A Review of Criticality Accidents A Review of Criticality Accidents

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9. Oak Ridge National Laboratory, 10 November 1961 53 235 U metal, paraffin reflected, assembly; single excursion; insignificant exposures. This power transient in about 75 kg of highly enriched (about 93% 235 U) uranium metal reflected with paraffin took place when one portion on a vertical lift machine was approaching the other stationary portion. The experiment was the last of a series during which uranium or paraffin had been added by increments to change the reactivity of the complete system; all previous experiments had been subcritical when fully assembled. In this case, the system became supercritical while the lift was in motion, leading to a yield of between 10 15 and 10 16 fissions. The closure speed of the lifting device was 16 inches per minute; delayed criticality was later determined to be at a separation distance of 2.7 inches. The sensitivity of the system at this point was 10. Sarov (Arzamas-16), 11 March 1963 54 84 8.6 $/inch. Thus, the reactivity insertion rate was 2.3 $/s and a lift slowdown, which became effective at 1.94 inches, did not affect the course of the transient. The reactivity and power histories must have been similar to those of the Jemima (II-B.4) accident, except that the pertinent scram delay time was only 50 ms in this case. The initial spike could not have exceeded 10 15 fissions, and the remaining energy must have been created during the subsequent plateau. The appearance of the metal (smooth, no oxide) and the fact that the paraffin did not melt qualitatively confirmed the yield estimate of 10 15 to 10 16 fissions. Personnel radiation doses were trivial, and the laboratory was ready for normal use within 1.5 hours. Plutonium, lithium deuteride reflected assembly; inadvertent closure; single excursion; two serious exposures. This accident occurred on 11 March 1963 as modifications and maintenance were being performed on a vertical split machine, MSKS. MSKS (Figure 51) was located within Building B at a reactor site (Figure 52) and was used to perform experiments other than critical approaches. It was limited to assemblies whose shape, composition, maximum subcritical multiplication (≤1,000), safe assembly–disassembly procedures, and the contribution of fixtures and equipment in use had already been verified by experiments on the vertical split machine, FKBN-1, located in the same Building B (Figure 52). MSKS was equipped with an automatic gravity driven scram system and closure of an assembly was performed remotely from a control panel behind concrete shielding (Figure 52). The assembly involved in the accident had a 135 mm outside diameter plutonium core surrounded by a ~350 mm outside diameter lithium deuteride reflector (Figure 51). The core was composed of a set of δ–phase plutonium metal hemishells in a thin (0.1 mm) nickel coating. The core had a 63 mm central cavity in which a neutron source (~10 6 n/s) had been placed. The neutron reflector was a set of LiD hemishells, ~107 mm thick. On 9 March 1963, the MSKS chief and operations engineer constructed the assembly on MSKS and conducted approach to critical experiments without first performing the required experiments on FKBN-1. Pu Core Neutron Source Liquid Damper Electromechanical Drive Figure 51. MSKS and assembly involved in the 11 March 1963 accident. LiD Reflector Steel Ring LiD Reflector Steel Cup Emergency Block Support Pipe Bedplate Electrical Drive Rod
On 11 March 1963, the facility chief and the operations engineer proceeded with pre-experiment operations with the assembly pieces still in place. The experimentalists attempted to make adjustments to the machinery of MSKS using unauthorized attachments in violation of the operating procedures. As they worked on MSKS adjusting the electromechanical lift mechanism, an excursion occurred. Seeing a flash of light, the experimentalists immediately exited and went to the control room, where the facility chief pushed the control button lowering the table. The automatic scram system did not activate because the detectors to which it was tied were not operating. The accident was the result of gross violations of the MSKS operating procedures by the facility chief and operations engineer. The excursion was due to the inadvertent closure of the assembly by the experimentalists. As a result of the accident investigation and analysis, it was judged that the assembly had approached or possibly even exceeded the prompt critical Neutron Detectors FKBN MSKS Neutron Detectors FKBN Control Room Figure 52. Building B floor plan. state. The number of fissions was estimated to be ~5 × 10 15 fissions. The nickel clad of the plutonium was not breached and therefore no contamination resulted. MSKS itself was undamaged and remained in long-term service. 54 Since the assembly parts were not damaged, they were used in a special set of experiments to more accurately estimate the exposure to personnel. The facility chief’s dose was ~370 rem, and the operations engineer received ~550 rem. Both experienced radiation sickness. The facility chief survived for 26 years, and the operations engineer was still alive 36 years after the accident. Both were immediately taken to a hospital for treatment. Eventually they returned to work at the VNIIEF Physics Division. Four other people working in adjacent area also received doses, although much lower (~7, ~1, ~1, and ~0.02 rem). MSKS Control Room FKBN Control Panel MSKS Control Panel Corridor Rail Track Neutron Source Entrance 85
