A Review of Criticality Accidents A Review of Criticality Accidents

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102 Figure 61. Mockup of the accident configuration for the 26 May 1971 accident.
In the final experiment, the critical number of fuel rods with the smallest pitch (7.2 mm) was measured. This number was 1,790 and exceeded the minimum critical number of rods for an optimum pitch by approximately 7 times. After the experiment was finished, the supervisor of the experimental team ordered the insertion of all control and emergency protection rods and the neutron source was removed from the core. Four staff members entered the critical assembly compartment for examination. The supervisor then ordered the water be removed through the fast dumping (emergency) valve. The water from the critical assembly tank could have been drained through the slow dumping valve in 15 to 20 minutes or through the fast dumping valve in 20 to 30 seconds. After the previous experiments, the water had been drained through the slow dumping valve. The plexiglas support plate almost completely covered the tank section, and the size of the gap between the plate edge and tank wall was less than the size of the fast dumping valve outlet. For this reason, upon initiating the fast dumping operation, the Plexiglas base plate sagged and the fuel rods fell out of the upper lattice plate (Figure 61). The fuel rods separated into a fan shaped array in which the pitch between the rods came close to optimum and the lattice went highly supercritical. The rate of reactivity insertion was calculated to have reached ~2 β eff /s. 13. The RA-2 Facility, 23 September 1983 79 The RA–2 Facility was located in Buenos Aires, Argentina. Control rods for this essentially zero power experimental reactor assembly were MTR elements in which 4 of the 19 fuel plates were removed and replaced by 2 cadmium plates. Just outside the fueled region (approximately 305 mm × 380 mm) was a graphite reflector approximately 75 mm thick. The large reactor vessel was filled with de-mineralized water during operations and was supposed to be drained during changes in fuel configurations when people were required to be present. The technician, a qualified operator with 14 years experience, was alone in the reactor room making a change in the fuel configuration. The moderator had not been drained from the tank, though required by During the criticality accident, two peripheral rows of fuel rods were destroyed by the energy release. Fragments of these fuel rods resembled welding rod fragments. Water splashed out of the tank. The energy release in the burst, estimated from the core radioactivity, was ~5 × 10 18 fissions. This value is, in all probability, universal for uranium–water systems in an open tank under conditions of rapid reactivity insertion. Excursions of this type are terminated due to fuel destruction or loss of water. Radioactive contamination of the critical assembly room was minimal and there was no contamination of the outer premises; however, the accident consequences were tragic for the personnel involved. A technician, who was close to the tank when the pulse occurred received a radiation dose of ~6,000 rem and died 5 days after the accident. The supervisor received 2,000 rem and died in 15 days. Two other staff members inside the critical assembly room received doses of 700 –800 rem. Physicians managed to save their lives but both suffered long term health effects. The construction of the critical assembly was the main cause of the accident. No calculations were performed for the components of the system and the construction as a whole. Improper and hasty actions by personnel during the final stage of the experiment also contributed to the cause of the accident. MTR–type fuel element, water reflected, assembly; single excursion; one fatality, two significant exposures. procedures. Two fuel elements had been placed just outside the graphite, instead of being removed completely from the tank. Two of the control elements, without the cadmium plates installed, were being placed in the fuel configuration. Apparently, criticality occurred as the second of these was being installed, since it was found only partially inserted. The excursion consisted of 3 to 4.5 × 10 17 fissions; the operator received an absorbed dose of about 2000 rad from gamma-rays and 1700 rad from neutron. This exposure was highly non–uniform, with the upper right side of the body receiving the larger exposure. The operator survived for 2 days. Two people in the control room received doses of about 15 rad from neutrons and 20 rad gamma–rays. Six others received lesser doses, down to 1 rad, and 9 received less than 1 rad. 103
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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
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additional reflection afforded by t
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15. Electrostal Machine Building Pl
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ased on a radiation survey, that th
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0.5 m 0.5 m 0.5 m 0.5 m 3.0 m 1.4 m
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The investigation identified severa
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19. Idaho Chemical Processing Plant
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20. Siberian Chemical Combine, 13 D
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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 and 98: Figure 50. Burst rod and several se
Page 99 and 100: On 11 March 1963, the facility chie
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: the room to drain the water out of
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 135 and 136: 1. Mayak Production Association, 15
Page 143 and 144: 9. Siberian Chemical Combine (Tomsk
Page 147 and 148: 13. Siberian Chemical Combine, 2 De
Page 149 and 150: 15. Electrostal Machine Building Pl
Page 153 and 154: 19. Idaho Chemical Processing Plant
Page 155 and 156: 21. Novosibirsk Chemical Concentrat
Page 157 and 158: This report has been reproduced dir
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