Each reactor will have some limits during shutdown that are based upon decay heat
considerations. These limits may vary because of steam generator pressure, core temperature,
or any other parameter that may be related to decay heat generation. Even during refueling
processes, heat removal from expended fuel rods is a controlling factor. For each limit
developed, there is usually some safety device or protective feature established.
Decay Heat Removal
Methods for removing decay heat from a reactor core can be grouped into two general categories.
One category includes methods which circulate fluid through the reactor core in a closed loop,
using some type of heat exchanger to transfer heat out of the system. The other category
includes methods which operate in an open system, drawing in cool fluid from some source and
discharging warmer fluid to some storage area or the environment.
In most reactors, decay heat is normally removed by the same methods used to remove heat
generated by fission during reactor operation. Additionally, many reactors are designed such that
natural circulation between the core and either its normal heat exchanger or an emergency heat
exchanger can remove decay heat. These are examples of the first category of methods for decay
If a reactor design is such that decay heat removal is required for core safety, but accidents are
possible that will make the closed loop heat transfer methods described above unavailable, then
an emergency cooling system of some sort will be included in the reactor design. Generally,
emergency cooling systems consist of some reliable source of water that is injected into the core
at a relatively low temperature. This water will be heated by the decay heat of the core and exit
the reactor via some path where it will either be stored in some structure or released to the
environment. Use of this type of system is almost always less desirable than the use of the
closed loop systems described above.
Students should research systems, limits, and protective features applicable to their own specific