A Review of Criticality Accidents A Review of Criticality Accidents

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On Friday, 24 July, at approximately 18:00 the operator assigned to work the solvent extraction columns asked his supervisor if it was necessary to wash some of the contaminated TCE. Since the contaminated TCE was to be used for rinsing a process column, he was told that washing the TCE was not necessary. Nevertheless, the operator proceeded to locate a bottle of TCE with the intention of washing it, perhaps only to obtain an empty bottle. Unfortunately, the operator mistook one of the bottles containing the high concentration solution for one containing TCE. The bottle was transported to the stairwell leading to the third floor location of the carbonate makeup vessel by cart, and then hand carried the rest of the way. The bottle’s label, which correctly characterized the contents as high concentration solution, was found after the accident on the floor near the cart. After arriving at the third floor, the operator poured the contents of the bottle into the makeup vessel already containing 41 l of sodium carbonate solution that was being agitated by a stirrer. The critical state was reached when nearly all of the uranium had been transferred. The excursion (1.0 to 1.1 × 10 17 fissions) created a flash of light, splashed about 20% of the solution out of the vessel and onto the ceiling, walls and operator. The operator who fell to the floor, regained his footing and ran from the area to an emergency building ~180 m away. An hour and a half after the excursion, the plant superintendent and shift supervisor entered the building with the intent of draining the vessel. When they reached the third floor, the plant superintendent entered the room and approached the carbonate reagent vessel while the supervisor remained behind in the doorway. The superintendent removed the 11 l bottle (still end up in the vessel) and turned off the stirrer. He then exited the room, passing the supervisor and preceding him down the stairs. Unknown to anyone at the time (the alarm was still sounding from the first excursion), the change in geometry, created as the stirrer induced vortex relaxed, apparently added enough reactivity to create a second excursion, or possibly a series of small excursions. The estimated yield of the second excursion was 2 to 3 × 10 16 fissions and no additional solution was ejected from the vessel. The two men proceeded down to the second and first 34 floors and began to drain the tank through remote valves. When the drain line became clogged with precipitate, the superintendent returned to the vessel, restarted the stirrer, and then rejoined the supervisor who was draining the solution into ~4 l bottles on the first floor. That the second excursion had occurred was not realized until dose estimates for the superintendent and supervisor were available. The supervisor received ~100 rad, while the superintendent received ~60 rad. Both doses were much higher than expected and were inconsistent with their reported actions. Only after significant analysis was it realized that the two had been exposed to a second excursion, which most likely occurred just as the superintendent passed the supervisor on the way down the stairs. The radiation dose to the operator as a result of the initial excursion was estimated to be about 10,000 rad. He died 49 hours later. Other persons in the plant received very minor doses. The investigation determined that there had been 2,820 g of uranium in 51 l of solution in the makeup vessel at the time of the first excursion. No physical damage was done to the system, although cleanup of the ejected solution was necessary. The total energy release was equivalent to 1.30 ± 0.25 × 10 17 fissions.
15. Electrostal Machine Building Plant, 3 November 1965 Uranium oxide slurry, U(6.5), in a vacuum system vessel; single excursion; insignificant exposures. This accident occurred in Building 242 that housed a production scale operation for the conversion of uranium hexafluoride to uranium oxide. The plant operated on four 6–hour shifts per day. Between 23 September 1964 and 19 October 1965, the facility had been converting 2% enriched material. However, because of the need to provide fuel for two newly commissioned uranium–graphite power reactors at the Beloyarskaya Nuclear Power Plant, it was necessary to begin processing 6.5% enrichment material. To perform this change over, the process was shutdown on 19 October 1965. During the following three days, the entire system was thoroughly cleaned out. The conversion process was restarted with the higher enriched material on 22 October 1965. The criticality accident occurred 12 days later. Figure 20 is a layout of the Building 242 uranium hexafluoride to uranium oxide conversion system and associated vacuum system. The uranium hexafluoride was burned in a hydrogen–air atmosphere in the conversion hopper. The resulting uranium oxides were collected at the bottom of the conversion hopper and then transferred by vacuum to the accumulation hopper. The vacuum system was then switched off and the oxides were loaded into geometrically favorable 20 l vessels by gravity. The vessels were then transferred from Building 242 to another location where the oxides underwent defluorination and complete reduction to UO 2 in a rotating calcination furnace. The vacuum system was located one floor level below the conversion