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. About 6 percent of the 200 MeV produced by an average fission is released with delay at some time after the instant of fission. 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. Most of these fission products are highly unstable (radioactive) and undergo further radioactive decays to stabilize itself. Absorption of this radiation in the fuel generates significant amount of heat even when a reactor is shut down.
When a reactor is shut down, fission essentially ceases, but decay energy is still being produced. The energy produced after shutdown is referred to as decay heat. The amount of decay heat production after shutdown is directly influenced by the power history (fission products accumulation) of the reactor prior to shutdown and by the level of fuel burnup (actinidies accumulation – especially in case of spent fuel handling). A reactor operated at full power for 10 days prior to 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, and then builds up slowly as the fission product inventory adjusts to the new power.
The decay heat produced after a reactor shutdown from full power is initially equivalent to about 6 to 7% of the rated thermal power. Since radioactive decay is a random process at the level of single atoms, it is governed by the radioactive decay law. Note that, irradiated nuclear fuel contains a large number of different isotopes that contribute to decay heat, which are all subject to the radioactive decay law. Therefore a model describing decay heat must consider decay heat to be a sum of exponential functions 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 obviously lose its 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 produces less decay heat, but will obviously lose its share more slowly. This decay heat generation rate diminishes to about 1% approximately one hour after shutdown. As was written, the decay comes from the beta and gamma decay of fission products and transuranic elements (+ alpha decay).
- Energy of delayed β– decay. About 6 MeV of fission energy is in the form of kinetic energy of electrons (beta particles). The fission fragments are neutron-rich nuclei and therefore they usually undergo beta decay in order to stabilize itself. Beta particles deposit their energy essentially in the fuel element, within about 1 mm of the fission fragment.
- Energy of delayed gamma decay. Gamma rays usually accompany the beta decay. The gamma rays are well attenuated by high-density and high Z materials. In a reactor core the largest share of the energy will be deposited in the fuel containing uranium dioxide, but a significant share of the energy will be deposited also in the fuel cladding and in the coolant (moderator). The range of gamma rays in a reactor vary according to the initial energy of the gamma ray. It can be stated the most of gammas in a reactor have range from 10cm – 1m.
Classification according to source material:
- Decay of fission products. Decay of fission products is dominating source directly after shutdown and in periods up to years.
- Decay of actinides produced by radiative capture (especially 239Pu). Decay of 239Pu and 240Pu is dominating source from long-term point of view (e.g. units or up to tens of years after shutdown – depending on the fuel burnup). 239Pu itself decays via alpha decay into 235U with half-life of 24 100 years. It significantly depends on the fuel burnup and it determines conditions for spent fuel handling. (See also: Fuel Depletion)
- Decay of activated structural materials by radiative capture. Decay of activated structural materials is minor contributor to overall decay heat.
See also: ANSI/ANS-5.1-2014, Decay Heat Power in Light Water Reactors. American Nuclear Society, 2014.
However, even at these low levels (about 1% after one hour), the amount of heat generated requires the continued removal of heat for an appreciable time after shutdown. Decay heat is a long-term consideration and impacts spent fuel handling, reprocessing, waste management, and reactor safety.
The design of the reactor must allow for the removal of this decay heat from the core by some means. If adequate heat removal is not available, decay heat 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.