ENERGY RELEASE FROM FISSIONDOE-HDBK-1019/1-93Atomic and Nuclear PhysicsBecause the 10 MeV of neutrino energy shown in Table 7 is not absorbed in the reactor, theaverage value of 200 MeV per fission is still accurate. Note in Table 6 that some fissionneutrons undergo radiative capture and the resultant gamma ray emission provides an additional10 MeV of instantaneous energy, which contributes to the total of 187 MeV instantaneousenergy.All of the energy released, with the exception of the neutrino energy, is ultimately transformedinto heat through a number of processes. The fission fragments, with their high positive chargeand kinetic energy, cause ionization directly as they rip orbital electrons from the surroundingatoms. In this ionization process, kinetic energy is transferred to the surrounding atoms of thefuel material, resulting in an increase in temperature. The beta particles and gamma rays alsogive up their energy through ionization, and the fission neutrons interact and lose their energythrough elastic scattering. Of the 200 MeV released per fission, about seven percent (13 MeV)is released at some time after the instant of fission. When a reactor is shut down, fissionsessentially cease, but energy is still being released from the decay of fission products. The heatproduced by this decay energy is referred to as "decay heat." Although decay energy representsabout seven percent of reactor heat production during reactor operation, once the reactor is shutdown the decay heat production drops off quickly to a small fraction of its value while operating.The decay heat produced is significant, however, and systems must be provided to keep thereactor cool even after shutdown.SummaryThe important information in this chapter is summarized below.Energy Release From Fission Summary Fission products have some general characteristics in common.They generally decay by b- emission.The most common mass numbers are grouped near 95 and 140.The energy released by fission can be calculated based on the difference in massbetween the masses of the reactants before fission and the fission fragments andfission neutrons after fission.Another method to determine the energy released by fission is based on thechange in binding energy per nucleon between the fissile nuclide and the fissionproducts.NP-01Page 62Rev. 0
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