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RFP-1621
March 21, 1972
A NEUTRON CRITICALITY DETECTION SYSTEM
William H. Tyree
DOW CHEMICAL U.S.A. ROCKY FLATS DIVISION
P. 0. BOX 888 GOLDEN, COLORADO 80401
U. S. ATOMIC ENERGY COMMISSION
CONTRACT AT(29-1)-1106
DlSl RISlHIOt! Gf THIS DOCUMENT IS UNLIMITED
DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
DISCLAIMER
Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.
Printed March 21, 1972
RFP~l621
UC-37 INSTRUMENTS TID-4500 - 56th Ed.
A NEUTRON CRITICALITY DETECTION SYSTEM
William H. Tyree
Research and Ecology
PRODUCT AND HEALTH PHYSICS GROUP
.----~-'----N 0T1· CE-------~ ·This ·report was· prepared as an a·ccount of work · sponsored by the United States Government. Neither
the United States nor the United States Atomic.Energy Commission, nor any of their employees, nor any of their contractors; subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability. ·or· responsibility for the accuracy, com-
1· pleteness or. usefulness of any information, apparatus, product or process .. disclosed; or represents that its use
·would not infringe privately owned rights. ·
DOW CHEMICAL U.S.A. ROCKY FLATS DIVISION
P. 0. BOX 888 GOLDEN, COLORADO 80401
Prepared under Contract AT(29-1I·1106 for the
Albuquerque Operations Office U. S. Atomic Energy Commission
I ' I v
DISTit8UllOM Of !KIS DOCUMEMl IS UMLIMllED~
J .. -
CONTENTS
Abstract
Introduction ......................................... .
Discussion ........................................... .
Alarm System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Neutron Detector Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Detector Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 3
Terminal Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Coincidence Input . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
·Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Neutron Alarm Output Signal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Maintenance ............ , . . . . . . . ..................... .
Detector Electrical Characteristics 1 ••••••••••••••••••••••••••
System Adjustments ................................... .
Coincidence Board
External Interfaces .. .' ................ · .............. .- .... .
Costs ............................................... .
System Tests .............................. ~ .......... .
Radiation ......................................... .
Environment ....................................... .
Routine Operational Tests ............................. .
Calculations for Detection Radius ........................... .
Detector Electronics
5
.5
11
11
11
11
11
11
11
16
16
16
Flux Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
RFP-1621
iii
( /. r .'-f
RFP-1621
(\ .. 'I..
iv
ACKNOWLEDGMENTS
Appreciation and thanks are expressed to the members of the Rocky Flats Health Physics Electronics group for their contributive efforts in the successful completion of the project. V. P. Johnson, J. B. Owen, and W. E. Chappell provided basic requirements defmition with continued support and encouragement during the early stages of development. T. H. Bell, C. H. Johnson, and D. C. McKinstry contributed ideas for the original concepts used in the prototype system design.
Former employees L. Ried and E. E. Overall with H. D. Marshall presently in the Plant's Research and Development, Coating group, while working on their own- design concepts assisted with thought-provoking discussions and observations, From the Industrial Hygiene and Bioassay group, I. B. Allen III fabricated the lithium foils and H. Urano of Facilities Engineering prepared the original drawings of the final system. Ray Pederson of H Division and Thomas Wimmett of N Division at the Los Alamos Scientific Laboratory at Los Alamos, New Mexico,' provided a useful and productive atmosphere for the neutron criticality alarm tests in Octobe~ of 1968.
~ I
.)
RFP-1621
A NEUTRON CRITICALITY DETECTION SYSTEM
William H. Tyree
' Abstract. A neutron criticality alarm system has been built which has applications for detecting high levels of neutron radiation. The system incorporates a thermal neutron-alpha reaction in a lithium-6 foil to produce pulses in a surface-barrier detector, and also includes a counting and sampling setup for noise rejection. As compared to earlier systems having a single detector, the described unit requires confirmation from a second detector before activating a general building-evacuation alarm.
The system operates from a common DC voltage for electronics and detector and witli an integral battery supply for power outages. The complete detector has been designed with costs below $130.00, while providing medium counting speed capability.
INTRODUCTION
A high-level neutron detection alarm system has been built which has advantages over previously used radiation instruments. The complete system and components are described and include the following features:
1. A sampling system with a reset period of one second.
2. A voltage level indication of the alarm condition of the detector unit.
3. A detector unit cost (including detector) of $125 ..
4. Coincidence requirements for any two-detector units.
5. Continuous operation during power outages or recesses.
The detector was tested in a cobalt-60 (6°Co) gamma ray field of 104 roentgens per hour(R/hour) without producing an alarm and detected a neutron criticality of approximately 5 x 1016 neutrons at 1900 feet from the.center of a critical assembly producing a burst width of 33 microseconds. (For metric conversion: 1 foot= 0.30 meters.)
