The Accident to the NRX Reactor on December 12, 1952

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DR-32 Series TA Special Distribution Copy No.

Technical Information Service, Oak Ridge l-25

15 ---- THE ACCIDENT TO THE NRX REACTOR ON DECEMBER 12, 1952

W. B. LEWlS Atomic Energy of Canada Limited Chalk River, Ontario

Report DR-32

ABSTRACT 1. INTRODUCTION

Because qf a complex concurrence of me- On Friday afternoon, Dec. 12, 1952, in a chanical defects in the shut-off-rod system and normal but not quite routine operation, the operating errors which alone would not have NRX 30-megawatt heavy-water research re- caused serious trouble. a power surge occurred actor at Chalk River was severely damagedow- in the NRX reactor during preparations for ex- ing to a concurrence of mechanical defects and periments at low &ou*e;-. Some of the cooling operating errors. It is very easy to prescribe arrangements at the time were adequate only measures to prevent any recurrence of the for low-power operation. Consequently some of accident, but it has to be remembered that the natural-uranium metal melted and ruptured this reactor has been characterized as having the aluminum sheathing and tubes which sepa- 900 devices for shutting it down but only one rated the heavy-water. air, and cooling-water for starting it up. Adding a few more safety systems. As a result some 10,000 curies of devices might prove to be the last straw in fission products from long-irradiated uranium preventing operation, or it might make the were carried by a flood of 1 ,OOO,OOOgal of cool- operators’ work so involved that some quite ing water into the basement. Fused masses of different accident would be provoked. Setting highly irradiated uranium and uranium oxide aside the simple self-evident lessons, there were left inside the calandria, and the core are some more subtle considerations which vessel of the reactor and tubes of the calandria may be of value for future reactor designs and were severely damaged. therefore give this article some interest be- In such a high-flux reactor where the tran- yond that assured by human emotions to all sient xenon poison may affect the reactivity by reporters of major misfortunes. 40 milli-k (mk), the shut-off rods have to cover What occurred, in brief, was that, during a reactivity range of about 70 mk. As one lesson preparations for reactivity measurements, the from the accident it appears preferable to with- draw the first or safeguard bank of shut-off rods soonajter shutting down, instead of making this the first step of the actual start-up. 4 W. B. LEWIS reactor was unexpectedly found to be divergent, lessons to be learned for the future. A second and at the same time there was some mechani- article will discuss the nature of the damage cal defect preventing shut-off rods from drop- and attempt to reconstruct the sequence of ping in. Even this would not have had serious events inside the reactor system. Further in- results if a number of the uranium rods had formation is given in references 1 to 7. not at the time a purposely reduced flow of cooling water. As the reactor was leveling off in power at about 1’7 megawatts, the cooling 2. CONTROL SYSTEM OF NRX REACTOR water of these rods boiled. thereby increasing the reactivity and the power. At the increased The normal sequence of operations to start power, some of the aluminum sheathing the up the reactor is (1) to set the level of the uranium melted. At least one rod blew itself heavy water at a predetermined level somewhat apart, and molten uranium poured out from the below that required for criticality; (2) to raise core of the upper part. Some of the tubes re- the shut-off rods; (3) to raise the one control taining the heavy water ruptured. All the fluid rod which gives only a fine control equivalent systems of cooling water, air, heavy water, to about 10 cm height of heavy water; and and helium were then in contact. The cooling finally (4) to raise the level of the heavy water water being under the highest pressure was slowly to that predicted for criticality. forced in, displacing air and helium, and helped The shut-off rods are thin steel tubes filled to bring the reactor below critical. Meanwhile, with boron carbide designed to be as light as however, the operators had been forced to their possible in wcirht. The rods are light to permit last resort; namely, to open valves which rapid acceleration and deceleration in being dumped the heavy water rapidly to storage tanks driven into position (a lo-ft travel) by air below. Within 60 set the power was back to pressure. The piston at the head of the rod is zero, but major problems of radioactive con- 17 in. long and has to be heavy to provide tamination had been set. radiation shielding. The total weight of the In the absence of radioactivity the damage moving parts (rod and piston) is 29 lb. would have been simple though tedious to re- The air pressures are manipulated by elec- pair, but the presence of large amounts of in- trical controls. In addition, the piston heads in tensely irradiated exposed uranium in very the up position are held by a solenoid magnet. inaccessible places presented a cleanup prob- The presence of each rod in a fully up position lem on a scale without precedent. There were is indicated by a red light on the control desk. many kilograms of uranium exposed as metal There were 12 shut-off rods, and the basis of or oxide containing over 3 kg/ton of fission their operation was that ‘7 in the down position products in the ruptured interior of a reactor were sufficient to hold the reactivity of the which was not designed to he repairable. Months reactor below critical for any approved charge later this material would still have a radio- of fuel and load. Actually all are not equal in ef- activity of about 1 curie/g. fect because of their differing positions in the At the time of writing the reactor had been reactor. On release from the solenoids the stripped of all significant amounts of uranium; rods are normally given an initial acceleration and the calandria, the aluminum vessel at the by 100 psi air pressure on the piston, over- core of the reactor, had been removed so that coming 13 psi upward-flowing cooling air. a new one could be .installed. Considerable The rods would normally also drop under problems of decontamination remained in the gravity alone against this upward air. With air- basement regions below the reactor, but the pressure drive the rods are halfway down in worst difficulties of excessive radiation hazard ‘/3 to t/r set after the trip signal; without head had been successfully passed. air pressure they take 3 to 5 set to drop all Following an explanation of relevant features the way. of the design of the reactor, a more detailed The 12 rods were electrically interconnected account of the operating sequence of events will in the following “banks” or groups which op- be given, together with indications of some erated together: THE ACC:DENT TO THE NRX REACTOR ON DECEMBER 12, 1952 s

