THE tSE OF TRI

UiUigL MFDXUM FOR T3J& OX$&

Do M c C O ~ O ~ Radiation Monitoring Section

Environmental Health & Engineering Department

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INTRODUCTION

The radiation protection program at a heavy water reactor such as the

Plutonium Recycle Test Reactor (PRTR) is 'complicated by the presence of

tritium, primarily in the form of tritium oxide, Neutron activation of

the deuterium in reactor systems produces tritiub which escapes to the

working environment through' system leaks and accidental heavy water spills, . .

The human.body readily assimilates tritium in the oxide form from the

environment by inhalation and direct absorption through the intact skin. '

The,presence of tritium in the air and on exposed surfaces is difficult

to detect with ordinary sampling techniques and monitoring instrumentation

because of the low energy of the beta particle emitted during the disinte- . .

gration of the tritium atom (0.018 Mev, maximum energy), This. paper

describe8 rapid, reliable, and simple methods of. sampling for airborne

and surface tritium contamination with a'high collection efficiency.

. . ! . .

'SUMMARY ' - A sampling technique for airborne tritium oxide was developed and tested,

Under the experimental conditions consi dered, the efficiency of this method :,

averaged 98%. The :technique employs a gas wash battle filled with ordinary

distille'd water through which air i s bubbled at a constant flow rate, The

method is rapid, reliable, and sensitive and has been proven under a

variety of field conditions, The minimum detectable tritium oxide air

concentration that can reliably be measured is 1 x log7 uCi /cc. Additionally,

a technique is described for determining'the amount of tritium oxide surface I . . .

contamination; Because of difficulties in measking the collection effi-

ciency of the technique, it is presently used' for qualitative measurements ,

as a trend indicator only, : .

111, SAMPLING AND MONITORING TECHNIQUES' FOR' TRITIUM

Among the methods employed to monitor for airborne tritium is an ion chamber

device through which air is continuously drawn after passing through an ion

trap, The chamber may be gamma-ray compensated if it is used in a high

gamma-ray backglrolled ares, or the sample may Be drawn from the work ore& to /

a low background location where the instr-ent is situateda This method , -------

does not provide any discrimination ag'*inst interfering airborne--riad< ./'

activity such as that emitted by noble gases produced during reactor

operation and serious errors in estimates:of .the air contamination status, .

may result.

An alternative method for tritium monitoring is to sample the moisture .

cdntained in the working atmosphere. A popular medi-urn for moisture

collection is a silica gel trap which collects water by adsorption and has . "

a low efficiency for interfering gases;' +he water i s driven off the

silica gel'by heating, collected, and .analyzed .for tritium content by . . ._li--..--%...::l

liquid scintillation counting or internal gas counting, Certain problems '

exist with the silica 'gel sampling technique for tritium. The procedure

i s cumbersome'snd tame consuming, requiring additional equipment such as a ,

furhace and water condenser, The collection efficiency of silica gel for

'water varies with use both in'successive sampling periods and during a ..

single sampling period.' Finally, all the water collected during a sampling

period is not dtstilled out of the silica gel which leads to dilutions or

concentrations of subeequent samples,

Another method sometimes used to sample air for tritium oxide employs a

freeze trap apparatus. Moisture is frozen out of the air which is drawn

through a chamber cooled by dry ice ox-;.li quid nitrogen. he s&nple i s then

, , .

analyzed f o r tritium content by l i q u i d s c i & i l l a t i o n o r i n t e r n a l gas count-

ing. This method requires @ long sampiing time t o obta in a sample of

adequate volume, p a r t i c u l a r l y i n atmospheres of low r e l a t i v e humidi t y . In

addt t ion , t h e 'method does not have a very.,high c o l l e c t i o n e f f i c iency

because ice crystals forwed i n the chwber mw Behave oe e m d l partgclee . .

