions to open o r clo se a p air of e le c tr ic a l co n tacts , either d irectly or via a re la y . Many p airs of co n tacts in s e r ie s m ust be closed to provide con tro l power for the beam sw itch. T here is a g re a t deal of v ariation in the interlocking arran gem en ts to be found in different neutron g e n e ra to rs . This is due to the inherent d ifferen ces among m ach in es, sp ecific space or com ponent lim itation s, and d ifferences in design philosophy, co st fa c to rs , and u se rs ' req u irem en ts. Ideally, the in terlock system should be as c lo se to 'fa il-s a fe ' in that d efects o r component failu res will prevent operation of the d evice.

The radiation in terlock sy stem has two g en eral functions: to p reven t a c c e s s to a re a s in which p erm issib le rad iation lev els a re being exceeded; and to p revent operation if such operation will exceed p erm issib le rad iation levels in any occupied a re a .

A cce s s to the ta rg e t room is e ith er through a m aze o r through a doorway, which is blocked during irrad iatio n by a shielding door.In either ca s e , it should be n e ce ssa ry for personnel entering the room to open a door which shuts off the a c c e le r a to r . A k ey-op erated sw itch should be in corp orated in the beam co n tro ls . The key to this sw itch should be req u ired to unlock the door or gate leading to the ta rg e t a re a . Thus the beam m ust be turned off and the key rem oved from the sw itch, b efore anyone can en ter the a re a of m ost ex trem e h azard . Additional sw itch es, which a re clo sed only when d oors, g ates , shielding plugs, and shielding doors a re in a safe condition, m ay be used in s e r ie s with the key sw itch.

P erso n n el m ust not depend on a door in terlock to turn off the beam . If the door in terlock fa ils , a serio u s radiation exposure is alm ost ce rta in to o ccu r. The sw itch on the con tro l console should alw ays be placed in the 'off' position before entering the radiation a re a . S im ilarly , the neutron g en erato r should n ever be turned on from any location other than the con tro l console.

One or m ore disabling sw itches should be installed in ta rg e t a re a s and exp erim ent ro o m s, so p erso n s in those a re a s can turn off the m achine or prevent the beam being turned on, in the event that they a re trapped when som eone else attem pts to turn on the beam . It is also advisable to have an in tercom o r telephone in the ta rg e t room for use by any individual trapped in the room .

The co n tro l c ir c u its should produce a warning by light or sound, o r both, for som e period a fte r the irrad iatio n ro o m s a re closed and b efore the beam can be turned on, to allow anyone trapped in a

19


room to re a ch the disabling sw itch. The irrad iatio n room door should be operable from the inside even afte r power fa ilu re . The door handle should open an in terlock , whether o r not it opens the door.

R adiation -d etection instrum ents should be installed to m onitor the radiation levels in various a re a s , including the con trol room and other roo m s norm ally occupied when the beam is on. M onitors m ay be installed in ta rg e t a re a s which a re not occupied but where the rad iation level is an im portant in d icator of g en erato r p erform an ce. S om etim es th ese radiation inten sities a re reco rd ed au tom atically .The in strum ents should turn off the beam an d /or give warning, if the p erm issib le rad iation level has been exceed ed. T hese instrum ents should be adjusted to the p rop er levels and tested p eriod ically .

Not to be overlooked a re in terlock sy stem s dealing with ex trem e e le c tr ic a l h azard s to personnel, p articu larly high-voltage d .c . power supplies. Sw itches on the a c c e s s doors to high-voltage com p artm en ts should in terru p t the input voltage to the co m p a rtm ent when the doors a re opened, and should d isch arge cap acito r banks. In g en eral, any e le c tr ic a l h azard which has a p rotective en closu re should have in terlock sw itches installed to open the power c ircu it if the en closu re is opened.

It should alw ays be rem em b ered that these in terlock s seldom a ct d irectly to rem ove the hazard , but ra th e r act through in te rm ediate re lay c ir c u its . Even though the in terlock sw itch in a door fram e is 'v isib ly ' in the open or safe position, the other p a rts of the in terlock c ircu it may fail to operate c o rre c tly , and the hazard m ay still ex ist. A lso , re la y s or sw itches may fail, or becom e m isaligned, so that th ese in terlock s a re not com pletely dependable.It should never be assum ed that the in terlock s have rem oved the h azard .

A com plete in terlock chain includes many p airs of co n tacts , arran ged in variou s m ain and subsidiary sequences; som e of these a re 'p ow er-on ' sequences, but som e may be 'pow er-off' sequences. The com plexity of an in terlock chain can lead to m isunderstanding on the p art of in exp erien ced op erato rs and tech n ician s. T h erefo re , com plete and a ccu ra te c ir c u it d iagram s of the en tire in terlock sy stem a re essen tia l. T hese drawings should be readily available to the o p era to rs and to the technicians who a re responsible for m aintenance and such p erson s should becom e fam ilia r with the c irc u its .

F au lty diagnosis or im proper re p a ir can c re a te a condition in which the in terlock chain ap pears to be operating c o rr e c tly , but im portant elem ents of the chain have been bypassed o r overrid den . Any re p a irs made to the in terlock c irc u its , esp ecially those

20


involving m odifications or substitutions of com ponents, should be carefully reco rd ed in log books and on the c ircu it d iagram s.

It is frequently a convenience and a m eans of con servin g o p erating tim e to circu m ven t a troublesom e in terlock component (such as a sticking re lay or an e r r a t ic con tact susceptible to vibration) by installing a jum per or bypass a c ro s s a cce ss ib le te rm in als in the co n tro l room , ra th e r than shutting down the neutron g en erato r until a rep lacem en t for the troub lesom e component can be obtained and in stalled . E xp erien ced o p erato rs may be able to do this and m onitor the bypassed in terlock function p erson ally , with adequate safety and with con sid erab le benefit to the work of the lab o rato ry . How ever, if the o p erator b ecam e c a re le s s and failed to re co rd the change, o r failed to make it conspicuously obvious to all oth ers con cern ed, an extrem ely hazardous condition could re su lt. Any tem p o rary change in the in terlock system should be made unm istakably evident to every p erson who could be endangered by the change.

Specific em ergency p roced u res should be established whenever in terlock s a re bypassed. This should include the p resen ce of at le a s t two individuals to verify the safety of the p ro ced u re . In addition, this change should be noted in a log book, signed by the p erson s p resen t, and norm al operation not resu m ed until the bypass is rem oved and the log book signed to indicate that it has been rem oved. A warning sign should also be posted at the con trol console to indicate that an in terlock or in terlock s have been bypassed.