DISCUSSION
Any new approach to a nuclear criticality alarm system must include a review of the existing philosophy in nuclear alarm systems. 1
' 2
• 3
The present nuclear criticality alarm system at the Rocky Flats Plant uses a scintillation detector to detect gammaray emissions. The system has be~n subject to false alarms because of high-level gamma sources sometimes located in the vicinity of the detector unit and as a result of the mechanical response of the meter relay used to actuate the electrical alarm circuit.
Use of a pulse-counting. system for neutron detection had been discounted during earlier alarm-development periods because of possible fast criticalities with short periods. A counting system capable of high-speed counting would require a complex amplifying system, would have poor noise immunity, and would be expensive.
A report4 summarizing the probable characteristics of plutonium criticalities-1014 fissions minimum, and a probable burst width range of 1 millisecond to 3 secondsprovided an incentive to reexamine the use of pulse-counting circuits for an alarm detector. The neutron criticality alarm described was the result of the study.
1 American Nuclear Society. American National Standard
Criticality Accident A /arm System. Report N 16.2. American National Standards Institute, New York. 1969.
2c. L. Schuske. "Criticality Control in a Chemical and Metallurgical Plant." Karlsruhe Symposium 1961-0rganization for Economic Co-operation and Development. European Nuclear Energy Agency, Karlsruhe, Germany. 1961.
3P. C. Friend and C. A .. Ratcliffe. "Neutron Sensitive Criticality
Detection System." Proceedings of the Institute of Electrical and Electronics Engineers, Nuclear Science Symposium. NS-14. B3ttelle Northwest Laboratory, Richland, Washington. February 1967. .
4K. J. Aspinall and J. T. Daniels. ~eview of USAEA Criticality
Detection and Alarm Systems 1963-64; Part 1: Provision and Design Principles (and its Amendment of Proposals Concerning Plutonium Systems dated November 10, 1965. Report AHSB(S}R92. Authority Health and Safety Branch, United Kingdo"m Atomic Energy Authority, London, England. 1965.
} '
';:\
; r,
'·• . J; ..
i. .. · ..
RFP-1621
2
REMOTE DETECTORS
-EJ RESET
, 2
'--- El ........... @
RESET
! 3 ....._ EB ~
@
I RESET
-- EB RESET
5
RESET
6
El '--- ~ @
RESET ~
I I
I I
I I
DETECTOR , RESET
'\T
....
....
EXTERNAL INTERFACE
--~ BUILDING
ALARM SYSTEM
J • • • • • 110 volts AC l -
I\ t:> 11om ALAnM ,
...-"_.,_;~..:....:..:...---.1-.i RELAY 110 volts AC ~.... I
6 volts Ir
COINCIDENCE .1. SYST~M ALARM ~XTFRNAt
L..J DATA I fTRANSMlrrER
r- - ,
I._ -f _J
RESET _._T_
I I I ~- \J 1/r- -, AMPLIFIER/ \
I
I
AUDIO-SIGNAL GENERATOR
I POWER TROUBLE --rffft""---- HAl"I l::HY 8Al.:K-UI' ::>Y::> I l::M ·.vAV
-~-.,I CHARGE
JJ° - -- . ~~~-1
+6 volts DC ·---\ j
POWER SUPPLY 1--------'I I
-,.. ----·-· n
BATTERY INDICATOR
GROUND
i.-------..... -----+-110 volts AC
' AC POWER ll\IDICATOR
I I
~----·--------
GUARD POST I
FIGURE 1. Typical Alarm System.
\.
Alarm System:
The system consists of two major blocks: a detector unit capable of detecting neutrons with an output control of the voltage level on the interconnecting line between the detector and a terminal unit.
Meters are used at the detector and in the terminal unit to indicate a normal condition [0.7 milliamperes (ma)] or an alarm condition (0.2 milliamperes). The 0.7-ma meter level represents 3.3 volts and the 0.2-ma meter level represents 1.0 volt. The normal power supply output is 6 volts.
The voltage level is sensed in the terminal unit to determine whether the detector produces an alarmed condition or not. The output of the terminal unit activates only if two detector units produce an alarm condition. A block diagram of the system is shown in Figure 1.
The line voltage drop of the wire connecting the two units should be kept low to provide as much change as possible in voltage between the normal and alarm conditions at the terminal unit. Additional individual and coincident alarm circuits are also included in the system to control remote alarm indicators. To obtain a normal alarm indication at the central terminal, the circuits need not be connected.
Neutron Detector Unit:
DETECTOR PHYSICS - During the past few years, rapid advances in solid-state detector technology have made available a low-cost surface-barrier detector with predictable performance even at low bias voltages. Such characteristics influenced the choice of the currently described neutron detection system.
The material ust:u fu1 the foil was chosen for its large rnaction energy available for the emitted alpha particle, as indicated in the following formula: 5
Lithium 6 + n ~ T + a+ 4.78 million electronvolts
Where: n =neutrons
T =Triton
a = alpha particle
The energy characteristic is necessary for pulse operation in the detector circuit with the low-field gradient present in the surface-barrier detector.