and pass r i g h t on through t h e chamber,

Because o f t h e above problems with each or t h e monitoring o r sampling

methods f o r t r i t i u m , a d i f f e r e n t technique was developed i n which a i r is

bubbled through d i s t i l l e d water, The water is then analyzed f o r tritium

by l i q u i d s c i n t i l l a t i o n counting and t h e quan t i ty r e l a t e d t o a i rborne

tritium concentration. A series o f tests irere devised t o i n v e s t i g a t e t h e . , . ". . . ...

c o l l e c t i o n e f f i c iency of t h e method under:various environmental conditions. ,

A I R SAMPLING 'I TEST 'PROCEDURE

A s e r i e s of experiments was devi sed t o t&t t h e e f fec t iveness o f . the sampling

technique f o r t r i t i u m oxide col lec t ion. The co l l ec t ion e f f i c i e n c y o f t h e

technique was measured f o r 'various combinations ,of sampling t i m e , tritium . . . . .

eoncentra t i on, ' and r e l a t i ve humidity. The "experf mental arrangement i s

shdwn i n Figure 1. Dried room a i r was drawn through a gas wash b o t t l e

containing approxfmately 100 mls, of d i l u t e tritium oxide of known co=en-

t r a t i o n . T h e a i r c a r r i e d . . . moisture from t h e , s o l u t i o n i n t h e gas wash b o t t l e

t o t h e sampler, ~ r o v i s i o n s were included t o . route t h e a i r throukh another.

gas wash b o t t l e while samples were being changedo The a i r was exhausted ,

through s i l i c a g e l t o remove most of t h e 'water leaving t h e experimental ,-. . .

set-up, A i r flow r a t e as measured with a rotanheter was maintained at - 3- ----- /-- -.

I l i t e r s p e r minute. The r e l a t i v e humidity of t h e r e s u l t i n g atkospheres was

. . hpproxiniately 80%. I n order t o measure' c b l l ~ c t i o n e f f i c iency a t lower

r e l a t i v e humidities:, t h e appartus was changed s l i g h t l y a s shown i n Figure 2

by t h e inc lus ion of a s i l i c a g e l column, 'The l i n e was branched downstream

of t h e gas wash b o t t l e conta in ing t h e tritium s o l u t i o n t o permit measure-

ments of the wet and dry bulb temperatures, Calcula t ions o f t h e moisture

' . weight pe r . un i t volume o f sir and r e l a t i v e humidity o f t h e a i r pass ing

through t h e sampler were made based on wetsand dry bulb temperatures,

Temperature measurements were made with a.0-50' C p rec i s ion thermometer,

It was assumed t h a t t h e moisture i n t h e air en te r ing t h e sampler had t h e

same t r i t i u m concentrat ion a s t h a t i n t h e . sp ike so lu t ion , Measurements of.

t h e tritium concentra t ions i n t h e water so lu t ions were done by l i q u i d

s c i n t i l l a t i o n counting, A t o t a l of 9 trials were completed, each t r i a l

. . l a s t i n g approximately 3 hours, .

A l l $as wash b o t t l e s and s i l i c a g e l co1umns:'were weighed before and a f t e r a . D

trial run t o measure t h e water balance ? f : t h e sybtem. I n add i t ion , t h e

amount o f tritium l o s t from t h e sp ike so lu t ion was measured and compared

t o t h e t o t a l amount o f tritium co l l ec ted , Resul ts o f t h e water balance . .

and t r i t i u m recovery measurements a r e given i n Table 1.

Vo TEST RESULTS '

The r e s u l t s o f t h e e f f i c i ency measurements are given i n Table 2, Sampling

time had l i t t l e e f f e c t Dn t h e c o l l ~ c t i o n e f f i c i ency of t h e system. A s

sampling time increased, ' the measured eff ic , iency dec reased s l i g h t l y due t o

water l o s s e s from t h e sampler, which amounted t o a O , l m l during a 5 minute

sampling per iod o r less. than 1% f o r a s,ample c o l l e c t e d f o r 30 minutes.

'There was no evidence. t h a t t h e c o l l e c t i o n e f f i cJency depended' upon t h e

r e l a t i v e humidity nor on tGitium concentrat ion i n t h e range 7 x log7 t o , '. .

