Study on charactelistics of transmutation of long-lived FPs

Yamaoka, Mitsuaki*; not registered

Some of fission products (FPs) in spent fuels have very long half lives as transuranic nuclides. Fast reactors have a potential to transmute these FPs into short-lived ones because of high neutron flux. As neutron capture cross sections of FPs increase in low energy region, one of the effective means to transmute them is to irradiate them using target assemblies that contain pins loaded with neutron moderating material such as ZrHx as well as FP loading Pins. A study has been performed on transmutation of long-lived FPs, Tc99 and I129 in fast reactors usig target assemblies. The Monte Carlo method (MC method) with continuous-energy cross section was mainly employed to analyze rigorously resonance absorption of FPs in target assemblies with large heterogeneity. Specications of target assemblies were investigated with a aim to transmute FPs effectively. The target assemblies were assumed to be located in the core region. Also, comparison of calculation methods was carried out, where results from deterministic methods (diffusion and transport calculations) were compared with those from the MC method. Further, the effect of heterogeneity upon resonance absorption was analyzed. The results are as follows: (1)As the number of FP pins increases with total number of FP pins and ZrHx pins kept constant, transmutation rate decreases monotonously, whereas amount of transmutation decreases after it shows its peak value. The transmutation rate is about 1-2%/year at the case with maximum transmutation amount. (2)Heterogeneity effect upon transmutation rate is very large ; the rate for heterogeneous geometry is 30% smaller than that for homogeneous geometly with the same average composition. This is caused by heterogeneity effect upon resonance absorption. (3)Amount of transmutation vary by about 20-30% with arrangement variation of FP pins. Dispersing FP pins increases transmutation rate. (4)The difference between absorption rates from the MC method and the ...