When a reactor is shut down, fission essentially ceases, but decay energy is still being produced. Absorption of this radiation in the fuel generates significant amount of heat even when a reactor is shut down. On average, after each U-235 fission the neutron-rich fission fragments must undergo 6 negative beta decays ( 6 neutrons must decay to 6 protons). Most of these fission products are highly unstable (radioactive) and undergo further radioactive decays to stabilize itself.
This energy comes from the beta and gamma decay of fission products and transuranic elements accumulated in the fuel rather than directly from the fission process itself. About 6 percent of the 200 MeV produced by an average fission is released with delay at some time after the instant of fission. In a nuclear reactor, the average recoverable energy per fission is about 200 MeV, being the total energy minus the energy of the energy of antineutrinos that are radiated away. In general, sources of these non-linearities that contribute to the residual heat are usually classified into four categories: In this case the thermal power output is not, by no means, proportional to nuclear power. On the other hand the thermal power decreases more slowly, for example, due to decay heat production. In other words, after reactor trip, the nuclear power drops almost immediately, since the chain reaction is stopped almost immediately. Thermal energy sources in power operation of pressurized water reactor The relationship between flux and thermal power, however, is not linear. To understand the processes that are after reactor shutdown, we have to explain the term “ residual heat”.Īt power operation, the fission reaction is responsible for the power generated in a nuclear reactor, and the fission reaction rate is proportional to the neutron flux, so you might expect that thermal power output is proportional to nuclear power. Decay heat is the heat released as a result of radioactive decay of fission products accumulated in the fuel. There is an extra energy source known as decay heat, that contributes to thermal power in power operation, but dominates in shutdown mode.
Why do reactors have to be cooled for long periods, while the chain reaction can be stopped almost immediately? Why can’t we just turn off nuclear reactors? This branch of nuclear engineering is of the highest importance in reactor safety, since it explains the question: In general, sources residual heat are usually classified into four categories: The amount of decay heat production after shutdown is directly given by the power history. Residual heat is the energy produced or transfered in the reactor core after its shutdown when the nuclear chain reaction is stopped.
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