W here fa il-sa fe operation of equipment cannot be otherw ise assu red , a duplicate system of in terlock s should be used. In this c a s e , m e ch a n ica l-e le c tr ica l in terlock sy stem s fo r doors e tc . should be c a r r ie d through in duplicate, with w iring, co n tacts , and re la y s , located so as to m ake sim ultaneous failu re of both c irc u its extrem ely unlikely. The h azard of som eone disabling the system (for exam ple, by placing a co n cre te block on a cable) is avoided by using widely sep arated cab les in the two sy ste m s. An im portant req u irem ent of a duplicate sy stem , how ever, is that failu re of e ith er of the two c ircu its should be conspicuously signalled, so that it can be c o rre c te d . The sy stem is safe only when both c irc u its indicate 's a f e 1; it m ust be broken when the two 'd isa g re e ' o r both indicate h azard . W hatev er type of sy stem is used, a frequent check of its p rop er operation is essen tia l.

\2 .7 . W arning devices

In addition to in terlock s to prevent inadvertent entry into hazardous a re a s during neutron g en erato r operation, warning

21


devices should be utilized to inform people of the m achine statu s. L ig h ts, signs and audible devices can be used.

(a) Status lights. The location of status lights (lights which indicate the state of the system ) is im portant. They should be located in the irrad iatio n ro o m s, co rr id o rs leading to the irrad iation ro o m s, and on the con tro l con sole. The status lights should indicate any conditions con sid ered im portant for the safety of operating p ersonnel. R egular ch eck s should be made to make su re the light bulbs have not burned out.

(b) Signs. Signs should be used to m ark a re a s where hazardous radiation levels could e x ist. A ppropriate radiation su rveys should be m ade.

(c) Horns o r other audible d evices should be used w here p ra c tica l to indicate that the beam is on o r is about to com e on.If the audible device m ust be on for long p eriod s, it should make an in term ittent tone which is low enough not to d is tra c t people working in the a re a . If the device is loud, it will probably be disconnected or modified to make it inaudible. A public ad d ress system announcem ent of impending tu rn -on can be used in addition to an audible warning signal.

3. RADIATION MONITORING AND IN TERPRETA TIO N OFM EASUREMENTS

The detection in strum ents which may be used for radiation su rvey s a re d escrib ed in this section . The d escrip tions a re intended to give the re a d e r an idea of the types of in strum ents available.Since the c h a ra c te r is tic s of each type of instrum ent may vary from one m an u factu rer to another, the m an u factu rers should be consulted for m ore detailed operating c h a ra c te r is tic s and recom m endations.

In selectin g the b est in strum ent for the m easu rem en t of ra d iation for a p articu lar d osim etry problem , the convenience of o p eration, portab ility , re liab ility , econom y, type of radiation detected, energy dependence, and exposure ra te independence of the instrum ent should be con sid ered . E a ch m onitoring assignm ent has its own individual se t of co n sid eratio n s. It is n e ce ssa ry to know the c h a ra c te r is tic s and lim itation s of any survey instrum ent or pocket d osim eter in o rd e r to use it intelligently.

It should be recogn ized that all in strum ents a re lim ited to m easu rin g ce rta in types of radiation within a fixed range of en erg ies , and that for other types of radiation or at ce rta in en ergies of ra d iation, th eir read in gs may not only be u se less but often m isleading.

22


It is extrem ely im portant that survey and m onitoring instrum ents be prop erly m aintained and calib rated , and sen sitive to th ose ra d iations being m onitored.

The radiation safety of any facility can be no b etter than the p erfo rm an ce of the survey and m onitoring in stru m en ts. Instrum ents should be calib rated at reg u lar in terv als using secondary stan d ard s. R eferen ce so u rce s a re available from som e lab o ra to ries which have the equipment n e ce ssa ry to ca lib ra te the so u rce s . W here p ossib le, in strum ents should be ca lib rated by the use of a so u rce whose activity , energy, and intensity a re s im ila r to those to be m easu red . E a ch tim e an instrum ent is used, the u se r should be a le r t for p ossible m alfunctioning, such as indicated by e r r a t ic reading, no reading, or ra te s of change in the readings that do not corresp on d to expected variatio n s in rad iation intensity. When such troubles o ccu r, the o p erator m ust not n e ce ssa rily assu m e that the m e te r is wrong, but instead should get another instrum ent or otherw ise attem pt to determ ine the tru e condition.

The following discu ssion on m onitoring instrum ents is n e ce ssa rily b rief since it is beyond the scope of this manual to co v er the subject in detail. B a sic concepts a re covered and other re fe re n ce s should be review ed to obtain fu rth er tech n ical con sid eration s.

3 .1 . T ritiu m m onitoring instrum ents

It is im portant that p rop er ventilation techniques be used to ensure that any re le a se of tritiu m is adequately exhausted from the facility . It is n e ce ssa ry to provide a ir m onitoring for re le a se of tritiu m gas.

When m onitoring for tritiu m , it is extrem ely im portant that the instrum ent be ca lib rated against a known con centration of tritiu m gas under actu al conditions of operation. The a ir flow m ust also be carefu lly determ ined. The m an u factu rer of such in stru m en tation should be req u ired to provide this inform ation at the tim e of p u rch ase . P erio d ic re ca lib ra tio n should be perform ed .

A com m on device for tritiu m a ir m onitoring is an ionization cham ber through which a ir is drawn at a fixed ra te . The ionization cu rren t is am plified and re la ted to the con centration of rad ioactive gas in the a ir . The ionization cham ber may be p reced ed by an ion c o lle c to r an d /or filte r in o rd er to rem ove other ions or rad ioactive m a te ria l.

T ritiu m contam ination can be detected and analysed through the use of sm e a r sam p les collected with filte r paper and properly m easu red . The m ost com m on technique is to p lace the filte r paper in a p rop erly ca lib rated p roportional counter or liquid scin tillation

23


d e tecto r . In gen eral liquid scin tillation m ethods a re p referab le sin ce with the weak beta radiation (about 18 keV m ax.) from tritiu m , g ro ss contam ination will be p resen t before detection with a p ro p o rtional counter becom es p ossib le. E xam p les of the distribution of tritiu m within an a c c e le ra to r obtained by taking sm e a r sam ples a re d escrib ed in the lite ra tu re [5, 10]. A survey of the techniques that have been developed for m easu ring tritiu m in various m edia is given in a recen tly published review paper [2 5],

3 .2 . Neutron m onitoring instrum ents

The flux density of neutrons outside the shielding of a sp ecific facility will depend on the use of the beam , the beam energy, the n u clear c h a ra c te r is tic s of the m a te ria l being irrad iated , and the n ature of the shielding m a te ria l. The purpose of a neutron radiation survey is to determ ine whether the flux density outside the shielding is within p erm issib le lim its .