Bank No. No. of rods only 12, no more than 4 may be set for condition (a). All other rods must satisfy condition 1 4 (b). To achieve a safe start-up in the shortest 2 3 time, as large a number as possible and the 3 2 most highly effective rods were in the safe- 4 1 guard bank. The reason for allowing always one 5 1 more than the minimum safe number is toallow 6 1 for one undetected failure in the safety system. It may be worth noting that in a high-flux Bank 1 was brought up by push button 1 at reactor, such as NFLX, the range of reactivity the control desk. The remainder were brought caused by the transient Xe13’ poison is over 4 up in the sequence of the bank numbers by per cent or in more convenient units 40 milli-k automatic interconnection after pressing push (mk). The maximum reactivity available might button 2. be 10 mk less than required to overcome the It may be noted that the bank brought up by peak poison. The shut-off-rod system had not push button 1 has to satisfy different conditions only to cover this range of 30 mk between the from those responding to push button 2. To reactivity available to overcome the transient stress this difference, the term “safeguard poison and that required for the unpoisoned re- bank” is applied to bank 1. As explained later actor but also that which might result fromloss the number of rods in the safeguard bank was of the cooling water from the system (estimated by design 1 greater than that in any other bank; as 25 mk). Cooling the reactor from its normal therefore, since the number chosen was 4, no operating power adds another 5 mk. The 12 other bank might contain more than 3. More- shut-off rods commanded a total of 70 mk. To over the 3 least effective in the safeguard bank avoid being poisoned out after a shutdown from had to be more effective than the total in any high power, it was designed that the reactor other bank. could be started up in about 10 min, requiring The safeguard bank was to be brought up a mean rate of removal of shut-off rods of ‘7 only from a condition in which all the shut-off mk/min. Actually this was unevenly spread in rods of the other banks were down. This had time, and the first 4 shut-off rods commanding been effected by a safety circuit involving 30 mk were arranged to be withdrawn together “limit” switches operated by each rod when in an operation which would be completed in fully down. Owing, however, to defects in these about 45 set, although the actual withdrawal switches and their being subject to flooding might take place in a few seconds. This rapid which could make them a hazard, this “safety” rate of withdrawal has been subject to criticism, circuit was not in operation at the time of the and it may therefore be of interest to note that, incident. The added responsibility was accepted in the reactor as originally designed and oper- by the operating supervisor. ated, the shut-off rods commanded a higher re- The other shut-off rods satisfied the different activity and were withdrawn in a shorter time, condition that none could be raised by the It is important to understand the force of the electrical controls unless the rods of the safe- arguments on both sides leading to the com- guard bank were all fully up and their head- promise. In a very high-flux thermal reactor gears charged to 80 psi air pressure. As ap- which is subject to incidental trips, safety pears later, however, there existedother means must be assured by measures other than a very of getting them up. slow maximum rate of withdrawing control The design reason distinguishing the safe- rods. guard bank is that, for safety, no shut-off rod To understand the course of events there is may be raised unless either (a) more than 7 also a practical detail of the shut-off-rod shut-off rods would be left fully down, or (b) system which has to be appreciated. This con- more rods are available for quick release than cerns the functions and location of four push are being raised at any time. To make start-up buttons at the control desk. possible, some rods must satisfy condition (a) Push button 4 is mounted on a wall panel at and not (b), and, if the total of shut-off rods is the left of the desk. It serves to charge air to 6 W. B. LEWIS the heads of the shut-off-rod assemblies. The 2 raises the remaining banks in automatic se- release of this air drives the rods down. quence. Push buttons 1, 2, and 3 are on Ihe panel To ensure satisfactory operation, push button shown in Fig. 1, centrally mounted on the con- 3 is pressed in conjunction with push buttons 4, trol desk. 1, and 2. If it is not pressed with push button 4, air may leak from the head system; if it is not pressed with 1 and 2, the shut-off rods may not be drawn fully home, and safety circuits would prevent the start-up operation from proceeding further.