~W. D. Allen. Neutron netection. Philosophical Library, Incorporated, New York. 1960.
RFP-1621
Since the output pulse obtained from a gamma-ray interaction is small, the detector assembly acts as an effective immunity device for high gamma-ray fluxes.
ELECTRONICS - The neutron detector electronics is mounted in a steel box of 6.35 centimeters (cm) high by 12. 7 cm wide by 24.1 cm in length. The unit contains two printed circuit boards for holding the detector and electronic circuitry and a meter to indicate the normalalarm condition of the unit. Complete with detector assembly, boards, meter, and hardware, the unit weighs 1.26 kilograms (kg). The unit without the cover is shown in Figure 2.
The detector system uses a neutron alpha-particle interaction in a lithium-6 (6 Li) foil to produce electrical pulses in a silicon surface-barrier detector circuit. The pulses are amplified and shaped in a 3-stage amplifier before triggering a monostable multivibrator. The amplifier includes a sensitivity control to set the amplitude of pulses triggering the multivibrator. The monostable multivibrator produces a 2-volt positive pulse, approximately I 0 microseconds wide. The pulses produced in the monostable multivibrator are stored in the data storage register. Figure 3 shows the electronic configuration of the detector unit.
The sampling pulse generator is adjusted to send a short positive-reset pulse to the reset line of the data storage register every one second. The sampling pulse generator consists of two circuits: an astable multivibrator, which determines the reset pulse period; and a monostable multivibrator which produces a short (5-microseconds) pulse for resetting the data storage register to zero.
An integrated-circuit gate produces an output signal when both binary stages of the data storage register have been triggered between reset pulses. The output signal from the gate triggers a SO-microsecond multivibrator which turns on the silicon-controlled rectifier. The silicon-controlled rectifier controls the level of DC voltage on the output line from the detector unit to the central control panel. The normal voltage goes to 3.3 volts and falls to about 0.9 volts when the silicon-controlled rectifier is turned on. A meter, included at each detector unit, monitors the voltage at the anode of the silicon-controlled rectifier (SCR).
Tetm.inal Unit:
The terminal unit includes the coincidence board, power supplies, and external alarm circuits. The inputs from the remote detectors are brought to the unit from locations throughout the building. The meters on the front panel monitor thP. linP.s from each detector, the master alarm, and the voltage levels from the power supplies. The meters
3
I
, •
RFP-1621
14730-1
FIGURE 2. Neutron Detector Unit with Components.
4
RFP-1621
8
IR39 13.9K
I I I I
+6V
+6 v
+6V
R24 620
+6V
+6 v
• ,. !
..... ---1t-------..... rP--<O +6 v S1 + l C14 + C15 _ ~ T
400 400 C16 50 V '-----\b-R~~ 0.01 ALARM,
...-------~~--..---..---~H - _,
R49 5K
07
R501K +6V
R52 014 1K 2N4125 c11 +
+ C13 R53
0.1 4.7'----+e 5K 35 v
Kl ~
SCR1 2N5063
...---<.i GROUND
I i.J i
(!
• FIGURE 4. Coincidence Terminal Board Diagra!ll.
monitoring the detector lines are in parallel with the meters in the detectors, and are used to provide a centi:al termi-nal monitor of the condition of the remote detectors. The block and schematic diagrams of the coincidence terminal board are shown in Figure 4.
Coincidence Input:
The coincidence terminal unit consists of an integrated circuit (IC) to interface the individual detector units to the input of the silicon-controlled rectifier. The coincidenceunit input circuit includes a meter for each detector signal line and a centrally located reset push button for each detector silicon-controlled rectifier (SCR).
The normal 3.3 volts on each detector signal line holds the integrated-circuit output circuit at zero volts. When the voltage on the signal line goes to 2 volts or less, the voltage in the output circuit goes up to about 4 volts. The adjustable Zener voltage regulators in series with each input line provide a means· of setting the input voltage required (2 volts) to produce an output voltage from the integrated circuit.
The voltage across the output potentiometer at the common junction from all the output circuits of the integrated circuit goes up to l volt. This is not sufficient to turn on the coincidence unit silicon-controlled rectifier. When any other detector unit produces a level of 2 volts or less un the signal line, the output of the coincidence unit produces approximately 2 volts across the potentiometer, an ample driving voltage for the SCR. The SCR turns on actuating the relay, which closes the external alarm circuit.
An adjustable Zener is placed in series with the SCR gate to set the voltage level required for SCR operation. The series circuit is set to 1. 7 volts.
Power Supply:
The power supply includes a continuous charging system of nickel-cadmium batteries to allow operation in the event of a power outage. This DC system would make possible criticality alarm operation without any auxiliary AC power. The output circuit for the power supplies and batteries appears in Figure 5.
The power supply consists of a conventional regulator for normal AC operation and a set of nickel-cadmium batteries producing slightly less voltage. When the AC power is removed from the unit, a diode OR gate connects the batteries to the power input to the system. A time constant in the coincidence circuit-power circuit requires 20 milliseconds to discharge which provides additional stability during the transition period between AC and battery-powered operations.
