STUDY ON THE CRITICALITY SAFETY EVALUATION METHOD FOR BURNUP CREDIT IN JAERI

In relation to burnup credit, thr ee tasks have been carried out at th e Japan Atomic Energy Research Institute (JAERI) for establishing the evaluation method of criticality safety for a spent-fuel system, such as storage ponds and transport casks. The first task is to prepare a b enchmark database of criticality experiments and nuclide compositions of spent fuels. The database of nuclide composition isformed by data m easured at JAERI and data collected from the literature. For the datab ase of criticality experiments, the effective multiplication factor of a spent-fuel assembly has been measured at JAERI. The next task is to develop computer codes. The burnup and criticality codes have been de veloped and validated by analyzing a targe number. of benchmarks store d in the aforementioned database. The last task needed to establish th e methodology in order to confirm the subcriticality of a spent-fuel s ystem applying burnup credit is described. A reference fuel assembly i s introduced so that the criticality of a system can be evaluated by u sing it, instead of modeling all fuel assemblies explicitly. To determ ine the nuclide composition of a spent fuel, a simple method is studie d utilizing a large number ofnuclide composition data stored in the da tabase. Further, the effects of the axial burnup profile and calculati on errors are discussed, and the remaining tasks are identified.