Neutron rad iation around the installation may be accom panied by re la tiv ely high levels of gam m a radiation . Consequently, in o rd er to m easu re the neutron flux density adequately it is n ecessary to have an instrum ent insen sitive to gam m a radiation.

In selectin g neutron survey in strum ents for a sp ecific a c c e le r a to r facility , som e g en eral a sp ects of the detection instrum ent should be con sid ered .

(a) The detection instrum ent may be calib rated either in f irs t collision neutron dose o r in neutron flux. In either c a s e , carefu l in terp retatio n of the m easu rem en ts m ust be made and the ca lib ratio n p roced ure m ust be known.

(b) The gam m a resp on se or d iscrim in ation of the d etector should be known in o rd er that the gam m a effects can be excluded from the n eutron-field m easu rem en t.

(c) Like the resp on se of any rad iation d etecto r, that of a neutron instrum ent is governed by s ta tis tic a l co n sid erations. Since shielding evaluation m ay ordinarily en com p ass se v e ra l o rd e rs of magnitude of intensity, the question of adequate sensitivity and linearity of perform an ce over the whole range to be m easu red is fundamental.

(d) Although absolute d osim etry of neutron fields is an in tr ica te problem re la tiv e m easu rem en ts a re frequently all that a re req u ired for shielding evaluation. Neutron flux density m easu rem en ts a re m ore useful fo r shielding evaluation than a re dose m easu rem en ts . How ever, even flux density m easu rem en ts req u ire a good referen ce

24


standard to allow m easu rem en t of d ep artu res from c a lib ration in the d e tecto r . The re fe re n ce standard should have a sp e c tra l com position approxim ating the neutron field being m easu red .

Ionization in strum ents a re usually not sa tisfa c to ry for m easu ring neutron fluxes or neutron dose, since they a re also sen sitive to gam m a radiation.

The boron triflu orid e (B F 3) proportional counter provides a sen sitive d etector for a neutron survey instrum ent and can be re la tiv ely insen sitive to gam m a radiation . The instrum ent uses B F3 gas enriched to m ore than 95% in the isotope 10B, and typically the d etecto r has about a 10 cm active length and 16 cm o v erall length.

The B F 3 counter is sensitive to th erm al neutrons. To m eet the req u irem en ts for a counter suitable for in term ed iate -en erg y and fast neutrons, the B F3 counter can be en closed by a polyethylene o r paraffin wax m od erato r which m od erates the neutrons before entering the B F3 g as . By suitable selection of the m o d erato r con figuration, it is possible to achieve reactio n ra te s which a re substan tially independent of neutron energy from 10 keV to nearly 10 MeV [25a].

The development of good scin tillation survey instrum ents to m easu re neutrons depends p rim arily on scin tillating m ate ria ls with the p rop er c h a r a c te r is tic s , since the c irc u its a re essen tially the sam e as those for m easu ring other kinds of radiation . E xam p les of scin tillating m a te ria ls used for neutron survey instrum ents a re organ ic phosphors or proton radiating m a te ria ls com bined with scin tilla to rs fo r fast neutrons, and boron or lithium bearing phosphors fo r th erm al neutrons. M ateria ls sensitive to th erm al neutrons can be com bined with m o d erato rs to provide a d etector which gives a rem resp o n se fo r th erm al, in term ed iate , and fast neutrons. S ev eral such neutron m onitoring in strum ents have been d escrib ed [2 6 -3 0 ],

T h ere a re other techniques for fast neutron detection which are beyond the scope of this m anual, e .g . those using film or p la s tics . The in terested re a d e r is re fe rre d to the publications in R efs [3 1 -3 8 ] for a m o re detailed inform ation on neutron dosim etry in gen eral.

3 .3 . X - r a y and gam m a-ray m onitoring instrum ents

Many hundreds of textbooks and tech n ical a r t ic le s have been w ritten on the subject of X -r a y and gam m a ray instrum entation and m easu rem en ts . A s exam p les, the re a d e r may re f e r to R efs [3 9 -4 1 ]

25


for a gen eral introduction. C om m ercially available instrum ents and the inform ation supplied by the m an u factu rer should provide adequate details for such m onitoring. As with other types of d e te cto rs , how ever, it is m ost im portant to understand the effect on the m easu rem en ts of the energy c h a ra c te r is tic s of the radiation being detected as well as the resp on se of the instrum ent to that energy [42]. And, the im portan ce of good instrum ent m aintenance and calibration cannot be overem phasized .

3 .4 . P erso n n el m onitoring

M onitoring the exp osure and dose to radiation w orkers can be accom p lished by using pocket d o sim eters , pocket ionization ch am b ers, film badges and therm olu m in escen t d o sim eters . D osim eters and pocket ch am b ers a re usually lim ited to beta and gam m a radiation. F ilm badges can be m ade sen sitive to gam m a and X -r a y s , e lectro n s, and neutrons. The use of f i lte rs make possible the sep aration of different kinds of radiation and different energy ran g es. Ionizing rad iation s produce blackening of the em ulsion, which can be d e te rmined photom etrically afte r developm ent. Since it is possible to prevent the disap p earan ce of the latent im age by suitable choice and treatm en t of the em ulsion, as well as by the a ir -tig h t en closu re of the film , an alm ost com plete in tegration of the radiation dose for se v e ra l weeks can be guaranteed . The continuous sensitivity and ease of handling of photographic film , as well as it being a perm anent re c o rd of the rad iation receiv ed , m ake film d osim eters p articu larly suitable for individual d osim etry and for periodic exam ination of the in tegrated exposure of p erson s constantly exposed to rad iation .

The gam m a dose re ce iv e d by the p ersonnel should be m easu red continuously by two independent m ethods, e .g . film badge and pocket d o sim eter. A p erson nel d osim eter sen sitive to neutrons should be included in this m onitoring. The lim its for the m axim um p e rm is sible doses have to be observed but the dose receiv ed should be as low as p ossib le.