3. EMERGENCY PROCEDURE

Since the events led to following the pre- scribed emergency procedures, it should be noted that these procedures had been set up and summaries were posted in various positions throughout the plant and laboratories and in- cluded as a full page in every copy of the project telephone directory. Acopyoftheprocedures is reproduced as Fig. 2, The procedures are designed especially for situations in which large amounts of radioactive material may be spilled or dispersed in the atmosphere and have the dual objective of safeguarding health directly and indirectly by limiting the spread of radioactive contamination particularly in areas where cleanup is difficult yet necessary.

4. ORGANIZATION

A number of the senior staff were absent from Chalk River at the time of the incident. The organization of those present and directly concerned is conveniently represented as shown in Fig. 3.

Fig. l-central panel on control desk. 5. REACTOR LOADING AND INTENDED OPERATION Push button 3 serves to increase temporarily the current through all solenoid magnets in the The experiment on hand was a series of shut-off-rod head system. This ensures good measurements of the reactor reactivity at low seating of the air-release valves and also power. The main object was to compare the draws the pistons on the shut-off rods fully reactivity of long-irradiated rods with that of home. The normal maintained current is ade- fresh rods. To avoid complications from di- quate to hold the rods and valves in position. mensional changes in the water-cooling chan- Excess current above the minimum would de- nels, it was necessary to blow the water out of lay release. some rods and substitute air cooling. At the As already mentioned push button 1 raises the time of the incident only one rod was air- safeguard bank of shut-off rods, and pushbutton cooled and that was a fresh unirradiated rod. THE ACCIDENT TO THE NRX REACTOR ON DECEMBER 12,1952 ERGENCYSlGI ’iAl§

STAYIN- RISINGand FALUNC SIGNAL :zz--

NOTE: Stay-in procedures will be carried out from time to time as drills.

EVACUATCONTION INUOUSSIGNAL Emergency lirenr Think of others! Are they alI out?

ALL CLEAR SERIESof INTERMITTENTBLASTS $&,

IWERGENCY PROCEDURE 1.

A-. iliE%& (a) STAY-IN - A Ac.sin~ and fallln~ nipal on tha .n*rgency alnns.

lb) EVACUATION - A Continuous l i@al on the mmr&mcy sirma. (cl ALLCLihQ - A ..ti.s of intermlct.nt b1.st.a on tha plant whistle l nmndIn& for thm* loInut*s.

