A Review of Criticality Accidents A Review of Criticality Accidents

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An excellent glossary of nuclear criticality terms has been available for several years. Rather than reproduce this document in part in an attempt to focus on terminology specific to criticality accidents, we decided to reproduce the entire document in this appendix. In this way it is hoped that this accident report will also serve to further standardize the definitions that practitioners use. The following is a reproduction of Glossary of Nuclear Criticality Terms by Hugh Paxton, LA-11627-MS. 3 For additional reference material cited in this document, please see Reference 94. This is a glossary of terms generally encountered in the literature of nuclear criticality and criticality safety. Terms sometimes misused are emphasized. The following two terms are so basic and so intertwined that they call for special consideration inconsistent with the body of this glossary. Consequently, they are given this introductory position. critical, criticality: Proper use is generally consistent with the following definition from Webster’s New International Dictionary, Second Edition, Unabridged: -ity. A suffix denoting state, condition, quality, or degree, used to form abstract nouns from adjectives, as in acidity, calamity. Thus, “delayed criticality” and “delayed critical state” are equivalent. “Critical” is not used as a noun but may seem so by implying “critical state” in legends of graphs or charts where space is at a premium. When the meaning of “critical” as an adjective may be misinterpreted, as in “critical terms” or “critical accidents,” “criticality” may be substituted for clarification. Use of “a criticality” for “a critical condition” or simply for “criticality,” as is sometimes heard, is unacceptable. See delayed criticality, prompt criticality. albedo, neutron: The probability under specified conditions that a neutron entering into a region through a surface will return through that surface. absorbed dose: The energy imparted to matter by directly or indirectly ionizing radiation per unit mass of irradiated material at the point of interest; unit of absorbed dose has been the rad and now in the International System of Units (SI) is the gray (Gy), 100 rad = 1 Gy. See rad, gray. absorption, neutron: A neutron-induced reaction, including fission, in which the neutron disappears as a free particle. The absorption cross section is 116 APPENDIX A GLOSSARY OF NUCLEAR CRITICALITY TERMS designated σa . See capture, neutron; cross section, neutron. alarm system, criticality accident: A system capable of sounding an audible alarm after detecting neutron or gamma radiation from a criticality accident. See criticality accident. alpha particle: A helium-4 nucleus emitted during a nuclear transformation. beta particle: An electron of either positive or negative charge that has been emitted in a nuclear transformation. buckling: For our purposes, algebraic expressions that relate critical dimensions of various simple shapes (sphere, cylinder, or cuboid) of cores of the same composition and similar reflectors. For example, the known radius of a critical sphere may be used to obtain the radius and length of a corresponding critical cylinder. See core, reflector. burst, prompt: Usually refers to the pulse of energy from fissions produced by a prompt burst reactor. See prompt burst reactor, spike (in a prompt power excursion). capture, neutron: Neutron absorption not leading to fission or other neutron production. The capture cross section is designated σa . See absorption, neutron; cross section, neutron. cent: A unit of reactivity equal to one-hundredth of the increment between delayed criticality and prompt criticality (a dollar). See dollar, reactivity. chain reaction, fission: A sequence of nuclear fission reactions in which fissions are induced by neutrons emerging from preceding fissions. Depending on whether the number of fissions directly induced by neutrons from one fission is on the average less than, equal to, or greater than unity, the chain reaction is convergent (subcritical), self-sustaining (critical), or divergent (supercritical). core: That part of a fissile system containing most or all of the fissile material, as distinguished from an external reflector. See fissile system, reflector. critical infinite cylinder: For specified fissile medium and surrounding reflector, the infinitely long cylinder with a diameter that would be critical. critical infinite slab: For specified fissile medium and reflector on each surface, the slab of infinite lateral dimensions with a thickness that would be critical.
