Neutron and Gamma Ray Source Evaluation of LWR High Burn-up UO2 and MOX Spent Fuels

Abstract

The axial neutron emission and gamma ray source distribution were measured for LWR high burn-up UO2 and MOX spent fuel rods. The gamma rays of 134Cs, 137Cs and 106Ru were measured on the fuel rods, and consequently compared with the results of the ORIGEN2/82 calculation, in which both the original library and ORLIBJ32 based on the JENDL-3.2 library were used. The effect of pellet radial gamma distribution on self-shielding factor was considered, and cesium radial migration was also discussed using the ratio of gamma ray intensity of 134Cs to 137Cs. The axial gamma ray measurement and calculation of 134Cs and 106Ru agreed within about 20% and 137Cs agreed within 13% after correction with pellet self-shielding factors. Two types of boundary curves associated with cesium migration were obtained by simple analysis using 134Cs/137Cs gamma intensity ratio. The axial neutron emission calculated by ORIGEN2/82 with ORLIBJ32 was generally smaller than the measured ones. The main neutron source in spent fuel is the spontaneous fission of 244Cm. Small buildup of 244Cm caused the underestimation of neutron emission. The main flow up to 244Cm is 241 Pu, 242Pu, 243Pu, 243 Am and 2440Am. The cross sections of 243 Am on the main flow and 244Cm have strong sensitivity on buildup of 244Cm. The re-evaluation of the cross sections of these nuclides will improve the prediction for 244Cm buildup.