ACCUMULATION OF GADOLINIUM ISOTOPES IN USED NUCLEAR FUEL

摘要

For the criticality safety control of damaged nuclear fuel, i.e., fuel debris, accounting for burnup credit (BUC) may become important if higher initial-uranium-enrichment fuel almost 5wt% loaded into the reactor. In this case, if the presence of fission products (FP) could be confirmed, the negative reactivity effect of FP allows us to treat a substantially greater quantity of fuel debris in one removal and storage operation. In the technical development of the criticality safety control for fuel debris from the Fukushima accident in Japan, adopting BUC assuming the presence of the FP is being considered because the fuel used in the Fukushima Daiichi Nuclear Power Plant had a higher initial ~(235)U enrichment, i.e., almost 5wt%, than the fuel used at TM1-2. The Expert Group on Burnup Credit Criticality Safety (EGBUC) under the Working Party on Nuclear Criticality Safety (WPNCS) in the OECD/NEA Nuclear Science Committee carried out international burnup calculation benchmarks "Phase-IIIB" and "Phase-IIIC" for BWR fuel assemblies. In these benchmarks, the differences in the ~(155)Gd calculation results of the participants generated keen interest because these differences were rather large. As the coordinators of the Phase-IIIC benchmark, the authors have conducted additional analyses on the accumulation of the gadolinium isotopes in used nuclear fuel during burnup. Without cooling time, the assembly-averaged amount of ~(155)Gd against the burnup value depends on the burnout property of gadolinium in burnable poison rods. This is because the amount of gadolinium isotopes accumulated owing to the fission reaction is less than the residual gadolinium in the burnable poison rods. After the burnout of gadolinium isotopes in the burnable poison rods, the amount of ~(155)Gd isotopes remains almost constant, which is defined by the balance between depletion due to the neutron capture reaction and generation due to beta-decay of ~(155)Eu generated by the fission reaction. However, after a few years of cooling, the amount of ~(155)Gd drasttically increases owing to ~(155)Eu decay. This effect is important for higher burnup fuel. In this case, the amount of gadolinium isotopes in the burnable poison rods is of less importance. This means that for better prediction of ~(l55)Gd, the adopted parameters and data pertaining to ~(155)Eu generation have significantly greater importance than the burnup treatment of the burnable poison rods. The release behavior of europium as well as gadolinium during the severe accident should be examined. This paper provides basic information for considering 155Gd in the criticality safety evaluation of fuel debris.