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The Accident to the NRX Reactor on December 12, 1952

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The Accident to the NRX Reactor on December 12, 1952 _ -- -- - 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 con- dition (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.
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