2017 年 16 巻 3 号 p. 107-118
Distributions of the TRU nuclide composition in spent fuels are important for radiation-shielding analyses in their storage and transport. In this research, detailed three-dimensional distributions of 242Cm and 244Cm compositions as the main neutron sources in spent fuel assemblies (BWR 8×8 and 9×9 types) were calculated with the three-dimensional neutron transport and fuel burnup code system. The distributions of 242Cm and 244Cm compositions become heterogeneous as fuel burnup increases. Horizontally, the distributions of both compositions in the fuel assemblies always show peaks at the corner rods. Vertically, in the corner rod, the distributions show peaks on the bottom nodes at the initial burnup, and the peak node gradually moves upward as fuel burnup increases until the fuel assemblies are unloaded. For example, in a 9×9 type fuel assembly at the final burnup (55 MWd/kgU), the horizontal peak ratio of the 244Cm composition was 1.78 at the corner rod and its vertical peak ratio was 1.67 on the upper node. 242Cm and 244Cm compositions mainly contribute to neutron emission rates in spent fuel. Accordingly, it is rational to consider those heterogeneous distributions for the neutron-shielding evaluation of spent fuels of higher burnup.
