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A criticality accident is an uncontrolled nuclear chain reaction. It is sometimes referred to as a critical excursion or a critical power excursion and represents the unintentional assembly of a critical mass of a given fissile material, such as enriched uranium or plutonium, in an unprotected environment. A critical or supercritical fission reaction (one that is sustained in power or increasing in power) generally only occurs inside reactor cores and occasionally within test environments; a criticality accident occurs when the same reaction is achieved unintentionally and in an unsafe environment. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed is still incapable of producing a nuclear detonation of the type seen in fission bombs, as the reaction lacks the many engineering elements that are necessary to induce explosive supercriticality. The heat released by the nuclear reaction will typically cause the fissile material to expand, so that the nuclear reaction becomes subcritical again within a few seconds. In the history of atomic power development, 60 criticality accidents have occurred, including 22 in collections of fissile materials located in process environments outside of a nuclear reactor or critical experiments assembly. Although process accidents occurring outside of reactors are characterized by a large release of radiation, the release is localized and has caused fatal radiation exposure only to persons very near to the event (less than 1 m), resulting in 14 fatalities. No criticality accidents have resulted in nuclear explosions. == Cause == Criticality occurs when sufficient fissile material (a "critical mass") is in one place such that each fission of an atom of the material, on average, produces a neutron that in turn strikes another atom causing another fission; this causes the chain reaction to become self-sustaining within the mass of material. Criticality can be achieved by using metallic uranium or plutonium or by mixing compounds or liquid solutions of these elements. The chain reaction is influenced by parameters noted by the acronym MAGIC MERV - for Mass, Absorption, Geometry, Interaction, Concentration, Moderation, Enrichment, Reflection and Volume. The calculations that predict the likelihood of a material going into a critical state can be complex, so both civil and military installations that handle fissile materials employ specially trained personnel to monitor operations and prevent criticality accidents. The calculations that predict the excursion characteristics can also be complex, as this requires knowledge of the likely process upset conditions. The assembly of a critical mass establishes a nuclear chain reaction, resulting in an exponential rate of change in the neutron population over space and time leading to neutron radiation and a neutron flux. This radiation contains both a neutron and gamma ray component and is extremely dangerous to any unprotected nearby life-form. The rate of change of neutron population depends on the neutron generation time, which is characteristic of the neutron population, the state of "criticality", and the fissile medium. A nuclear fission creates approximately 2.5 neutrons per fission event on average.〔Lewis, Elmer E. ''(Fundamentals of Nuclear Reactor Physics ). ''Elsevier, 2008, p. 123. ISBN 978-0-12-370631-7〕 For every 1000 neutrons born in fission, 7 are delayed neutrons which are emitted from the fission product precursors, called ''delayed neutron emitters''. This delayed neutron fraction, on the order of 0.007 for uranium, is crucial for the control of the neutron chain reaction in reactors. It is called one dollar of reactivity. The lifetime of delayed neutrons ranges from fractions of seconds to almost 100 seconds after fission. The neutrons are usually classified in 6 delayed neutron groups.〔 The average neutron lifetime considering delayed neutrons is approximately 0.1 sec, which makes the chain reaction relatively easy to control over time. The remaining 993 prompt neutrons are born very fast, approximately 1 μs after the fission event. Nuclear reactors operate at exact criticality. When at least one dollar of reactivity is added above the exact critical point (the point where neutrons produced is balanced by neutrons lost per generation) then the chain reaction does not rely on delayed neutrons, and the rate of change of neutron population increases exponentially as the time constant is the prompt neutron lifetime. Thus there is a very large increase in neutron population over a very short time frame. Since each fission event contributes approximately 200 MeV per fission, this results in a very large energy burst as a "prompt critical spike". This spike can be easily detected by radiation dosimetry instrumentation and "criticality accident alarm system" detectors that are properly deployed. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「criticality accident」の詳細全文を読む スポンサード リンク
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