A Review of Criticality Accidents A Review of Criticality Accidents

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6. Los Alamos Scientific Laboratory, 3 February 1954 42,44,47 Lady Godiva reactor; bare 235 U sphere; control rod incorrectly operated; single excursion; insignificant exposures. 7. Los Alamos Scientific Laboratory, 12 February 1957 47,48,49,50,51,52 Lady Godiva reactor; bare 235 U sphere; added reflection; single excursion; insignificant exposures. These two excursions occurred in the Lady Godiva assembly, an unreflected metal reactor fabricated in three principal sections that when assembled formed a sphere. Figure 48 shows Godiva in the scrammed state. The central section was fixed in position by small tubular steel supports, while the upper and lower sections were retractable by means of pneumatic cylinders, thus providing two independent scram mechanisms. The critical mass was about 54 kg of uranium enriched to 93.7% 235 U. It was operated remotely from a distance of 1/4 mile. The first accidental excursion occurred during preparations for a scheduled prompt burst, part of a program to measure the parameters associated with excursions. Normally, a burst was initiated by establishing delayed criticality. This was accomplished by adjusting control rods, lifting the top section to reduce reactivity and allow decay of the neutron population, and lowering the top section into position and rapidly inserting a burst rod worth slightly more than 1 $. A power excursion typically creating about 10 16 fissions in 100 µs followed; in 40 ms the system would be scrammed safely. Because the only source of neutrons was spontaneous fission, it was customary to assemble to an excess reactivity of about 70 ¢ to generate sufficient neutrons to determine the settings for delayed criticality in a reasonable time. This accidental excursion was caused, apparently, because additional reactivity was inserted by error after assembly to 70 ¢, but before a fission chain started. The excursion yield was 5.6 × 10 16 fissions, about six times the yield of the average burst. There was no radiation hazard, contamination, personnel exposures to radiation, or significant damage to the major uranium parts. One piece was slightly warped and required remachining. Several light steel supporting members were bent or broken (Figure 49). The second accidental excursion occurred during preparations for an experiment in which Godiva was to provide a pulse of fast neutrons. Again, the burst occurred during assembly to establish, in this case, a fiducial point at about 80 ¢ excess reactivity. Control rods were to be adjusted on the basis of this period. The extra reactivity is thought to have been contributed 80 by a large mass of graphite and polyethylene that was to be irradiated. This mass had just been moved close to Godiva, and either the change in reflection was underestimated or the material slumped further toward Godiva. The burst yield was 1.2 × 10 17 fissions, about 12 times the standard excursion. The uranium metal was severely oxidized, warped, and apparently had been plastic near the center. The central burst rod was nearly ruptured and, at its center, must have been within 100°C of the uranium melting temperature. Figure 50 shows several of the pieces. External damage was limited to the supporting structure; radioactive contamination consisted of oxide scale; cleanup proceeded rapidly. Repair of Lady Godiva was not practical; therefore construction of Godiva II 59 specifically designed for burst operation, was accelerated. Despite the severity of the excursion, operating personnel received no significant radiation exposures because of the large distance between the reactor and the control room. The behavior of the Godiva system during superprompt critical power excursions is well understood both experimentally and theoretically. 47,51,52 Lady Godiva experienced well over 1,000 safe, controlled bursts. A coupled hydrodynamics-neutronics code describes the behavior of the system adequately. The first excursion (5.6 × 10 16 fissions) must have had a period of 6.4 seconds, equivalent to a reactivity excess over prompt criticality of 15 ¢. The excess reactivity of the larger excursion (1.2 × 10 17 fissions) was 21 ¢ above prompt criticality, corresponding to a period of 4.7 µs. The fission yield of 1.2 × 10 17 in the second accident is equivalent to the energy contained in 1.7 lb of high explosive (HE), but the damage was much less than would have been caused by that quantity of HE. The above mentioned code can predict the fraction of fission energy converted to kinetic energy; in this case, only about 1.4% of the energy, equivalent to 0.024 pounds of HE, was available as kinetic energy to do damage. The damage was consistent with this figure, and it is evident that most of the fission energy was deposited as heat.
Figure 48. The Los Alamos Scientific Laboratory Lady Godiva assembly (unreflected enriched-uranium sphere) in the scrammed configuration. 81
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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,
Page 43 and 44: heater and stirrer were turned off
Page 45 and 46: 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: At the equatorial plane, the two ha
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 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
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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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