RFP-1621
The power required for each detector unit.is 156 milliwatts . The 4-ampere-hour batteries will allow six units to operate . for at least 16 hours without AC power. The power supply· panel includes meter readouts for monitoring the 6-volt, DC power line and the condition of the batteries.
Neutron Alarm Output Signal:
The relay closure, produced by anode current flow in the SCR, controls an audio-signal generator located with the public address system in each building. The audio generator provides a signal simulating a klaxon horn or a warbling signal for aural alarm. The circuit is shown in Figure 6.
MAINTENANCE
Detector Electrical Characteristics: The detector unit is set to produce an output pulse from the monostable circuit on the amplifier-trigger board with a 12-millivolt positive pulse at the input to the test connector (BNC). (See Figure 3.)
The pulse width from the monostable multivibrator in the amplifier-trigger circuit is approximately I 0 microseconds.
The pulse width from the monostable multivibrator triggering the SCR is 50 microseconds. The rese~ generator pulse period is 1 second. The pulse width is 5 microseconds.·
A series of three' pulses within l second is normally required to produce triggering of the SCR controlling the line voltage at the input of the coincidence board at the terminal when the norm-test switch (S 1 of Figure 3) on the detector unit is in the norm position.
The test position is used to permit use of a low-level flux neutron source to trigger the detector unit SCR. The test position disables the sampling pulse generator. As a result, the counter stores the three pulses in an indefinite period of time. A source of unmoderated high-energy/neutrons having an output of 105 neutrons per square centimeter per second {n/cm2 /sec) produces SCR triggering within 2 minutes.
The first prototype detector units used a toggle switch to set the norm or test condition for the detector unit. After tests in routine field use, several instances of false alarms occurred from the detector when the test switch was inadvertently left in the test position. The switch has been changed to a 3-circuit phone jack. Thus personnel testing the unit are required to insert a 3-circuit plug in before beginning the test. [Refer to J3 (test) in Figure 3.)
5
RFP-1621
~ l~T-;,D CIRCUI; B~ARDS
I + 1 o~7µf 1R06K~----. I 15 V ND1 ND2
C1 10 pf 1 KV
R1 100
C2 0.01µf
R5 820 K
C3 0.01µf
TEST CONNECTOR (BNC)
C4 220 pf 1 KV
RB R9 10 K 1 K
2.4 v DC
R11 R12 22 K 470
01 2N4123
2.4 v DC
BLOCK DIAGRAM
DETECTOR ND1 ND2
6
AMPLIFIER
RESET . GENERATOR.----
DUAL J-K
MONOSTABLE M,UL Tl VIBRATOR
(8 microseconds)
..
GATE MS
FIGURE 3. Electronic Configuration of Detector Unit.
C18 C19 · - C20 470 pf. 470 pf 470 pf~· 1 KV 1 KV 1 KV )I
3.3 v DC
RESET
SCR
'"1 GROUND l
I
CR2 3.3V R14
47 K
3.3 v DC
C10 .10V 390 pf 0.1 µf
1 KV C11
R17 68 K
C13 +10µf 50V
50 K R19
+
R18 47 K
+Cl2 10 µf
C17 400µf 10V
RFP-1621
R20 3.9 K
C23 0.1 µf +
"f M1
R15 47 K
' ( 15V
r12-· ·_ ~ -- ~ -· -11 IC2 ,--. 5--. 3--6 ~ol _ ___....__ ___ ,.._-+-_._-=--J-'3
14 - MC717P 130-AiM-..--+.~-·=--=-.. =·-·...,.··=--=·-*·-~~--~ R16
MC776P 10 K
2N5063
+V,
Sl \ r-2-12~10·· 47K R21
a 6 IC4 . R22 47 K .-----t-----1t-----C:J I
TEST CONNECTOR
(3-circuit) 13 B 14 10 V 1 K '---I-I 0.1 µf C15 Dl D2 'R23
SCR1
C14 lc)j 1 KV
I- · r_ _· -b~o2r~ JIN2069
2 1 J 9 11 0 12 14 C16
IC3 - o 1 f -MC717P 4 · µ
10V
· Cl. .. C24 - capacitors RI. .. R23 - resistors CRl, CR2 - diodes
Nl>l and ND2 --forms a detector I\. I. .. 1\.4 - intr.erntr.rl rirri1its
Q l. .. Q - transistors SCR - silicon-con trolled rectifier
J 1. . . 13 - insulated jacks Sl - switch MS - monostable multivibrator
urn - milliamperes K - 1000 µf - microfarads pf - picofarads V - volt& (AC or DC) Ml -··meter
FIGURE 3. (continued)
.... ' .. ~
I SCR RESET
I
+. C24
100 µf J2 6V
' 7
RFP-1621
~ l~T-;,D CIRCUI; B~ARDS
I + 1 o~7µf 1R06K~----. I 15 V ND1 ND2
C1 10 pf 1 KV
R1 100
C2 0.01µf
R5 820 K
C3 0.01µf
TEST CONNECTOR (BNC)
C4 220 pf 1 KV
RB R9 10 K 1 K
2.4 v DC
R11 R12 22 K 470
01 2N4123
2.4 v DC
BLOCK DIAGRAM
DETECTOR ND1 ND2
6
AMPLIFIER
RESET . GENERATOR.----
DUAL J-K
MONOSTABLE M,UL Tl VIBRATOR
(8 microseconds)
..