(dl TEST - The ainns till b. t.H,,d on th. ‘t,, 3und.y of ..sv month at 1100 hourr, local tlm.. 5. &&t&l on star-Ip (a) Pmcwd to n.‘nst bulldlng md r.a.In th.n until Instructad othnrIs*. (b) Closm all windows md doors .nd t.k. mfh spclal action as b.m b.an laid dam for th. bulldInt. c) Pmcead with normal work .a far .a poosslble. I d) Do not uae.tha tblmphone. 6. Action on HWIC~

(a) Make a11 off1e.s end Lboratori..s mfe, and lock up a11 meret documents, *CC. lb) Walk quickly to tha Rat., holdiny: . hmdksrchl~f 0v.r mouth and nom. 7. Cwtain ~rsons, such .m btildlng h.ads and all Branch heads, till hmva bean RIvm BPCIal lnstluct?onl to c.rry out In tha .v.nt of .n amer~‘“cy. If you are one of them. famIllwI~0 yoursslf with ,Our dutlas,

Fig. 2- Emergency procedures. 8 W. B. LEWIS

All enriched fuel rods, adjuster (cobalt load) The supervisor at the control desk noticed this rods, special assemblies, and isotope loads were because the red lights came on. He phoned to out of the reactor except one thorium and ura- the operator in the basement to stop and went nium sample rod in an outer region. down himself to investigate and rectify the situ- A full complement of normal uranium rods ation, leaving his assistant at the control desk. was in position. Certain of these rods were to He recognized the operator’s mistake and was be moved between measurements and had only horrified at the possible consequences if the temporary cooling by means of hoses. Such operator had continued to open these wrong cooling is adequate for low-power operations. valves (actually he could not have opened all

PROJECT HEAD I I I I RESEARCH DIRECTOR OF GENERAL SUPERINTENDENT OF OPERATIONS PHYSICISTS BIOLOGY RESEARCH I AND RADIATION REACTORS BRANCH SUPERINTENDENT I. HAZARDS CONTROL I ASSISTANT SUPERINTENDENT I REACTOR SUPERVISOR I ASSISTANT TO REACTOR SUPERVISOR

Fig. 3- Organizational chart of personnel onduty and responsible for operation of the reactor at the time of the incident.

As the reactor had not been up to power for valves since some handles had been removed several days, transient poison had decayed, a for safety). The supervisor rectified all valves necessary condition for the experiment. and checked air pressures. He assumed that The heavy-water level was at 260 cm and all shut-off rods would drop back into position, was to be raised to 277 cm for the planned ex- but, on account of unexplained mechanical de- periment. The reactivity changes by about 1 mk fects, it is apparent from subsequent events for 3 cm change of heavy-water level in this and inspection that two. or three did not drop range. back, although they slipped down sufficiently to Becauue of the experiments in hand, re- clear all the red lights on the control desk. search physicists were present in the reactor The supervisor then phoned his assistant to control room, but the reactor was operated by press buttons 4 and 1. He had intended to say 4 the reactors branch personnel who alone have and 3, but under normal circumstances 4 and 1 authority for this. The reactor loading to be should have been safe (all the shut-off-rod red used was recommended by the physicists and lights were out). His assistant therefore did so. approved in writing by the reactors branch Having to leave the phone to reach simultane- superintendent. ously with two hands the two buttons, he could not be recalled to correct the mistake. Button 3 not having been pressed, the air pressure 6. DESCRIPTION OF THE INCIDENT brought up by button 4 leaked away. Up in the control room it was soon evident The immediate chain of events which led to when the first bank of shut-off rods was raised the accident began with an error by an operator by button 1 that the reactor was above critical, in the basement who opened by mistake three or which was of course a complete surprise. four bypass valves on the shut-off-rod air It takes a few Beconds for this tobe apparent. system, thereby causing three or more shut-off There was surprise but no alarm for thi next rods to rise when the reactor was shutdown. step would be to trip the reactor and thus drop THE ACCIDENT TO THE NRX REACTOR ON DECEMBER 12, 1952 9 back the shut-off rods. This the assistant did was already reaching for the dump switch and about 20 set after pushing button 1. But two of beat the others to it. . the red lights stayed on, and in fact only one of However by this time the reactor power was the four rods of the first bank droppedback into up in the tens of megawatts, and the dumping the reactor and that over a period of about 11/2 took a few seconds to become effective. Then a min. Even though, as it appeared, the air pres- fear arose that they might be dumping too fast sure had leaked from the header, all shut-off as the helium pressure had dropped back rods should have nevertheless dropped back sharply, and they envisaged danger of col- under gravity. lapsing the calandria by vacuum. The assistant