In addition to the p ersonnel m onitoring of ex tern al radiation, u rin alysis for the detection of tritiu m should be c a rr ie d out when n e ce ssa ry , e .g . after work with tritiu m contam inated com ponents.

4 . REQ UIREM EN TS FO R AN E F F E C T IV E S A FE T Y PROGRAMME

R espon sib ilities for the establishm ent of a safety p rogram m e and an organization for safety at p artic le a c c e le ra to r facilities

26


have been d escrib ed [43] and those which a re pertinent to neutron g en erato r facilities a re presen ted .

No neutron gen erato r facility should op erate without an adequate and effective safety p rog ram m e; how ever, one plan fo r a safety organization will not fit all types of fa c ilitie s . The p rogram m e can be effective only through the actions of a safety organization which m eets the needs of the facility .

The safety p ro gram m e should be designed to p ro tect personnel from injury and equipment from dam age. It should: (i) m aintain safe working conditions; (ii) enable the facility to m eet its statutory and legal obligations; (iii) estab lish p ro ced u res and organizations to deal with em erg en cies , such as f ire s , explosions and radiation accid en ts; (iv) conduct n e ce ssa ry inspections; and (v) in stru ct p erson nel in safe attitudes and p ra c tic e s .

P erso n n el req u ire train in g to becom e aw are of a ll the hazard s which exist or may develop, and to becom e fam iliar with the ap prop riate safe p ra c tic e s . This is especially tru e of rad iation h azard s.It is also tru e of the many conventional h azard s which may e x ist in equipment and surroundings.

The establishm ent and support of an effective safety p rogram m e is , ultim ately, the resp on sib ility of the d ire c to r of the facility . He should c le a rly estab lish the a re a s and levels of authority for the actu al conduct of the p ro g ram m e, sin ce everyone in the facility m ust contribute if it is to be su ccessfu l.

S u p erviso rs at a ll levels should be resp on sib le fo r safety and should prom ote the safety p ro gram m e. P erso n n el should be inform ed through ap propriate train in g of the h azard s and recom m ended safe p ra c tice s re la ted to th eir work. Individuals should recogn ize the h azard s encountered, and take p recau tion s in th eir work.

A safety p rog ram m e m ust balance between m inim izing risk s and m axim izing the use of the facility . At the sam e tim e a p rop er safety p rogram m e should cau se negligible in terferen ce with the work of the facility . When this sm all in terferen ce and expense is com pared with the co st of possible accid en ts , no one should object to a p roperly planned and ad m in istered safety p rogram m e. The establishm ent and rigo rou s support of operating and radiation safety p roced u res in operation and m aintenance is of utm ost im portan ce. The safety p rogram m e should assu re :

(a) That an adequate organization is established to form u late , advise on and im plem ent safety policy, beginning at the tim e the facility is f i r s t proposed.

(b) That buildings and facilities provide an environm ent for safe conduct of exp erim en ts, and lim it the extent of damage cau sed by any equipment m alfunction.

27


(c) That safety is in tegrated in a p ro ject from its beginning and that the p ro ject is p eriod ically review ed by responsible p erson s to ensure that continuing con sid eration isbeing given to problem s of safety .

(d) That th ere is an identification and evaluation of h azards at all stag es of an experim ent or irrad iatio n p ro ce ss .

(e) That em ergency plans a re developed and im plem ented to include p rogram m es for im m ediate decontam ination p ro ce d u res, cooperation with health authorities and notification of fire officials of potential h azards which could exist.

4 .1 . Safety organization

O rganization for safety in a neutron g en erato r installation should provide for radiation safety , equipment safety , and gen eral safety .A safety com m ittee should review proposed exp erim en ts, facility changes o r deviations from standard operating p ro ced u res. The safety com m ittee should also com pile and publish ru les and p ro ce dures for safe p ra c tice s in th eir facility . The functions of the safety com m ittee should include the following:

(a) F o rm u la te safety policy(b) E stab lish review p ro ced u res and standards(c) C o-ord in ate and review safety activ ities(d) E xam in e req u ests fo r v arian ces(e) Advise the d ire c to r , staff and u ser groups on safety m a tte rs .

A typ ical constitution of a safety com m ittee would include the head of the departm ent in which the facility is located , the radiation safety engineer, the fire officer, the health physics sp ecia lis t, and a sen io r m em b er of the operations staff.

4 .2 . Radiation safety

R espon sib ilities for rad iation safety should include:

(a) Inventory and con tro l of rad ioactive so u rce s , ta rg e ts and other activated m a te ria ls

(b) O bservation and con tro l of radiation hazards(c) R adiation w aste sto rag e and disposal(d) G eneral radiation m onitoring p roced u res(e) In struction of personnel in observation of ru les and

m onitoring p roced u res(f) M aintenance of re co rd s re la ted to exp osures and accu m u

lated doses rece iv ed by the p ersonnel

28


(g) P erio d ic routine survey of the installation(h) Surveys of new exp erim ental set-u p s(i) Survey of unusual conditions including conditions during

m aintenance op erations.

4 .3 . Neutron g en erato r safety

The person in ch arg e of safety (safety engineer) should have an adequate background of exp erien ce re la ted to the g en era to r . He should be resp on sib le fo r the m ech an ical and e le c tr ic a l safety re la ted to:

(a) T a rg e ts(b) A uxiliary m ech an ical equipment(c) Special f ire protection(d) C ontrol sy stem .

He should be responsible for instruction of o p erato rs and tech n ician s, should keep the o p erator adequately inform ed of the radiation fields, and should dissem inate inform ation about safety p roced u res and sp ecia l h azard s.

4 .4 . G eneral safety

R espon sib ilities for gen eral safety p roced u res should include:

(a) S torage and safe use of ch em icals(b) S torage and safe use of gases(c) Safety supplies(d) Elim ination of fire and m ech an ical h azards(e) E m erg en cy lighting and power(f) M aintenance and use of sp ecia l safety equipment, such as

ventilation sy ste m s, re s p ir a to rs , safety g la sse s , se lf - contained breathing apparatus and a ir sampling equipment.

(g) Review of handling p roced u res involving toxic m a te ria ls .

Specific guidelines for the safe operation of sm all p article a c c e le r a to rs have been published [44, 45] and provide useful consid eratio n s for neutron g en era to r facilities as well. Individual national lab o ra to ries often have available safety inform ation which can be helpful in establishing an effective p rog ram m e.

