Decay heat production is a particular problem associated with nuclear
reactors. Even though the reactor is shut down, heat is produced from
the decay of fission fragments. Limits for each particular reactor are
established to prevent damage to fuel assemblies due to decay heat.
DEFINE the term decay heat.
Given the operating conditions of a reactor core and the
necessary formulas, CALCULATE the core decay heat
DESCRIBE two categories of methods for removing
decay heat from a reactor core.
Reactor Decay Heat Production
A problem peculiar to power generation by nuclear reactors is that of decay heat. In fossil fuel
facilities, once the combustion process is halted, there is no further heat generation, and only a
relatively small amount of thermal energy is stored in the high temperature of plant components.
In a nuclear facility, the fission of heavy atoms such as isotopes of uranium and plutonium results
in the formation of highly radioactive fission products. These fission products radioactively
decay at a rate determined by the amount and type of radioactive nuclides present. Some
radioactive atoms will decay while the reactor is operating and the energy released by their decay
will be removed from the core along with the heat produced by the fission process. All
radioactive materials that remain in the reactor at the time it is shut down and the fission process
halted will continue to decay and release energy. This release of energy by the decay of fission
products is called decay heat.
The amount of radioactive materials present in the reactor at the time of shutdown is dependent
on the power levels at which the reactor operated and the amount of time spent at those power
levels. The amount of decay heat is very significant. Typically, the amount of decay heat that
will be present in the reactor immediately following shutdown will be roughly 7% of the power
level that the reactor operated at prior to shutdown. A reactor operating at 1000 MW will
produce 70 MW of decay heat immediately after a shutdown. The amount of decay heat
produced in the reactor will decrease as more and more of the radioactive material decays to
some stable form. Decay heat may decrease to about 2% of the pre-shutdown power level within
the first hour after shutdown and to 1% within the first day. Decay heat will continue to
decrease after the first day, but it will decrease at a much slower rate. Decay heat will be
significant weeks and even months after the reactor is shutdown.