DECAY HEATHeat TransferDECAY HEATDecay heat production is a particular problem associated with nuclearreactors. Even though the reactor is shut down, heat is produced fromthe decay of fission fragments. Limits for each particular reactor areestablished to prevent damage to fuel assemblies due to decay heat.EO 2.7 DEFINE the term decay heat.EO 2.8 Given the operating conditions of a reactor core and thenecessary formulas, CALCULATE the core decay heatgeneration.EO 2.9 DESCRIBE two categories of methods for removingdecay heat from a reactor core.ReactorDecayHeatProductionA problem peculiar to power generation by nuclear reactors is that of decay heat. In fossil fuelfacilities, once the combustion process is halted, there is no further heat generation, and only arelatively 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 resultsin the formation of highly radioactive fission products. These fission products radioactivelydecay at a rate determined by the amount and type of radioactive nuclides present. Someradioactive atoms will decay while the reactor is operating and the energy released by their decaywill be removed from the core along with the heat produced by the fission process. Allradioactive materials that remain in the reactor at the time it is shut down and the fission processhalted will continue to decay and release energy. This release of energy by the decay of fissionproducts is called decay heat.The amount of radioactive materials present in the reactor at the time of shutdown is dependenton the power levels at which the reactor operated and the amount of time spent at those powerlevels. The amount of decay heat is very significant. Typically, the amount of decay heat thatwill be present in the reactor immediately following shutdown will be roughly 7% of the powerlevel that the reactor operated at prior to shutdown. A reactor operating at 1000 MW willproduce 70 MW of decay heat immediately after a shutdown. The amount of decay heatproduced in the reactor will decrease as more and more of the radioactive material decays tosome stable form. Decay heat may decrease to about 2% of the pre-shutdown power level withinthe first hour after shutdown and to 1% within the first day. Decay heat will continue todecrease after the first day, but it will decrease at a much slower rate. Decay heat will besignificant weeks and even months after the reactor is shutdown.HT-02 Page 52 Rev. 0
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