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Appendix I

While it is the purpose of this m anual to em phasize the ra d iation h azard s, it is advisable to briefly point out som e other h azard s which m ay e x ist at a neutron g en erato r facility [43].

The p rin cip al fire hazard is asso cia ted with that of e le c tr ic a l f ire s . F i r e prevention m easu res and fire protection equipment for e le c tr ic a l f ire s should be provided. It m ust be borne in mind that, for exam ple, com bustible or inflam m able liquids, solvents and g ases a re used in operation and m aintenance.

F i r e prevention and p rotection m e asu res comm only include location of hazardous equipment in non-com bustible s tru c tu re s , adequate drainage and ventilation, elim ination of a ll com bustible m a te ria ls from the a re a and fire detection d evices.

Som e recom m ended fire and explosion prevention m easu res for exp erim en tal a re a s include the following:

(1) A periodic review of all cu rren t experim ents should be m ade to evaluate the possible in teraction of the hazards of each , with the equipment and m a te ria ls of other exp erim en ts.

(2) B efore approval is given for equipment and m a te ria ls , as well as exp erim ental p ro ced u res, con sid eration should be given to layout and design with re g a rd to confining any potential incident to the sm allest a re a p racticab le . Thus dam age will m o re likely be adsorbed within one a re a withr out propagating to o th ers .

(3) The quantity of flam m able m a te ria ls in the exp erim ental a re a should be m inim ized.

(4) The fa c ility 's e le c tr ic a l , ventilation, a la rm , and other sy stem s should be evaluated for each experim ent, and p a rticu la r attention be given to possible in teraction of any of th ese sy stem s in an em ergency.

O bjects which a re not bolted to the floor should be b raced or arran g ed so that they can r e s is t side fo rce s of at le a s t a fifth of th e ir weight without overturning. P ip es , panels, or other components which could be stepped on under any circu m stan ce should eith er be b raced to withstand such load s, or be guarded against being stepped on.

NON-RADIATION HAZARDS AND SAFETY CONSIDERATIONS

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M echanical vacuum pumps a re frequently o il-sea led vane pumps. The a re a around an o il-sea led pump tends to becom e slippery as a resu lt of condensed oil vapour and will p resen t a fire halzard unless carefu lly m aintained. Oil spills and leaks also contribute to the problem . Mounting pumps in m etal tra y s can confine the oil to a re a s which a re norm ally not walked upon.

Diffusion pumps use oil or m ercu ry vapour. P erso n n el who handle or a re exposed to such fluids should acquaint th em selves with the degree of flam m ability or toxicity of the fluid being used. P a rtic u la r c a re should be taken when m e rcu ry is handled.

Ion pumps use high voltages to ionize and trap gas m o lecu les. The e le c tr ic a l term in als to such pumps typically op erate at se v e ra l thousand volts and should be trea ted as a serio u s e le c tr ic a l h azard .

A m ost serio u s danger is that of fatal e le c tr ic shock. Howe v e r, non-fatal e le c tr ic shocks can cau se se v e re injury as a re su lt of falls or im p acts again st nearby equipment. E le c tr ic accid en ts frequently cau se serio u s damage to equipment, including dam age by fire and sm oke.

E v ery situation o r event will req u ire good judgement and intelligent application of the n e ce ssa ry safety m e a su re s to the p a rticu la r problem of the m om ent. E ach facility should develop its own safety p roced u res based on its own needs. T hese safety p ro ced u res should include the following:

(1) N ever work alone on hazardous e le c tr ic equipment.(2) N ever take ch ances — always assu m e c ircu its a re

en ergized until you have checked them .(3) Avoid form ing a c ircu it to ground through your body.

Stand on insulating m a te ria l and do not lean on m etal cabinets or s tru c tu re s . Use one hand only.

(4) Follow approved tag -o u t and lock -ou t p ro ced u res.R em ove fuses whenever possible to prevent accid en tal application of pow er.

(5) Make su re th ere is adequate light and free a c c e s s to the equipment. Avoid working in confined or uncom fortable sp a ce s .

(6) P rovid e and use grounding hooks on cap acito r banks and high-voltage equipment.

(7) Follow good housekeeping p ra c tic e s .

B ecau se p ersonnel do not expect high voltages in con trol c irc u its , it is im portant to keep such voltages out of the con tro l cab in ets. P ow er c irc u its should also be kept out of con tro l cabinets b ecause the techniques and instrum ents used in con trol

31


c irc u its and th eir testin g can fail dangerously on power c irc u its , it is a lso im portant that all con tro l power com e from a com m on so u rce and that in terlock co n tacts operated by power co n tacto rs or b reak ers be supplied from the con trol power and not from the power c ircu it through the sw itch gear to which they a re attached.

The o p e ra to r 's console and all other en clo su res with which personnel can com e into con tact should be secu rely grounded or earthed to prevent e le c tr ic shock in the event of s h o r t 'c ir c u its or. component fa ilu res inside the equipment. H igh-voltage and power cab les should be run in sep arate cable ducts (wire ways) from co n tro l c ir c u its .

O perating and m aintenance p ersonnel should always rem em b er that console and m e te r indications p resen t only in d irect inform ation about the condition of the c ircu its they m onitor. T h ere a re often se v e ra l in term ed iate components between the actu al c ircu it elem ent or voltage which is a sso cia ted with the indication, and the in dicator itse lf . T hese com ponents may include r e s is to r s , ca p a cito rs , re lay s e tc . in addition to the panel light o r m eter itse lf. The fact that a m eter in dicates zero voltage, or that a light is out, does not p resen t conclusive evidence. No c ircu it should be assum ed to be d e-en ergized , and th erefo re safe, m erely because that condition is rep resen ted by the in d icato rs .

A toxic m a te ria l is any substance which has the cap acity to produce p erson al injury or illn ess through ingestion, inhalation or absorption through any body su rfa ce . All ch em icals should be regard ed with suspicion until proven to be n on-to xic. Many ch em ic a ls , solven ts, and m etals have known to xic p rop erties and standard handbooks on to xic m a te ria ls should be available for easy re fe re n ce .

It is im portant to use good com m on sen se when evaluating safety p ra c tic e s . A constant aw aren ess of possible h azard s is m ost im portant.