system. To prevent oxide from entering the vacuum system, two filters were located in the vacuum line connecting the accumulation hopper to the (liquid ring) vacuum pump. Both the primary and the secondary filters used Lavsan, a fluorinated plastic material woven into a cloth–like fabric. Written procedures required that personnel on each shift open and visually inspect the secondary filter. This inspection was performed to ascertain the level of oxide accumulation and to look for mechanical defects in the Lavsan itself. If the operator was unable to see through the Lavsan of the secondary filter, in addition to replacing it, he was also required to open and visually inspect the primary filter. Procedures also required personnel on each shift to have a sample of the vacuum system water analyzed for uranium content. Sample assay results were usually available about 1.5 hours after the sample was taken. There was no nondestructive assay equipment in place or routinely used to determine if uranium oxide was accumulating in the vacuum system. The components of the vacuum system were a vacuum pump, a water pump, a vacuum supply vessel (where the accident occurred), and a shell–and–tube heat exchanger. The vacuum supply vessel was a vertical axis, right circular cylinder, with a diameter of 650 mm and a height of 900 mm (~300 l). The vessel was equipped with a water level glass site gauge. On 3 November 1965 at 11:10, the criticality accident alarm system sounded in Building 242. All personnel in Building 242 immediately evacuated. The alarm systems of the adjacent buildings did not activate. The facility’s chief physicist made the first re– entry into Building 242 about 50 minutes after the building had been evacuated. Using a portable gamma– ray detector, he quickly determined that an accident had occurred in the vacuum supply vessel. At this time he recorded a gamma exposure rate of 3.6 R/h at a distance of 1.5 m from the surface of the vessel. Recovery operations, performed by operations personnel under the direction of a health physicist, were conducted in a manner that minimized the likelihood of causing additional excursions. A long rod Conversion Hopper First Floor Ground Floor Primary Filter Vacuum Supply Vessel Heat Exchanger Accumulation Hopper 20 Vessel Secondary Filter Vacuum Line Seal Water Pump Vacuum Pump Figure 20. Layout of UF 6 to uranium oxide conversion equipment and associated vacuum system. 35
Page 1 and 2: LA-13638 Approved for public releas
Page 3 and 4: A Review of Criticality Accidents 2
Page 5 and 6: PREFACE This document is the second
Page 7 and 8: Tom Jones was invaluable in generat
Page 9 and 10: C. OBSERVATIONS AND LESSONS LEARNED
Page 11 and 12: FIGURES 1. Chronology of process cr
Page 13 and 14: 51. MSKS and assembly involved in t
Page 15 and 16: A REVIEW OF CRITICALITY ACCIDENTS A
Page 17 and 18: 3 Black Sea Baltic Sea Obninsk Norw
Page 19 and 20: North Atlantic Ocean Irish Sea Wind
Page 21 and 22: 1. Mayak Production Association, 15
Page 23 and 24: time of the accident. However, the
Page 25 and 26: determined by a radiation control p
Page 27 and 28: consequences of this accident, the
Page 29 and 30: Figure 10. Drum in which the 1958 Y
Page 31 and 32: The entire plutonium process area h
Page 35 and 36: During the next shift, radiation sa
Page 37 and 38: 9. Siberian Chemical Combine, 14 Ju
Page 39 and 40: The accident investigation determin
Page 41 and 42: 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: additional reflection afforded by t
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 and 98: Figure 50. Burst rod and several se
Page 99 and 100:
On 11 March 1963, the facility chie
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13. Chelyabinsk-70, 5 April 1968 57
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14. Aberdeen Proving Ground, 6 Sept
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completed the construction of the l
Page 107 and 108:
C. MODERATED METAL OR OXIDE SYSTEMS
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63,65,66, 67 4. Chalk River Laborat
Page 111 and 112:
7. Centre dÉtudes Nucleaires de Sa
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a technician prescribing the loadin
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the room to drain the water out of
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In the final experiment, the critic
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3. Los Alamos Scientific Laboratory
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III. POWER EXCURSIONS AND QUENCHING
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Figure 64. Energy model computation
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References 1. Stratton, W. R. A Rev
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50. Paxton, H. C. “Godiva Wrecked
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92. Stone, R. S., H. P. Sleeper, Jr
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criticality accident: The release o
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prompt criticality: State of a fiss
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1. Mayak Production Association, 15
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Page 139 and 140:
5. Los Alamos Scientific Laboratory
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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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