GATE MS
FIGURE 3. Electronic Configuration of Detector Unit.
C18 C19 · - C20 470 pf. 470 pf 470 pf~· 1 KV 1 KV 1 KV )I
3.3 v DC
RESET
SCR
'"1 GROUND l
I
CR2 3.3V R14
47 K
3.3 v DC
C10 .10V 390 pf 0.1 µf
1 KV C11
R17 68 K
C13 +10µf 50V
50 K R19
+
R18 47 K
+Cl2 10 µf
C17 400µf 10V
RFP-1621
R20 3.9 K
C23 0.1 µf +
"f M1
R15 47 K
' ( 15V
r12-· ·_ ~ -- ~ -· -11 IC2 ,--. 5--. 3--6 ~ol _ ___....__ ___ ,.._-+-_._-=--J-'3
14 - MC717P 130-AiM-..--+.~-·=--=-.. =·-·...,.··=--=·-*·-~~--~ R16
MC776P 10 K
2N5063
+V,
Sl \ r-2-12~10·· 47K R21
a 6 IC4 . R22 47 K .-----t-----1t-----C:J I
TEST CONNECTOR
(3-circuit) 13 B 14 10 V 1 K '---I-I 0.1 µf C15 Dl D2 'R23
SCR1
C14 lc)j 1 KV
I- · r_ _· -b~o2r~ JIN2069
2 1 J 9 11 0 12 14 C16
IC3 - o 1 f -MC717P 4 · µ
10V
· Cl. .. C24 - capacitors RI. .. R23 - resistors CRl, CR2 - diodes
Nl>l and ND2 --forms a detector I\. I. .. 1\.4 - intr.erntr.rl rirri1its
Q l. .. Q - transistors SCR - silicon-con trolled rectifier
J 1. . . 13 - insulated jacks Sl - switch MS - monostable multivibrator
urn - milliamperes K - 1000 µf - microfarads pf - picofarads V - volt& (AC or DC) Ml -··meter
FIGURE 3. (continued)
.... ' .. ~
I SCR RESET
I
+. C24
100 µf J2 6V
' 7
RFP-1621
8
IR39 13.9K
I I I I
+6V
+6 v
+6V
R24 620
+6V
+6 v
• ,. !
..... ---1t-------..... rP--<O +6 v S1 + l C14 + C15 _ ~ T
400 400 C16 50 V '-----\b-R~~ 0.01 ALARM,
...-------~~--..---..---~H - _,
R49 5K
07
R501K +6V
R52 014 1K 2N4125 c11 +
+ C13 R53
0.1 4.7'----+e 5K 35 v
Kl ~
SCR1 2N5063
...---<.i GROUND
I i.J i
(!
• FIGURE 4. Coincidence Terminal Board Diagra!ll.
monitoring the detector lines are in parallel with the meters in the detectors, and are used to provide a centi:al termi-nal monitor of the condition of the remote detectors. The block and schematic diagrams of the coincidence terminal board are shown in Figure 4.
Coincidence Input:
The coincidence terminal unit consists of an integrated circuit (IC) to interface the individual detector units to the input of the silicon-controlled rectifier. The coincidenceunit input circuit includes a meter for each detector signal line and a centrally located reset push button for each detector silicon-controlled rectifier (SCR).
The normal 3.3 volts on each detector signal line holds the integrated-circuit output circuit at zero volts. When the voltage on the signal line goes to 2 volts or less, the voltage in the output circuit goes up to about 4 volts. The adjustable Zener voltage regulators in series with each input line provide a means· of setting the input voltage required (2 volts) to produce an output voltage from the integrated circuit.
The voltage across the output potentiometer at the common junction from all the output circuits of the integrated circuit goes up to l volt. This is not sufficient to turn on the coincidence unit silicon-controlled rectifier. When any other detector unit produces a level of 2 volts or less un the signal line, the output of the coincidence unit produces approximately 2 volts across the potentiometer, an ample driving voltage for the SCR. The SCR turns on actuating the relay, which closes the external alarm circuit.
An adjustable Zener is placed in series with the SCR gate to set the voltage level required for SCR operation. The series circuit is set to 1. 7 volts.
Power Supply:
The power supply includes a continuous charging system of nickel-cadmium batteries to allow operation in the event of a power outage. This DC system would make possible criticality alarm operation without any auxiliary AC power. The output circuit for the power supplies and batteries appears in Figure 5.
