Study on advanced fast reactor; Analysis of burning Characteristics of long-lived FPs

Yamaoka, Mitsuaki*; Kawashima, Masatoshi*

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 ZrH as well as FP loading Pins. In the work carried out in 1994, a study was performed on transmutation of long-lived FPs, assuming that FP target assemblies are loaded in the core region of fast reactors. As a result, it was found that neutron moderation tends to improve transmutation rate whereas it causes to significant power spike in the adjacent core fuel. So as to suppress the power spike, FPs' loading at the outer periphery of the core was suggested. In the present study, an analysis was carried out to transmute FPs in fast reactors, assuming that target assemblies containing ZrH pins and Tc pins are loaded at the outer periphery of the core. The analysis was mainly performed using the continuous energy Monte Carlo code that is effective for rigorous trestment of rcsonance absorption of Tc99 in the target assembly with large heterogeneity. The results are shown below. (1)To increase absorption rate of Tc99, the neutron spectrum where absorption in the resonance region is more than in the thermal region is advantageous. This is because the spectrum helps to suppress absorption by structural material. Therefore, the appropriate loading mass of moderator depends upon the moderating power. (2)The target assembly selected from the survey indudes 19 ZrH pins with large diameter and 36 Tc pins with small diameter. The transmutation rate of Tc99 is about 4-5%/year and the mass is about 15-20kg/year. (3)The effect of loading target assembly upon main core characteristics were analyzed. It was found that the ...