criticality accident: The release of energy as a result of accidentally producing a self-sustaining or divergent fission chain reaction. See reaction, fission. criticality safety: Protection from the consequences of a criticality accident, preferably by prevention of the accident. Encompasses procedures, training, and other precautions in addition to physical protection. See criticality accident. criticality safety standards: These standards describe criticality control practices for which there is industry-wide consensus. Consensus is established through procedures of the American National Standards Institute. cross section (σ), neutron: The proportionality factor that relates the rate of a specified reaction (such as capture or fission) to the product of the number of neutrons per second impinging normally onto a unit area of a thin target and the number of target nuclei per unit area. It may be considered a small area assigned to each target nucleus, usually expressed in barns, i.e., 10-24 cm2 . See absorption, neutron; capture, neutron; fission, nuclear. decay, radioactive: A spontaneous nuclear transformation in which particles or gamma radiation is emitted, in which x-radiation is emitted following orbital electron capture, or in which the nucleus undergoes spontaneous fission. See fission, nuclear; gamma radiation. delayed criticality: State of a fissile system such that keff = 1, the steady-state condition. See multiplication factor. delayed neutrons: Neutrons from nuclei produced by beta decay following fission. They follow fission by intervals of seconds to minutes. See prompt neutrons. dollar: A unit of reactivity equal to the increment between delayed criticality and prompt criticality for a fixed chain-reacting system. See reactivity. dose equivalent: The absorbed dose multiplied by the quality factor and other less significant modifying factors, so that doses from different radiations (alpha, beta, gamma, slow neutron, fast neutron) can be summed to provide an effective total dose at the point of interest. The conventional unit of dose equivalent has been the rem, and now in the International System of Units (SI) is the sievert (Sv), 100 rem = 1 Sv. See rem, sievert. dose rate: Absorbed dose delivered per unit time. See absorbed dose. excursion, nuclear: An episode during which the fission rate of a supercritical system increases, peaks, and then decreases to a low value. excursion, prompt-power: A nuclear excursion as the result of a prompt-critical configuration of fissile material. In general, a sharp power spike followed by a plateau that may be interrupted by smaller spikes. See excursion, nuclear; spike (in a prompt power excursion). excursion period (T): The reciprocal coefficient of t, where fission power in a nuclear excursion increases as et/T before a quenching mechanism becomes effective. See excursion, nuclear; quenching mechanism. exponential column: A subcritical block or cylinder of fissile material with an independent neutron source at one end. Under appropriate conditions, the response of a neutron detector decreases exponentially with distance from the source. From the logarithmic rate of this decrease and lateral dimensions of the column, critical dimensions of an unreflected assembly of the material may be deduced. exposure: A measure of the ionization produced in air by x-rays or gamma radiation; the sum of electric charges on all ions of one sign in a small volume of air when all electrons liberated by photons are completely stopped, per unit mass of the air. Note that exposure refers to the environment, not absorbing material. The unit of exposure is the roentgen. See gamma radiation, roentgen. Alternatively, exposure is the incidence of radiation on living or inanimate material. favorable geometry: Geometric constraint of fissile material in which subcriticality is maintained under anticipated conditions. Examples are limited diameter of pipes intended to contain fissile solution or limited volumes of solution containers. fissile nucleus: A nucleus capable of fission by thermal neutrons, provided the effective neutron production cross section, νσ f exceeds the effective absorption cross section, σ a . The common fissile nuclei are 235 U, 239 Pu, and 233 U. See absorption, neutron; fission, nuclear. fissile system: A system containing 235 U, 239 Pu, or 233 U nuclei and capable of significant neutron multiplication. See fissile nucleus; multiplication, subcritical. fission, nuclear: Disintegration of a nucleus (usually Th, U, Pu, or heavier) into two (rarely more) masses of similar order of magnitude accompanied by a large release of energy and the emission of neutrons. Although some fissions take place spontaneously, neutron-induced fissions are of major interest in criticality safety. The fission cross section is 117
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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
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on a 0.4 m square pitch grid. The b
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competing reactivity effects proved
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Figure 35. The precipitation vessel
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B. PHYSICAL AND NEUTRONIC CHARACTER
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Material Fissile Mass: Fissile mass
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235 U Spherical Critical Mass (kg)
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Table 10. Accident Fission Energy R
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Page 125 and 126: References 1. Stratton, W. R. A Rev
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