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Abstract
Fission Products were not considered in conventional fast shielding analyses that were predominantly developed in clean core experiments. However, in power reactors with high burn-up, the accumulation of FP nuclides affects the neutron balance mainly due to the absorption reaction and it cannot be neglected when calculating the neutron flux of a high burn-up reactor. In this study, the lumped group constants of FP nuclides used for a fast reactor shielding calculation were computed with the JENDL-3.2 library and compiled to the JSDJ2/JFTJ2 set. The effect of considering FP nuclides on the neutron flux calculations was evaluated in the JOYO experimental fast reactor. These tests showed conventional calculations that ignored FP nuclides overestimated neutron flux by about 2%. The effect on reaction rate calculation of the spent fuel in IVS (in-vessel storage rack) is a maximum of 10%.