32


Appendix II

CONSIDERATIONS FOR A NEUTRON GENERATOR LABORATORY

C erta in b asic guidelines can be furnished that will provide a ssis ta n ce regard in g co st estim ates and shielding considerations for individuals contem plating the installation of a neutron gen erato r lab o rato ry . It m ust be understood, how ever, that every such installation will have its own unique c h a ra c te r is tic s and thus any final plans an d /o r p rogram m es will req u ire carefu l and intensive study and evaluation.

COST ESTIM ATES

Often the co st of shielding m a te ria l fo r a lab orato ry can be equal to or g re a te r than the initial co st of a neutron gen erator itse lf . F o r the y e a r 1974, th ese co sts w ere found to v ary from as little as about $5 0 0 0 to g re a te r than $2 5 000, depending on the type of g en erato r (e .g . sealed -tub e or pumped C ockcroft-W alton type), the a c c e s s o ry equipment d esired and the design of the lab oratory (shielding for a sealed -tub e gen erato r or placed underground could co st as little as $3 000 to $4 0 0 0 ) in a typ ical industrialized country.

M onitoring instrum entation and radiation protection equipment can be expected to vary from $ 2 5 0 0 up to $ 1 0 000, again depending upon the sp ecific application and facility design for the neutron g en erato r.

T Y P IC A L SHIELDING ARRANGEMENTS [2]

One of the m ost effective ways to shield fast neutrons is to com pletely surround the ta rg e t a re a with a hydrogenous m a te ria l such as w ater or paraffin w ax, which absorbs the neutrons before they have a chance to escap e into the surrounding environm ent.This principle is applied in the arran gem en t shown in F ig .A -II -1 .The ta rg e t is in the cen tre of a five-foot cu bical w ater tank which is surrounded by a 0 .25 in (6 .4 m m ) lay er of b oric acid . Hydrogen in the w ater th erm alizes the fast neutrons. B o ro n -10 in the b oric acid lin er cap tu res m ost of the slow neutrons which leak out of the tank. A 40 in (~ 1 m ) wall of co n crete blocks is situated between the w ater tank and the op erator con sole.

33


luuj 90

FIG- A -II-1 . Installation used for fast and thermal activations.


The co n crete around the tank provides additional shielding again st both fast neutrons which escap e the w ater and gam m a ra y s produced by th erm al cap tu re in the w ater.

F o r operation of the neutron g en erato r at a level of 4X 1010n<s"1, the to tal dose at the console due to fast neutrons or gam m a ra y s is below 2 .5 m R -h '1 (6 .45 X 10-1C•kg"1-h"1); the dose due to slow neutrons is well below this value. In the a c c e le r a to r room , the fast neutron dose is considerably above the m axim um p erm issib le dose.

The shielding m a te ria l shown in F ig .A -II -1 does not rem ove all neutrons. Many neutrons a re slowed down and subsequently captured in the shield. A significant fractio n of neutrons escap e the shield, but m ost of th ese have been degraded in energy, and their contribution to the to tal dose is much le s s than the higher energy neutrons. An effective shield need not stop all neutrons, but it m ust be efficient in low ering th eir en ergy. F o r the arran gem en t shown in F ig .A -I I -1 , the average energy of the neutrons at the outside su rface of the shield is only a few electro n v olts. How ever, the m ajo r contribution to the dose re su lts from the relatively sm all num ber of fast (> 0.5 MeV) neutrons p resen t. F o r purposes of dose estim ation , the m axim um p erm issib le fluence of fast neutrons at any point outside the w ater tank may be assum ed to be about2 0 n-cm "2-s"1.

In m ost c a s e s , if the shielding is sufficient to reduce the level of fast neutrons (> 0.5 MeV) below the m axim um p erm issib le dose, the dose due to slow neutrons and gam m a ra y s will be below to leran ce .

A fter a prolonged period of tim e, the shielding m a te ria l itse lf m ay becom e rad io active due to neutron activation . If the shielding is adequate to keep the prom pt radiation at a safe level, the radiation lev el due to induced rad ioactiv ity will be negligible, excep t perhaps at the v ery outside su rface of the shield.

The w ater tank shown in F ig .A -II -1 se rv e s as a neutron m o d era to r as well as a shield. The w ater surrounding the ta rg e t slows the 14 MeV neutrons to th erm al or n e a r-th e rm a l e n erg ies . A sam ple to be activated by slow neutrons is sim ply low ered into the w ater and placed clo se to the ta rg e t. The sam ples a re tra n sfe rre d to and from the ta rg e t a re a by a pneum atic tra n s fe r system . With the ta rg e t a re a com pletely surrounded by w ater, the th erm al flux n ear the ta rg e t is in the sam e o rd er of magnitude as the fast flux, and, hence, activation with fast neutrons cannot easily be c a rr ie d out. P ro vision is made for fast activation by using a 'd ry w ell'. The dry well is a thin polyethylene cylinder of about 14 in-dia. (3 56 mm dia.) in serted into the w ater and placed around the ta rg e t. The cylinder contains an in sert that fits around the ta rg e t and allows it to be

S5


xx

xx

yy

xy

yw

BRICK FACING -127mrrv 203mm 203 mm

f 2 0 3 m m l \ f w

CONTROLCONSOLE-

DATA ROOM

203 mm

3.02 m

FIG. A -I I-2 . Complete activation analysis laboratory.

203 mm

203 mm


positioned in the cen tre of the cylin d er. The purpose of the dry well is to rem ove the m oderating m a te ria l (H20 ) from the im m ediate vicinity of the ta rg e t. The polyethylene cylind er is lined with a cadm ium sheet, which cap tu res many slow neutrons that otherw ise would leak into the dry w ell. The sam ple to be irrad ia ted is contained in a pneum atic tube which is situated inside the well and positioned an inch o r le ss from the ta rg e t. With the dry w ell in p lace, the ra tio of the fast to th erm al flux, at a distance of about 2 5 m m from the ta rg e t, is 150 to 1.

The type of shielding configuration d iscu ssed above has se v e ra l advantages:

(1) Optimum utilization of space(2) V ersatility — can be used for both slow and fast irrad iatio n s(3) Inexpensive(4) Not perm anent — the co n cre te blocks and tank can be moved.

F ig u re A -II-2 shows an activation analysis lab o rato ry . The a c c e le ra to r room is a re s tr ic te d a re a . The data room and the outside w alls a re below m axim um p erm issib le lev els for operation at an output of 4 X 1010 n 's '1. The co n cre te w alls a re stacked blocks (density = 2 .0 g-cm "3). A sliding co n crete door sep ara tes the a c c e le r a to r room from the data room . The b rick facing lay er is a few inches thick, and the Haydite co n cre te blocks a re hollow, so these add v ery little to the shielding.

