{"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":"
<\/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 <\/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 <\/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 Classification according to the source material<\/strong>:<\/p>\n See also: ANSI\/ANS-5.1-2014, Decay Heat Power in Light Water Reactors. American Nuclear Society, 2014.<\/p>\n However, even at these low levels (about 1%<\/strong> after one hour), the amount of heat generated requires the continued removal of heat for an appreciable time after shutdown. Decay heat<\/strong> is a long-term consideration and impacts spent fuel handling, reprocessing, waste management, and reactor safety.<\/p>\n The reactor’s design must allow for the removal of this decay heat<\/strong> from the core by some means. If adequate heat removal is not available, decay heat<\/strong> will increase the temperatures in the core to the point that fuel melting and core damage will occur, as in the case of Three Mile Island and Fukushima. The degree of concern with decay heat will vary according to reactor type and design. Therefore, there is little concern about core temperature due to decay heat for low power, pool-type reactors.<\/p><\/div><\/div> Methods for removing decay or residual heat from a reactor core can be grouped into two general categories:<\/p>\n The amount of decay heat being generated in a fuel assembly at any time after shutdown can be exactly calculated by the determination of the number of fission products present at the time of shutdown. This is a fairly detailed process and is dependent upon power history<\/strong>. The concentrations, decay energies, and half-lives of fission products are known for a given type of fuel. By starting from a known value, based on power history at shutdown, the decay heat generation rate can be calculated for any time after shutdown. An exact solution must consider the fact that hundreds<\/strong> of different radionuclides<\/strong> are present in the core, each with its concentration and decay half-life. For this purpose, the SCALE\/TRITON code can be used for depletion calculations. The SCALE\/ORIGEN-ARP code can be used to calculate decay heat rates at specific initial fuel composition and discharge burnup levels.<\/p>\n See also: Brian J. Ade, Ian C. Gauld. Decay Heat Calculations for PWR and BWR Assemblies Fueled with Uranium and Plutonium Mixed Oxide Fuel Using Scale, \u00a0ORNL\/TM-2011\/290, OAK RIDGE NATIONAL LABORATORY, 2011.<\/p>\n It is also possible to make a rough approximation<\/strong> by using a single half-life that represents the overall decay of the core over a certain period. An equation that uses this approximation is the Wigner-Way formula<\/strong>:<\/p>\n\n
\n
Decay Heat Removal<\/h2>\n
\n
\n
\n
Calculation of Decay Heat – Wigner-Way formula<\/h2>\n