Fig. 4- Lower header room below reactor after the accident with water gushing down.

The galvanometer spot indicated that the superintendent halted the dumping after about 1 power level was still climbing up. The assistant min but after a little thought resumed it. How- telephoned the supervisor in the basement urging ever, in 10 to 30 set after starting to dump, the him to do something to the air pressure to get instruments were back on scale, and the power the rods down. rapidly dropped to zero. The assistant super- Others in the control room were worried: the intendent went to report to the superintendent, physicists, the assistant superintendent of the but the consequences were only beginning. reactors branch, and a junior supervisor. At In the basement the door into the chamber least two thought of the last resort; namely, to under the reactor (the lower header room) was “dump the polymer.” All were familiar with the open. Through this an operator sa.w watergush- process as it had been done the previous day ing down (Fig. 4), and immediately he called for experimental purposes. The assistant super- the supervisor. Their instant reaction was to intendent gave the word; one of the physicists suspect any water as being heavy water; there- 10 W. B. LER’IS

fore the supervisor and operator rushed in 7. TIME RECORD SUMMARY with a bucket and collected a sample, which was soon found to be light water but radioactive. The time of pressing push button 1 will be The assistant superintendent, returning to the taken as 1507 hr. Times are in most cases control room, was met by an operator who re- very approximate, and certain discrepancies ported a rumble and a spurt of water up through are known to remain. The sequence of events the top of the reactor. is indicated in Table 1. Then the air activity began, and automatic radiation-level alarms sounded in the reactor 8. THE POWER SURGE building. A phone call to the control room from the adjoining chemical extraction plant reported Although all relevant instruments went off atmospheric activity off-scale and requested scale, it proved possible to piece together data the emergency stay-in procedure. The sirens to lconstruct reasonably well-timed curves of for this were sounded. The radiation hazards power and reactivity. This reconstruction is control branch got busy reading instruments, described by W. J. Henderson, A. C. Johnson, making surveys, and collecting reports. Some and P. R. Tunnicliffe in Report NEI-26, and a minutes later the activity inside buildings with summary of their conclusions is given here. forced ventilation was found higher than out- Before the first bank of shut-off rods was side; therefore on the advice of the Biology raised the reactor was more reactive than and Radiation Hazards Control Director the supposed owing to a number of shut-off rods Project Head gave the order for the plant evacu- not being down. This unsuspected extra re- ation procedure, and that went into effect. activity was about 10 mk. Raising the first Meanwhile in the reactor system not earlier bank made the reactor overcritical by about than 30 set before the dumping began, helium 6 mk, and it diverged with a doubling time of began to leak at a rate of 140 cu ft/min. After about 2 set, reaching a power of the order of 3’/4 min, by which time the reactor power had 100 kw. At this point the reactor trip circuit been down to a negligible level for 2 min, the opened, but only one shut-off rod fell slowly in. reserve gasholder was almost empty. Then The reactor continued to diverge but at a rate suddenly in less than 30 set the 585 cu ft decreasing with time in such a way as to sug- gasholder rose to its fullest extent. The change gest that it would have leveled off at about 20 of direction of motion of the gasholder was so megawatts. (The scale for power is nominal abrupt on the record and its motion so well- owing to unknown shadowing effects by shut-off timed by pen marks at 15-set intervals that it rods on the ion chambers.) At 17 megawatts on can be deduced with certainty that within a this scale boiling is presumed to have occurred period of 15 set the gasholder became con- in some of the temporarily cooled rods, ex- nected presumably to a mass of gas at high pelling light water from the reactor and in- enough pressure to give a large acceleration creasing the reactivity by at least 2 mk. The to the massive gasholder. Further discussion reactor continued to diverge for a period of 10 of this will be given in the second article. to 15 set and reached a power between 60 and About the same time that the gasholder was 90 megawatts when it was checked by opening forced up, the radiation level in the reactor the heavy-water dump valves and also possibly building became high. Respirators were issued by ingress of light water through ruptures in to those in the control room. All not concerned the cooling-water tubes. with the reactor operation were evacuated The reactor power was greater than 1 mega- from the building. watt for less than 62 sec. Holding discussions in gas masks is difficult It is to be noted that the powers in the mega- so after a few further minutes those concerned watt range quoted here are from a Leeds and with reactor operation also went to an ad- Northrup Micromax recorder (1 ma full scale) jacent building and planned further steps, re- operated from an ion chamber and amplifier. turning to the reactor building to put them The full-scale deflection normally corresponds intn DffQd. to a reactor power of 60 megawatts. The tran- THE ACCIDENT TO THE NRX REACTOR ON DECEMBER 12, 1952 11