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LA-13638 Approved for public releas
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A Review of Criticality Accidents 2
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PREFACE This document is the second
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Tom Jones was invaluable in generat
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C. OBSERVATIONS AND LESSONS LEARNED
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FIGURES 1. Chronology of process cr
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51. MSKS and assembly involved in t
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A REVIEW OF CRITICALITY ACCIDENTS A
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3 Black Sea Baltic Sea Obninsk Norw
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North Atlantic Ocean Irish Sea Wind
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1. Mayak Production Association, 15
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time of the accident. However, the
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determined by a radiation control p
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consequences of this accident, the
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Figure 10. Drum in which the 1958 Y
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The entire plutonium process area h
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During the next shift, radiation sa
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9. Siberian Chemical Combine, 14 Ju
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The accident investigation determin
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10. Hanford Works, 7 April 1962 18,
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heater and stirrer were turned off
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vessel 64-A was added. The dissolut
Page 47 and 48: additional reflection afforded by t
Page 49 and 50: 15. Electrostal Machine Building Pl
Page 51 and 52: 16. Mayak Production Association, 1
Page 53 and 54: ased on a radiation survey, that th
Page 55 and 56: 0.5 m 0.5 m 0.5 m 0.5 m 3.0 m 1.4 m
Page 57 and 58: The investigation identified severa
Page 59 and 60: 19. Idaho Chemical Processing Plant
Page 61 and 62: 20. Siberian Chemical Combine, 13 D
Page 63 and 64: To Glovebox 6 7 8 6 From Glovebox 6
Page 65 and 66: on a 0.4 m square pitch grid. The b
Page 67 and 68: competing reactivity effects proved
Page 69 and 70: Figure 35. The precipitation vessel
Page 71 and 72: B. PHYSICAL AND NEUTRONIC CHARACTER
Page 73 and 74: Material Fissile Mass: Fissile mass
Page 75 and 76: 235 U Spherical Critical Mass (kg)
Page 77 and 78: Table 10. Accident Fission Energy R
Page 79 and 80: • Accidents in shielded facilitie
Page 81 and 82: • Senior management should be awa
Page 83 and 84: Table 11. Reactor and Critical Expe
Page 85 and 86: 3. Oak Ridge National Laboratory, 2
Page 87 and 88: 5. Oak Ridge National Laboratory, 3
Page 89 and 90: eflector of 236 kg when he noticed
Page 91 and 92: 3. Los Alamos Scientific Laboratory
Page 93 and 94: At the equatorial plane, the two ha
Page 95 and 96: Figure 48. The Los Alamos Scientifi
Page 97: Figure 50. Burst rod and several se
Page 101 and 102: 13. Chelyabinsk-70, 5 April 1968 57
Page 103 and 104: 14. Aberdeen Proving Ground, 6 Sept
Page 105 and 106: completed the construction of the l
Page 107 and 108: C. MODERATED METAL OR OXIDE SYSTEMS
Page 109 and 110: 63,65,66, 67 4. Chalk River Laborat
Page 111 and 112: 7. Centre dÉtudes Nucleaires de Sa
Page 113 and 114: a technician prescribing the loadin
Page 115 and 116: the room to drain the water out of
Page 117 and 118: In the final experiment, the critic
Page 121 and 122: III. POWER EXCURSIONS AND QUENCHING
Page 123 and 124: Figure 64. Energy model computation
Page 125 and 126: References 1. Stratton, W. R. A Rev
Page 127 and 128: 50. Paxton, H. C. “Godiva Wrecked
Page 129 and 130: 92. Stone, R. S., H. P. Sleeper, Jr
Page 131 and 132: criticality accident: The release o
Page 133 and 134: prompt criticality: State of a fiss
Page 143 and 144: 9. Siberian Chemical Combine (Tomsk
Page 147 and 148: 13. Siberian Chemical Combine, 2 De
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15. Electrostal Machine Building Pl
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19. Idaho Chemical Processing Plant
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21. Novosibirsk Chemical Concentrat
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