The w ater tank is equipped with a 'd ry ' well of the type d escrib ed above. A pneum atic system tra n s fe rs sam p les from the irrad iation site to the counting a re a in the data room . The data room contains the con trols for the neutron g en era to r , the sam ple tra n sfe r system , and variou s detecting equipment. R ad ioch em ical sep aration and low -level counting a re done in the rad io ch em istry room .

F ig u re A -II-3 shows an in stallation w here the neutron g en erato r is being used with a su b critica l assem b ly . With the ta rg e t in the cen tre of the su b critica l tank (no uranium slugs in the su b critica l) the m easu red dosages at the north wall and at the console a re shown in the table of F ig .A -I I -3.

T h ese m easu rem en ts w ere m ade with a to tal yield of 14 MeV neutrons of approxim ately 4 X 1010n -s"1. The su b critica l tank was filled with w ater when th ese m easu rem en ts w ere m ade. Note that the shielding for this installation is sim ila r to the arran gem en ts d escrib ed above (F ig s A -II-1 and A -II-2 ) .

C erta in applications, such as neutron sca tte rin g exp erim en ts, req u ire that the region of the ta rg e t be as free as possible fxf sca tte rin g m a te ria ls . The shielding arran gem en ts previously d iscu ssed a re not applicable. In neutron sca tte rin g exp erim en ts it is

37


COCO

NORTH WALL DOOR WITH INTERLOCK

—1.02 m - CONCRETE

1.02 m

1.02 m DIA SUB- CRITICAL TANK

3.05'HIGH VOLTAGE POWER SUPPLY

/NEUTRON GENERATOR

- 5.12 m --------------------

CONCRETET ~1.02m

IMACHINE SHOP

POSITION DOSE"FAST THERMAL GAMMA

A 1.5 0.2 0.2F 0 0.2 0.1

-3.05 m

CONSOLES

uin u n its o f m re rrv h 1 (1 0 5 J - k g “ ' h 1 d o se eq u iva len t).

FIG. A -I I .3 . Installation where the neutron generator is used to pulse a subcritical assembly.


MACHINE SHOP

FIG. A -II-4 . Arrangement employing distance as a shielding factor.


d esirab le to employ distance as a shielding fa c to r . The disadvantage of utilizing distan ce is the co st of building sp ace rela tiv e to the co st of shielding m a te ria ls ; space always seem s to be at a prem ium .A typ ical arran gem en t where the neutron so u rce is sep arated 30 ft (9 .14 m) from the o p erato r console is shown in F ig .A -I I -4 . The height of the room is 15 ft (4 .57 m) and the shielding w alls A and B a re 10 ft (3 .05 m) high. With this arran gem en t, c a re m ust be taken to m inim ize sca tte rin g of neutrons from the floors and w alls into p erson nel operating a re a s . The movable walls m arked A are positioned so that neutrons cannot s c a tte r through the opening to the console room . The ceiling is made of thin m etal sheet, and 'skyshine' (sca tterin g from the ceiling) is m inim ized. The dotted line se p a ra te s the re s tr ic te d a re a s from the u n restricted a re a s .

If the distance from the sou rce to the console w ere 50 ft (15 .24 m) instead of 30 ft (9 .14 m ), the thickness of each of the shielding walls could be reduced by about 6 in (152 m m ). The shields m arked A are mounted in a fram e on c a s te r s which can be rolled from one location to another. A m ovable shield is very convenient for this type of arran gem en t sin ce it facilita tes optimum positioning of the shielding m a te ria l. This type of shield is also useful if the neutron gen erato r is to be used in se v e ra l applications or different location s.

R E F E R E N C E S

[1] CSIKAI, J . , "Use of sm all neutron generators in science and technology", At. Energy Rev.11 (1973) 415 .

[2] PRUD’ HOMME, J . T . , Texas Nuclear Corporation Neutron Generators, Texas Nuclear Corporation, Austin, TX. (M ar. 1964).

[3] Operation Manual for the Model A -1001 Neutron Generator, Kaman Nuclear, Colorado Springs, CO (O ct. 1965).

[4] LENIHAN, J .M .A ., e t a l . , Advances in Activation Analysis, 2, Academ ic Press, London and New York (1972).

[5] NELLIS, D .O ., e t a l . . Tritium Contamination in Particle A ccelerator Operation,PHS Publication No. 999-RH -29, Superintendent of Docum ents,U .S.Governm ent Printing O ffice, Washington, D C (N ov. 1967).

[6] JOHNSON, A .G ., Tritium Considerations Associated with the Operation of Cockcroft-W alton Type Neutron Generators, USAEC, Region V, Div. of Com pliance (Apr. 1965).

[7] PINSON, E .A ., LANGHAM, W .H ., Physiology and toxicology o f tritium in man, J . Appl. Physiol. 10 (1957) 108.

[8 ] PINSON, E .A ., T heoretical Considerations of Acute Exposure of Man to HTO or T 20 Activity in the Atmosphere Environment, USAEC Rep. LA -1469 (1952).

[9] Report of ICRP Com mittee II on Permissible Dose for Internal Radiation, 1959, Health Phys.3 1 (1960).

40


[10] STEINBERG, R-» ALGER, D .L ., A New Pumping System for a 150-k ilovolt Neutron Generator to Reduce the Present Tritium Hazard, Report publishedby U .S . Department of Health, Education and W elfare, Public Health Service, Rockville, M D (O ct. 1972).

[11] Protection against Neutron Radiation, National Council on Radiation Protection andMeasurement, Rep. N o .38 (Jan . 1971).

[12] ICRU, Dose Equivalent, Suppl. to ICRU Rep. 19 (1973).[13] BURRILL, E .A ., D irect Nuclear Accelerators, High Voltage Engineering Corporation,

Burlington, MA (Jun. 1965).[14] CLOUTIER, R .J . , Fast neutron generators: Radiation levels and shielding requirements,

Industr. Hyg. J- (Sep ./ O ct. 1963) 497 .[15] HACKE, J . , Dosimetry and shielding with a 14-M eV neutron generator, Int. J . Appl.