The power supply consists of a conventional regulator for normal AC operation and a set of nickel-cadmium batteries producing slightly less voltage. When the AC power is removed from the unit, a diode OR gate connects the batteries to the power input to the system. A time constant in the coincidence circuit-power circuit requires 20 milliseconds to discharge which provides additional stability during the transition period between AC and battery-powered operations.
RFP-1621
The power required for each detector unit.is 156 milliwatts . The 4-ampere-hour batteries will allow six units to operate . for at least 16 hours without AC power. The power supply· panel includes meter readouts for monitoring the 6-volt, DC power line and the condition of the batteries.
Neutron Alarm Output Signal:
The relay closure, produced by anode current flow in the SCR, controls an audio-signal generator located with the public address system in each building. The audio generator provides a signal simulating a klaxon horn or a warbling signal for aural alarm. The circuit is shown in Figure 6.
MAINTENANCE
Detector Electrical Characteristics: The detector unit is set to produce an output pulse from the monostable circuit on the amplifier-trigger board with a 12-millivolt positive pulse at the input to the test connector (BNC). (See Figure 3.)
The pulse width from the monostable multivibrator in the amplifier-trigger circuit is approximately I 0 microseconds.
The pulse width from the monostable multivibrator triggering the SCR is 50 microseconds. The rese~ generator pulse period is 1 second. The pulse width is 5 microseconds.·
A series of three' pulses within l second is normally required to produce triggering of the SCR controlling the line voltage at the input of the coincidence board at the terminal when the norm-test switch (S 1 of Figure 3) on the detector unit is in the norm position.
The test position is used to permit use of a low-level flux neutron source to trigger the detector unit SCR. The test position disables the sampling pulse generator. As a result, the counter stores the three pulses in an indefinite period of time. A source of unmoderated high-energy/neutrons having an output of 105 neutrons per square centimeter per second {n/cm2 /sec) produces SCR triggering within 2 minutes.
The first prototype detector units used a toggle switch to set the norm or test condition for the detector unit. After tests in routine field use, several instances of false alarms occurred from the detector when the test switch was inadvertently left in the test position. The switch has been changed to a 3-circuit phone jack. Thus personnel testing the unit are required to insert a 3-circuit plug in before beginning the test. [Refer to J3 (test) in Figure 3.)
5
BLOCK DIAGRAM
,--COINCID~NCE ~~A-R_D __ --
- r I l ----- ~ - ---r· . 2o-'.:°--4>-----1-....1
VARIABLE VOLTAGE -----ZENERS
5-r ----- -- ---
r ----- -----
ALARM RESET
+6V -o-- - - - - -i>-- - -
I
-
ISUMMINC NETWORK
-
VARIABLE. VOLTAGI ZENER
SCR
-
SPST RELAY
RFP-1621
I I
NO l © co~ l® NC'®
I I
L ___ '--_-_-_-_-_--'_ -- c1~o'!c)~c)l.?0§0F __ -- ----- ----•> - - _J TO EXTERNAL INTERFACE @TO METERS ON REMOTE
DETECTORS
Cl. .. Cl7 - capacitors (values in m!crofarads)
Dl. .. 07 - diodes Ql. .. Ql4 - transistors Rl. •. R54 - resistors
(values in ohms) @ ... @ - off-board connectors
ICl - integrated circuit Kl - relay
I '
Legend
NO ·- normally open COM - common
NC - normally closed V - volts K - 1000
FRONT PANEL
SPST Relay - single pole single throw · SCRl - silicon-controlled rectifier
Sl - switch
FIGURE 4 .. (continued)
9.
\ .
RFP-1621
110 v AC
10
r- BATTERY PACK NO. 5 ----.
1 2 3 4 ,-- -.~--_____;;..n ----s I +6.5 v D9
DB DC
PRINTED CIRCUIT BOARD
--------BLACK
WHITE
AC
I I
liA_: u
Dl
Rl
D2
R2
GROUND I I
ELECTf10§.TATICS POW_ER SUPPLY _ _j
Legend
Rl. .. R4 - re&iBtor (values in ohms)
01. .. D9 - diodes SI =switch V -volts
FIGURE 5. Output Circuit for Power Supply and Batteries.
D7
D6
D5 I I
R4 12J
Meters are used throughout the system to provide visual indication of the alarm condition of the detectors and terminal unit. They also provide the capability of indicating normal voltage levels or abnormal trends. The normal voltage level read on the meters is indicated by a meter reading of 0.7 milliamperes (ma). This represents the normal voltage level of 3.3 volts. The alarm level indicated by the meters will be approximately 0.2 to 0.3 ma, or a voltage level of approximately 1 volt. '
SYSTEM ADJUSTMENTS
Coincidence Board: The adjustable Zeners on the coincidence board must be set to the proper levels or the board will malfunction. (Refer to Figure 4.)
