Gamma Decay Heat Distribution in Core: A Known Issue Revisited

Decay heat in fission reactors is almost equally subdivided into two parts, one part due to beta rays and the other due to gamma photons. Beta rays are absorbed practically where they are generated while gamma photons travel some distance in core before being absorbed. The decay power peaking factor is, in fact, affected by this phenomenon of gamma decay heat redistribution. Calculations have been performed by the Monte Carlo MCNP5 computer code on the experimental LOFT reactor and on a larger 1000 MWe PWR using various initial power distributions with variable power peak sharpness (midheight peak width). The results indicate that an average peak energy reduction ratio of 0.82 for gamma (18% peak reduction) can be used with tolerable error up to a midheight width of the produced energy peak (neutron flux shape during operation) of 120 cm. Beyond this value, no peak energy reduction is warranted. This phenomenon of absorbed γ power redistribution in core may be very significant (100 to 150∘K reduction in calculated PCT).