Radiat. Isotopes 18 (1967) 33.[16] THOMAS, R .H ., Neutron Shielding Properties of Various Materials in the Energy Region

1-100 MeV, Rutherford Laboratory Internal Report NIRL/M/13 (M ar. 1961).[17] Concrete in Attenuating Neutron Radiation Produced by a 5-BeV Photon Beam, USAEC

Report CEAL-1007 (D ec. 1963).[18] WALLACE, R ., Cyclotron: Neutron Emission and Attenuation, in Engineering Compendium

On Radiation Shielding 3, Springer-Veil&g, Berlin (1910) 15T.[19] CHAPMAN, G .T . , STORRS, C .L ., Effective Neutron Removal Cross-Sections for Shielding,

USAEC Report AECD-3978 (1955).[20] PRICE, B .T . , HORTON, C .C . , SPINNEY, K .T ., Radiation Shielding, Pergamon Press,

Oxford (1957)*[21] ALLEN, F . J . , FUTTERER, A .T . , Neutron Transmission Data, Nucleonics 21 8 (1963) 120.[22] DAVIDSON, B -, Neutron Transport Theory, Oxford (1958).[23] JAEGER, R .G ., (Ed.), Engineering Compendium on Radiation Shielding,Springer-Verlag,

Berlin 3_(1968), 3 (1970).[24] PATTERSON, H .W ., THOMAS, R .H ., A ccelerator Health Physics, A cadem ic Press,

New York and London (1973).[25] BUDNITZ, R .J . , Tritium instrumentation for environmental and occupational monitoring —

A review, Health Phys. 26 (1974) 165.

[25a] BLIZARD, J .B . , Reactor Handbook, 2 n d e d ., 3, Part B — Shielding, IntersciencePublishers, New York (1962).

[26] HANKINS, D .E ., "New methods of neutron-dose-rate evaluation". Neutron Dosimetry (Proc. Symp. Harwell, 1962) 2 V IAEA, Vienna (1963) 123 .

[27] ANDERSON, I .O . , BRAUN, J . , "A neutron rem counter with uniform sensitivity from0 .0 2 5 eV to 10 M eV", Neutron Dosimetry (Proc. Symp. Harwell, 1962) 2, IAEA, Vienna (1963) 87 . ~

[28] BRAMBLETT, R .L ., EWING, R .I . , BONNER, T .W ., A new type of neutron spectrometer, N ucl. Instrum. Meth. £ 1 (1960).

[29] McGUIRE, S .A . , A Dose Monitoring Instrument for Neutrons from Therm al to 100 MeV,Los Alamos S cien tific Laboratory, NM, Rep. LA-3435 (1965).

[30] HANKINS, D *E ., A modified-sphere neutron detector, Los Alamos Scien tific Laboratory,NM, Rep. LA-3595 (1966).

[31] KATHREN, R .L ., Dosimetry o f 14-M eV neutrons with Eastman NTA personnel monitoring film , Health Phys. 13 (1967) 1039.

[32] NISHIWAKI, Y . , TSURUTA, T . , YAMAZAKI, K ., D etection of fast neutron by etch-p it method of nuclear track registration in plastics, J . N ucl. S c i. T echnol. 8, 3 (M ar. 1971) 162.

[33] IAEA, Neutron Dosimetry (Proc. Symp. Harwell, 1962), IAEA. Vienna (1963) V o l.I 653 pp, V o l .I I6 1 5 p p .

41


[34] IAEA, Biological Effects of Neutron Irradiation (Proc. Symp. Neuherberg, 1973) IAEA, Vienna (1974) 484 pp.

[35] IAEA, Californium-252 in Teaching and Research, Technical Reports Series No. 159,IAEA, Vienna (1974) 139 pp.

[36] IAEA, Biomedical Dosimetry (Proc. Symp. Vienna, 1975) IAEA, Vienna(1975) 708 pp.[37] SHAW, K .B ., STEVENSON, G.R., THOMAS. R.H ., Evaluation of dose equivalent from

neutron energy spectra, Health Phys. 17 (1969) 459.[38] de KERVILER, H., Dosimetrie des Neutrons au Moyen de Compteurs Proportionnels £ Paroi

Equivalente aux Tissus, Rapport CEA-R 2891 (1965), Centre d'Etudes Nucleaires de Saclay.[39] ATTIX, F .H ., ROESCH, W .C ., Radiation Dosimetry I — Fundamentals, Academic Press,

New York and London (1968).[40] ATTIX, R .H ., ROESCH, W .C ., Radiation Dosimetry II — Instrumentation, Academic

Press, New York and London (1966).[41] IAEA, Radiation Protection Monitoring (Proc. Regional Seminar Bombay, 1968), IAEA,

Vienna (1969) 556 pp.[42] KIEFER, H., Maushart, R., Strahlenschutztechnik,Verlag G. Braun, Karlsruhe (1964).[43] Particle Accelerator Safety Manual,U.S.Dept, of Health, Education and Welfare, National

Centre for Radiological Health, Rockville, MD, Rep. MORP 68-12 (Oct. 1968).[44] GUNDAKER, W .E ., BOGGS, R .F ., Recommendations for the Safe Operation of Particle

Accelerators,U.S.Dept, of Health, Education and Welfare, National Centre for Radiological Health, Rockville, MD, Rep. MORP 68-2, (Feb. 1968).

[45] American National Standard ANSI N 43.1 - 1969, Radiological Safety in the Design and Operation of Particle Accelerators, American National Standards Institute, New York, NY (Jun. 1970).

B I B L I O G R A P H Y

Publications of the IAEA where additional information may be found:

IAEA, The Use of Film Badges for Personnel Monitoring, Safety Series No. 8, IAEA, Vienna(1962).

IAEA, Basic Safety Standards for Radiation Protection, Safety Series No.9, IAEA, Vienna (1967).

IAEA, The Basic Requirements for Personnel Monitoring, Safety Series No. 14, IAEA, Vienna (1965).

IAEA, Personnel Dosimetry Systems for External Radiation Exposures, Technical Reports Series No. 109, IAEA, Vienna (1970).

IAEA, Handbook on Calibration of Radiation Protection Monitoring Instruments, Technical Reports Series No. 133, IAEA, Vienna (1971).

IAEA, Monitoring of Radioactive Contamination on Surfaces, Technical Reports Series No. 120,IAEA, Vienna (1970).

IAEA, Training in Radiological Protection for Nuclear Programmes, Technical Reports Series No. 166, IAEA, Vienna (1975).

42


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