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. Fuel that has been removed
from the reactor will also require some method of removing decay heat if the fuel has been
exposed to a significant neutron flux. Each reactor facility will have its own method of removing
decay heat from both the reactor core and also any irradiated fuel removed from the core.
Calculation of Decay Heat
The amount of decay heat being generated in a fuel assembly at any time after shutdown can be
calculated in two ways. The first way is to calculate the amount of fission products present at
the time of shutdown. This is a fairly detailed process and is dependent upon power history.
For a given type of fuel, the concentrations, decay energies, and half lives of fission products are
known. 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 take into account the fact that there are hundreds of different
radionuclides present in the core, each with its own concentration and decay half-life. It is
possible to make a rough approximation by using a single half-life that represents the overall
decay of the core over a certain period of time. An equation that uses this approximation is
= decay heat generation rate at some time after shutdown
= initial decay heat immediately after shutdown
= amount of time since shutdown
= overall decay half-life of the core