The Zener for the SCR gate voltage level must be set first and should be adjusted to 1.7 volts. The Zener may be set to that voltage by shorting one of the inputs to ground. Four volts are produced at the output of integrated circuit. The potentiometer (R49) may then be set to produce
. 1.7 volts. With this voltage level present on the input to the Zener gate circuit, adjust the Zener control (R53) until the SCR fires and closes the relay. The voltage at the potentiometer will decrease because of the increased load to the gate,. a normal reaction.
After setting the output Zener, remove the short to ground at the input. Replace the short with a 100-ohm potentiometer with clip leads soldered to the center terminal and one end. Clip these leads to one input and ground of the coincidence board. Adjust the potentiometer to produce 2 volts at the input to the Zener at that input. Adjust the Zener {R4, Rl 1, RIB, R29, R36, R'1'1) to produce an outp1_1t voltage of 4 volts from its corresponding output of the integrated circuit. Measure the result al the cathode side of the OR gate diode connected from the terminal of the integrated circuit. Repeat the steps for each of the other five inputs.
After finishing the setup of the six Zeners, remove the clipped-on potentiometer and substitute two grounding clips to anY. two inputs. Then adjust the output potentiometer (R49) to produce SCR triggering with the two inputs grounded. Allow approximately 0.1-volt extra level to provide reliable triggering for any comhination of the six inputs. To confirm the proper voltage level for the potentiometer setting.if necessary, reset the SCR anode circuit. The coincidence board setup is thus completed.
RFP-1621
EXTERNAL INTERFACES
Each connection to an external interface shown in Figure 7 has a series Zener to the base of the transistor driver for a single pole double throw {SPOT) relay. The Zener control must be set to produce current flow when the input voltage to the interface is 3 volts or higher.
The interface Zeners may be set to 3 volts by clipping a potentiometer from positive 6 volts to ground and adjusting the wiper to produce 3 volts output. ·
The Zener control {R2) should be adjusted to just produce relay operation with 3 volts present at the interface input. Each Zener input {R2 1-6) should be adjusted to produce relay operation with the 3-volt output from the potentiometer~ The system should not remain locked up when power is removed from the input to each of the interface Zeners.
COSTS
The cost for each detector unit, excluding semiconductor detector and foil, amounts to $90. The detector and foil combination came to an estimated cost of $35. The total amount for the detector unit with. detector assembly would be about $125.
SYSTEM TESTS
Radiation:
The detector unit of the alarm system has been tested in colbalt-60 (6°Co) gamma radiation fields of up to 104
roentgens per hour (R/hour) without producing a unit alarm.
The detector unit produced an alarm during a 5 x 1016
neutron burst, lasting 33 microseconds over a distance of. 1900 feet (about 579 meters). Another detector unit was placed 39 inches {99 cm) from the critical assembly for three bursts of not less than 5 x 1016 neutrons each. The unit was able to be reset and performed normally after each burst.
Environment:
The system has been tested over a temperature range of 0 to 60 °C, and has performed normally. The introduction of. the prototype system into the Plant noise environment produced several false alarms before additional shielding and circuit filtering eliminated them. The present noise test includes exposure of the shielded detector unit to a rotating spark-gap operating at 5000 volts. The detector units are placed within 6 ~ches (15.2 cm) of the gap and must not.respond.
11
RFP-1621
+7.6 v
+24V
CRI IN4738
12
+
400 10 v I Cl
RCA 40407 03 .
R16 820
+ -
C4 "40U
YELP
r-1
R3 4.7K
C2 = 0.1
C9 = 0.1
R4 47K
R5 4.7K
I IC1 MOTOROLA MC9719P I L--r-%---------
R17 R18' R20 R22
220K 4.7K 220K 4.7K
2N4123 05
04 2N4123
R19 15K R21 15K
CB 10
+
FIGURE 6. Schematic of Alarm-Signal Simulator.
R32 220K
06 2N4123
RB 4.7K
R91K
R24 4.7K
R261K
07 ·2N4123
SCHEMATIC
M1 I u to 1 milliamperes)
2N4124 01
R43 1K
QB 2N4123
R131K
R15 100K
-©--0-1· I I I _____ ...J
R35 2.2K
010 2N4123.
C10 .05
R30 680
R42 10K
TAPC-52 T1
2
II 1
09 2N4124
R31 1K
•
RFP-1621
~~4 >oll----: ..... 3 ~
Legend
@.@ .. @- off-board connectors
Sl - switch R36 Cl. .. ClO - capacitors 560 (values in
miw:if:1rad~)
R l. .. R45 - resistors (values in ohms,
K = 1000) CRl, CR2 - diodes Ql. .. Qll - transistors
K - 1000 V - volts
IC l - integrated circuit
C7 Ml - meter (values, 1- 0 to l
milliamperes) M - 106 ohms
G
---~. ~~~~~~~~~~~---FIGURE 6. (continued)
13
RFP-1621
TO INDICATORS
I) I) (> I) 1) I) I> 1) ) I> I> I) I>
; I I I I I I I I
K3 I I I POWER I I
SPST 0
RELAYS I 6V TROUBLE ~ I I I I
RELAY I I I I I SPST 1 I 2
I 3 I 4 I
I I I I
• .