sient was too rapid to record properly, but ex- increased to 3 in./min. In order to estimate amination of the trace showed that the stationary the reactivity increase which occurred at 17 positions of the pen during successive tentative megawatts, the simulator was leveled off at balances by the instrument could be readily dis- this power and then the reactivity was increased tinguished. Since the time intervals between by a known amount and the “reactor” allowed to successive balance attempts are well defiiied, diverge. For an excess reactivity of 2.5 mk or

Table I-Approximate Time Sequence of Various Phases of the Incident

Time Activity or condition noted Time Activity or condition noted

1507 + 00 set Push button 1 pressed 1537 Electric fans added to steam fan 1507 + 20 set Manual trip operated extracting air from reactor; 1507 + 30 set Power - 17 megawatts; reactivity radiation level around steam suddenly increased by 2.5 mk; fan 900 mr/hr helium leak started 1537 Level in heavy-water storage 1507 + 44 set Dump started tanks rose so dump valves again 1507 + 49 set Power - 100 megawatts; instru- closed; water level in calandria ments indicating activity of air rose to 134 cm and remained so passing to the stack off scale 1520 to 1545 Air-filter samples taken outside 1506 + 08 set Low power restored plant area - 500 counts/min 1509 Radiation level by Cutie Pie 40 py from 3 to 4 m3 mr/hr generally around control 1547 Plant evacuation signal given room and 90 mr/hr at door 1615 Air-filter sample in Building 300 leading to top of reactor adjacent to reactor showed no 1510 200 mr/hr at top of reactqr detectable activity 1511 Gasholder rose suddenly 16-40 Weir box raised to top of calandria ’ 1511 900 mr/hr on bridge at top of re- to prevent light water entering actor near door of control room storage tanks by way of the weir 1512 Circulating pumps and constant- box overflow line level pump turned off 1700 Air-filter sample outside (5 m3) 1515 Wearing respirators advised showed nothing detectable above 1517 Stay-in emergency signal given background of 1000 counts/min 1517 Ventilating air to reactor building 1800 Flood water at foot of steps to turned off auxiliary equipment room in 1527 5 r/hr in auxiliary equipment basement highly active room (basement) near north wall 2045 Air-filter sample main reactor by pencil chambers; similar floor (14 m3) gave 20 mr/hr radioactivity at door of lower on Tracerlab SUIB instrument, header room but no alpha activity was 1537 Air-filter sample in radiation detectable hazards control room (1.4 m3) gave >20,000 counts/min