1 x loo3 pCi/cc, From t h e da ta i n able '2, a f i v e minute sampling time i s . . . .

TABLE 1 I

Water Balance and Tritium Recovery of t h e Experimental Ar.rangement

Water ~ a l & c e - gms Tritium Recovery Run Tota l Weight Tota l Weight pCi H-3 Lost. u C i H-3 Gained Percent

Number before Run a f t e r Run from Spike i q. Equipment Recovery_ - 1 Not Determined ' . . s 0 9 5 -0 86

TABLE 2

Percent Collect ion Eff ic iency of t h e Bubbler Technique i n Sampling f o r Airborne Tritium Oxide

Calculated Tritium A i r

concentrat ions Relatf ve Sampli ng Times pCi/cc Hmf d i ty $ Minutes 10 Minutes' 20 Minutes 30 Minutes

.

adequate to achieve an average collection:efficiency of.98% in the range ,

of air eoncentrations studied, . .

. . . . .

VI, DISCUSSION AND FIELD EXPERIENCE

The largest e'rror in the test pro=edure involved measurement of the tem-

perature,, he thermometer$ used had inaicated precisidn of 2 /

Calculations of the water volume per unit weight of dry air were completed --.

..- J'

rather than determinations of the water:volume per unit wei-ght of moist

lira This error is estimated to..be less, *hair l%b(l) The maximum error in

the 'collection efficiency measurements is. estimated to be 10%.

The minimum tritium concentration.in a water solution detectable by liquid

scintillation counting is 1.3 x log5 p~i/cc.*(~)' For a five-minute sampling

period, this yields a tritium air concentration of -1 x log7 pCi/cc, The P .

normal levels encountered at the PRTR ranee from 10'' to log5 pCi/cco . , . .

The bubbler sampler developed for field .use. is shown in Figure 30 The -.,-.-- .--. .

vacuum source is a small diaphragm pump which.draws.an air sample through

" 100 mls, of distilled water at .the rate of 3 liters/minute, The gas wash . .

bottle has a 125 mlo capacity,. The. systep.has . . been used routinely at the :

PRTR under a variety of operating and outage conditions, The main virtue

of the system is the simplicity and ease with which a sample may be col- . .

leeted, Experience has shown that results 'of. a sample analysis can be . . . . ,

available in as little as 30 minutes after the sample is collected, The

'typical air eoncentrat'ions measured with this system through an operation-

\. outage cycle at the PRTR are shown in Ffgure 4 , A plot of the parti culate I

beta-gamma airborne.radioactivity coliected at a location near the tritium

*For a sample volume of 1 m l , and a 3 6 minute counting period, the accuracy is 2 25% and the counting precision is 2.20% at. the 95% confidence. level,

- 7 0 . :' BNWL-CC-547

. . r . . :

sampling s t a t i o n i s given f o r compariscin. 1 ~ i r b b r n e . t r i t ium contamination

l e v e l s 'remain r e l a t i v e l y constant during..,operation, then increase a t t h e

beginning of an outage gnd gradually decrease t o pre-operational l e v e l s ,

The. average tritium a i r contamination l e ~ e l during operat ion has been

3 x loo6 l~Ci /cc , while during shutdowns i t increases t o an kverage vaiue

' \

of 1 x 10'~ pCi/cc, The airborne betaOg&nma contamination (composed

of f i s s i o n gases and t h e i r daughter i s .somewhat I ;

higher during operat ion but falls o f f r ap id ly at t h e s t a r t of an outage,

The highes t a i rborne tritium l e v e l s normally occur i n t h e v i c i n i t y of an .

i r r a d i a t e d heavy water s p i 11, The m a x i m b m concentrat ion measured t o da te

i n t h i s s i t u a t i o n i s 3 x loo3 pCi/cco '

'

V I I , ' SURFACE CONTAMINATION SAMPLING .. '

A " d i f f e r e n t technique f o r determining se&-quantiSatively t h e amount o f . . . . ,

smearable tritium surface contamination yas a l s o developed, The technique

involves t ak ing a wet smear of a known surface tares with a dampened . . . .

:.