---'"" I I I I
I I I I I
I I I I I I RELAY DRiV~HS I I I I I
I I I I 1 I 2 I 3 I 4 I
I I I I
. • K4
6V
SONALERT . POWER ALARM RELAY SPOT '
I I I I I I I I I I I I I I
6V VARIABLE VOLTAGE ZENERS ':..!::' - I I I I
' I I I I
1 I 2 I
3 I 4 I I I I I
J4 , •• I II I
1)
P4r 0 \,) I) I)
2 1 . 3 4 6 7 8
14
SPST Relay - single pole single throw (K3) .
SPOT Relay - single pole double throw (K4)
Legend
Rl. .. R4 - resistors (K = 1000 ohms)
)(-volts @. . cy}- off-board connectors Ql. .. Q4 - transistors
P4 - plug J4 - jack
FIGURE 7. External Alarm Circuit Interface.
I> 0 )
I
I I I I I
5 I 6 I
I I
I i I I
5 I 6 I
a
I I I I I I.
5 I 6 I
• I
I) c) n 9 10 5
-----.-.
2·o J4 P4
1 0 < +6V
3
4
Rl-1 R2-1
Rl-2 R2-2
Rl-3 R2-3
Rl-4 R2-4
Rl-5 R2-5
Rl-6 R2-6
·' ,,,,
SCHEMATIC
R4-1
R4-3
R4-4
R4-5
R4-6
FIGURE 7. (continued)
RFP-1621
N
------~ p A 0 0 R
M L
K J
H F
E D
15
RFP-1621
An anode circuit of the SCR which is exposed to noise spikes from long interconnecting lines between the detector unit and coincidence terminal unit may require a lowfrequency filter in extreme noise environments. A lowimpedance millihenry choke may be used to provide fast rise-time rejection of high voltage pulses.
Routine Operational Tests: The system in normal field operations is tested each month for individual detector operation and total system performance.
The detector unit is tested with a moderated plutoniumfluoride (PuF) source having a total output (Q) of 1.6 x 106
neutrons per square centimeters per second. The unit under test must produce an alarmed condition within one minute. The normal time for the detector to trigger to the alarm condition is 20 to 30 seconds.
Several combinations of coincidences between detectors are used to provide the input conditions for the terminal unit to test its alarm capability and response.
CALCULATIONS FOR DETECTION RADIUS
The following values and assumptions were used for the . calculations of the effective detection radius of the neutron criticality alarm system. The useful radius for the neutron detector system exposed to an unattenuated 1014 -fission plutonium criticality is 350 feet (106 meters).
Detector Electronics:
1. Pulses required for the initiation of the alarm circuit -+ = 6.
2. Pulses from the solid-state detector having the minimum trigger amplitude for the detector digital-board electronics-+ = 30 percent.
3. The foil geometry relating the window area of the solid-state detector to the foil area >
4. The foil efficiency for converting
= l 0 percent.
thermal neutrons to alpha particles -+ = 10 percent. (calculated= 16 percent)
5. Detector area normally 20 millimeters; . therefore one fifth of one square centimeter for flux calculation -+
6. The portion of neutron energy spectrum available for producing thermal neutron interaction in the foil adjacent to the detector -+ = 10 percent.
16
Flux Requirements:
1. Pulses required for detector alarm -> = 3 minimum. Pulses required for calculation-+ = 6 minimum.
2. Total number of output pulses from detector to produce 6 usuable pulses-+ = 20.
3. The foil geometry requires 200 alpha particles to produce 20 electrical pulses in tht: detector.
4. The foil efficiency requires 2,000 neutrons to produce 200 alpha particles.
5. Detector area is 20 percent of unit flux area; therefore 101000 neutrons are required per square centimeter to provide 2,000 incident on foil.
-6. Neutron spectrum produced either from an unattenuated source or moderated fast spectrum is probably not above 10 percent thermal; therefore the total flux required at. lhe detector is 105 neutrons per square centimeters pee second (n/cm2 /sec).
The· calculation follows where Q is the total number of neutrons produced in a criticality excursion. Refer to the work of Aspinall and Daniels6 wruch has determined that the most probable minimum is 1014 fissions for a metal plutonium system. This is for a probable excursion time period of one millisecond to three seconds. The neutron flux, F, is the number of neutrons crossing unit area (I cm2 )
per second normal to the incident direction; and R is the radius in centimeters.
F Q
41TR2
Q = 1.5 x 1014 neutrons
1.5 x 1014
F = 41TR2
1.5 x 1014 1.5 x 1014
R2 =----41TF 41T(105 )
R = 1.192 x 108
1.09 x 104
•
R = ~ 356 feet 2.54 centimeters per inch x 12 inches per foot
Minimum expected metal criticality in air= 1014 fissions
Minimum expected solution criticality = 1015 fissions (see Aspinall and Daniels, Page 1)
6 Loe. cit.
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