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Iodine pit, also called iodine hole and xenon pit, is a temporary disabling of a nuclear reactor due to buildup of short-lived nuclear poisons in the core of a nuclear reactor. The main isotope responsible is xenon-135, mainly produced by natural decay of iodine-135. Iodine-135 is a weak neutron absorber, while xenon-135 is the most powerful known neutron absorber. When xenon-135 builds up in the fuel rods of a reactor, it significantly lowers their reactivity, by absorbing a significant amount of the neutrons which provide the nuclear reaction. The presence of iodine-135 and xenon-135 in the reactor is one of the main reasons for its power fluctuations in reaction to change of control rod positions. The buildup of short-lived fission products acting as nuclear poisons is called reactor poisoning, or xenon poisoning. Buildup of stable or long-lived neutron poisons is called reactor slagging. ==Fission products decay and burnup== One of the common fission products is tellurium-135, which undergoes beta decay with half-life of 19 seconds to iodine-135. Iodine-135 itself is a weak neutron absorber. It builds up in the reactor in the rate proportional to the rate of fission, which is proportional to the reactor thermal power. Iodine-135 undergoes beta decay with half-life of 6.57 hours to xenon-135. The yield of 135Xe for uranium fission is 6.3%; about 95% of xenon-135 originates from decay of iodine-135. 135Xe has a huge cross section for thermal neutrons, 2.6×106 barns, so it acts as a neutron absorber or "poison" that can slow or stop the chain reaction after a period of operation. This was discovered in the earliest nuclear reactors built by the American Manhattan Project for plutonium production. Fortunately the designers had made provisions in the design to increase the reactor's reactivity (the number of neutrons per fission that go on to fission other atoms of nuclear fuel). 135Xe reactor poisoning played a major role in the Chernobyl disaster. Xenon-135 is the most powerful known neutron absorber. Its buildup in the fuel rods significantly lowers reactivity of the reactor core. By a neutron capture, Xe-135 is transformed ("burned") to xenon-136, which is stable and does not significantly absorb neutrons. The burn rate is proportional to the neutron flux, which is proportional to the reactor power; a reactor running on twice the power will have twice the xenon burn rate. Xenon-135 beta-decays with half-life of 9.2 hours to caesium-135; a poisoned core will spontaneously recover after several half-lives. For some reactors, the 135Xe concentration will be equal to its equilibrium concentration at full power. After about 3 days of shutdown, the core can be assumed to be free of 135Xe, without it introducing errors into the reactivity calculations.〔 The increase of the 135Xe concentration during lowering the reactor power can lower the reactivity enough to effectively shut down the reactor. As there are not enough neutrons to offset their absorption by 135Xe, nor to burn the built-up xenon, the reactor has to be kept in shutdown state for 1–2 days until enough of 135Xe decays. The inability of the reactor to be restarted in such state is called xenon precluded start up or dropping into an iodine pit; the duration of this situation is known as xenon dead time, poison outage, or iodine pit depth. Due to the risk of such situations, in the early Soviet nuclear industry, many servicing operations were performed on running reactors, as downtimes longer than an hour led to xenon buildup that could keep the reactor offline for significant time, lower the production of valuable weapon plutonium-239, and cause an investigation by a committee and punishment of the operators. 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「iodine pit」の詳細全文を読む スポンサード リンク
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