{"id":18726,"date":"2018-09-23T08:32:49","date_gmt":"2018-09-23T08:32:49","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=18726"},"modified":"2023-02-08T13:03:41","modified_gmt":"2023-02-08T13:03:41","slug":"decay-heat-decay-energy","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/reactor-operation\/residual-heat\/decay-heat-decay-energy\/","title":{"rendered":"Decay Heat – Decay Energy"},"content":{"rendered":"
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\"Energy<\/a><\/p>\n

In a nuclear reactor, the average recoverable energy<\/a><\/strong> per fission is about 200 MeV<\/strong>, being the total energy minus the energy of the energy of antineutrinos<\/a> that are radiated away. About 6 percent of the 200 MeV produced by average fission is released with the delay<\/strong> at some time after the instant of fission. This energy comes from the beta<\/strong> and gamma decay<\/strong> of fission products<\/a> and transuranic elements accumulated in the fuel rather than directly from the fission process itself. Most of these fission products are highly unstable<\/strong> (radioactive) and undergo further radioactive decays<\/a> to stabilize themselves<\/a>. On average, after each U-235 fission, the neutron-rich fission fragments must undergo 6 negative beta decays<\/strong>\u00a0(6 neutrons must decay to 6 protons<\/strong>). Absorption of this radiation in the fuel generates a significant amount of heat even when a reactor is shut down.<\/p>\n

\"Decay<\/a>When a reactor is shut down, fission essentially ceases, but decay energy<\/strong> is still being produced. The energy produced after shutdown is referred to as decay heat<\/strong>. The amount of decay heat production after shutdown is directly influenced by the reactor’s power history<\/strong> (fission products accumulation) prior to shutdown and by the level of fuel burnup<\/strong><\/a> (actinides accumulation – especially in case of spent fuel handling). A reactor operated at full power for 10 days prior to the shutdown has much higher decay heat generation than a reactor operated at low power for the same period. On the other hand, when the reactor changes its power from 50% to 100% of full power, the ratio of decay heat to neutron power drops to roughly half its previous level. It then builds up slowly as the fission product inventory adjusts to the new power.<\/p>\n

\"Decay<\/a>The decay heat produced after a reactor shutdown from full power is initially equivalent to 6 to 7%<\/strong> of the rated thermal power. Since radioactive decay<\/a> is a random process<\/strong> at the level of single atoms, it is governed by the radioactive decay law<\/a>. Note that irradiated nuclear fuel contains many different isotopes that contribute to decay heat<\/strong>, all subject to the radioactive decay law. Therefore a model describing decay heat<\/strong> must consider decay heat to be a sum of exponential functions<\/strong> with different decay constants and initial contribution to the heat rate. Fission fragments with a short half-life are much more radioactive (at the time of production) and contribute significantly to decay heat but will lose their share rapidly. On the other hand, fission fragments and transuranic elements with a long half-life are less radioactive (at the time of production) and produce less decay heat but will lose their share more slowly. This decay heat generation rate diminishes to about\u00a01%<\/strong> approximately one hour<\/strong> after shutdown. The decay comes from the beta<\/a> and gamma decay<\/a> of fission products and transuranic elements (+ alpha decay<\/a>).<\/p>\n