'2" x 2" gauze pad, adding 9 known amount o f d i s t i l l e d water t o t h e gauze 0 . .

pad i n a conta iner , and allowing e q u i l i b r a t i o n of t h e t r i t i u m i n t h e pad

and t h e water t o take p lace over a pre-determined per iod of t imeo !l!he

d i s t i l l e d water i s then,drawn o f f and analyzed f o r tritium content by

l i q u i d s c i n t i l l a t i o n counting,

Studies were compldted t o def ine t h e parameters involved i n t h e smear

technique, The f i rs t study x& designed t o determine t h e quant i ty of

t r i t i u m t h a t would be 'recovered i n t h e added d i s t i l l e d water, I"o'tr gauze/

pads were moistened with 0,5 m l . of so lu t ions 'containing known amounta of-- --- '

. . *' i

tritium. Approximately 80% of t h e tritium edded t o t h e ga<;= pads was

'de tec ted i n t h e d i s t i l l e d water 'in t h i s ,andcsubsequent s tud ies , The . .

r e s u l t s of t h e test are given i n Table 36. . .

TABLE. 3

Percent Recovery Measured for the Wet Pad Tritium Smear Technique

Equilibration Solution - 10 m l . Equilibration Time - 10 min,

Spike Trf tfw 'Tritium Detected Concentration Added to id Equilibration Percent

pci/cc Pad - pCi Solution - pCi Recovery

Another study was designed to determine the variation in results with . ,

various quantities of distilled water equilibration solutions, The results

are given in Table 4, The data does not 'fnd4cate any appreciable variation

in results with the various amounts of equilibration solution used,' However,

.P

there is not sufff cient data to indicate. thi s with certainty,

TABLE .4

Variation of Percent Recovery with Different Amounts of.Equ9libration Solution

Equilibration Time - PO minutes Spike - O,5 ml, of 64 pCi/c,c tritium solution

. : .Tritium ~etected

Equi librati on in Equilibration Percent Solutf on - m l , Solution -.pCf . . Recovery

5 19 , : . 6 o , 10 19 .. ' 6 0

' 20 28 . . . 88 30 ? 1 66'

. . . ' \

~:.,Pinal study was done to estimate the time necessary for equilibrium. ! !

The results given in Table 5 indieate that the equilibration time is short ; !

. .

and that 10 minutes is adequate for equilibrium to be established,

TABLE 5

'Variation of Percent Reco~ery with Different Equilibration Times

Equilibration Solution - 10 ml, Spike - 0,5. ml , of 64 pdi/cc tritium solution

Tritium Detected Equf lf brat Pon in Equilibration Percent Time - Mi n, Solution - uCi Recovery

. .

Because of difficulties in preparing a surface with a known quantity of

tritium oxide contamination and because .'the collection efficiency depends ' .

on the characteristics of the surface being smeared, coLlection efficiency

measurements were not completed, The technique has some value qualitatively .....

as a trend indicator. The sampling technique presently used involves a smear

of a 4 sq, ft, area, an equilibration solution of 10 m l , of distilled water,

and an equilibration time of 10 minut='s.: It is estimated that the minimum . .

smearable tritium surface contamination detectable by this method is . ,

5 x 10' pCi/sq, ft, for i 4 sq, fto area, Normal smearable tritium oxide

surface contami,nation at the PRTR has been approximately 0,03 u~i/sq. ft,

. .

REFERENCES

. . 1 , Unpublished work of R. J: Erigelman.

2, Unpublished work of L, S, Kellogg,

3, Do McConnon, A Review .of the Air ~0nitori.k~ Program at the PRTR, HW-82705, May 15, 1964,

To Vacuum

A i r Intake

FIGURE 1 \

Experimental Arrangement for Determining Trit ium Oxide Collection Efficiency . . I. .

To Vacuum . .

I FIGURE 2

. . . . . .

Air Intake

I I Modifications of Experimental Arrangement for Determining Tritium Oxide Collection Efficiency

' Time - ~ a ~ s

FIGURE 4

Typical Air contamination. Levels at the PRTR