a good graph could be made of instrument greater, the recorder ran at its maximum rate. reading against time. This is shown in Fig. 5. The estimate that the reactor power did not The recorder and its amplifier were re- exceed 90 megawatts (on the scale of this in- moved and set up to operate from an electronic strument) is based on the observation that a reactor simulator. The response of the amplifier sufficient overload signal would jam the indi- to transient input signals showed that the limit- cating galvanometer and the recorder would ing time constant lay in the recorder. The out- remain at full-scale deflection. This did not put from the simulator was therefore fed di- occur in the incident as shown by Fig. 5. rectly to the recorder. The chart speed was Confirmation of the maximum power reached THE ACCIDENT TO THE NRK REACTOR ON DECEMBER i2.1932 :3-14 from the basement to a storage tank. The REFERENCES total water collected amounted to about 1 ,OOO,OOO gal and contained about 10,000 curies of long- 1. F. W. Gilbert, Canadian Facilities for Isotope Pro- lived fission products. This water was suc- duction and Bombardment, Nucleonics, 10: 6 (1952). cessfully disposed of by pumping it through a 2. D. G. Hurst, Chalk River N.R.X. Nuclear Reactor, Elec. Eng., 70: 476 (1951). l’/,-mile pipeline to a trench system in a dis- 3. W. B. Lewis, The NRX Pile at Chalk River, Phys. posal ground where it was allowed to seep away. Today, 4: 12 (1951). A check was kept on activity in water draining 4. W. B. Lewis, Interim Report at 9 January 1953 on from this area, but no detectable activity was Problems Arising from the NRK Incident of 12 De- found even in the creek draining the area to a cember 1952, Report DR-29. small lake. 5. W. B. Lewis and A. G. Ward, An Appreciation of the Problem of Reactor Shut-off Rods with Special Reference to the NRK Reactor, Report DR-31. 10. IMMEDIATE CONCLUSIONS 6. W. J. Henderson, A. C. Johnson, and P. R. Tunni- cliffe, An Investigation of Some of the Circum- Since none of the operating errors made ap- stances Pertinent to the Accident to the NRX Re- pears to be outside the normal range of human actor of December 12, 1952, Report NEI-26. error and since design and management aimed 7. I. N. MacKay, Shut-off Rods, CRE 400 series, especially CRE 400F. at setting conditions which would be safe despite normal human errors and mechanical faults, it appears necessary to take note of improvements which may be possible in all respects. To reduce the risk of human error and me- ABOUT THE AUTHOR chanical failure, no doubt a better system of review and inspection should be established. W. Bennett Lewis, C.B.E., F.R.S., is Vice-Presi- TMs should relate the design considerations to dent, Research and Development, of the Atomic En- the current practice. ergy of Canada Limited, Chalk River, Ontario. He In the design of a shut-off-rod system an in- was born in England in 1908 and took the Natural teresting point emerges, namely, that it may be Science Tripos at Cambridge in physics in 1930. He studied fine structure and energies of alpha-ray safer to plan and set experiments or normal groups with Lord Rutherford at Cavendish Laboratory operations with the safeguard bank raised out of and received the Ph.D. degree in 1934. Dr. Lewis the reactor. If the safeguard bank had been out continued research at Cavendish Laboratory as Uni- when the operator in the basement made his versity Demonstrator and Lecturer, working with Sir initial mistake and blew up extra shut-off rods, John D. Cockcroft onnuclear disintegrations by parti- the reactor would then have become critical, but cles accelerated by high voltages and then in the con- the consequent dropping in of the safeguard struction and operation of the Cambridge cyclotron. bank would have averted any serious accident, He is the author of “Electrical Counting,” published even if the serviceability of the safeguard bank in 1942. In 1939 he was loaned to the Air Ministry for radar work and continued in this at Telecommunica- was as low as it subsequently proved to be. tions Research Establishment until September 1946, Moreover all would have been alerted to the serving as Chief Superintendent from 1945 to 1946. hazardous condition that rods had not dropped In 1946 Dr. Lewis succeeded Sir John D. Cockcroft as back. Director of Research at Chalk River.