Heat TransferDECAY HEATThe design of the reactor must allow for the removal of this decay heat from the core by somemeans. If adequate heat removal is not available, decay heat will increase the temperatures inthe core to the point that fuel melting and core damage will occur. Fuel that has been removedfrom the reactor will also require some method of removing decay heat if the fuel has beenexposed to a significant neutron flux. Each reactor facility will have its own method of removingdecay heat from both the reactor core and also any irradiated fuel removed from the core.CalculationofDecayHeatThe amount of decay heat being generated in a fuel assembly at any time after shutdown can becalculated in two ways. The first way is to calculate the amount of fission products present atthe 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 areknown. By starting from a known value, based on power history at shutdown, the decay heatgeneration rate can be calculated for any time after shutdown.An exact solution must take into account the fact that there are hundreds of differentradionuclides present in the core, each with its own concentration and decay half-life. It ispossible to make a rough approximation by using a single half-life that represents the overalldecay of the core over a certain period of time. An equation that uses this approximation isEquation 2-16.(2-16)QQo12timehalf lifewhere:= decay heat generation rate at some time after shutdownQ= initial decay heat immediately after shutdownQotime = amount of time since shutdownhalf-life = overall decay half-life of the coreRev. 0 Page 53 HT-02
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