ANALYSIS OF BURNUP CREDIT ON SPENT FUEL TRANSPORT / STORAGE CASKS - ESTIMATION OF REACTIVITY BIAS

Chemical analyses of high bumup U02 (65 GWd/t) and MOX (45 GWd/t) spent fuel pins were carried out. Measured data of nuclides' composition from U234 to Pu242 were used for evaluation ofORIGEN-2/82 code and a nuclear fuel design code (NULIF). Criticality calculations were executed for transport and storage casks for 52 BWR or 21 PWR spent fuel assemblies. The reactivity biases were evaluated for (I) axial and horizontal profiles of burn up, and historical void fraction (BWR), (2) operational histories such as control rod insertion history, BPR insertion history and others, and (3) calculational accuracy of ORIGEN-2/82 on nuclides' composition. This study shows that introduction ofbumup credit bas a large merit in criticality safety analysis of casks, even if these reactivity biases are considered. The concept of equivalent uniform bumup was adapted for the present reactivity bias evaluation and showed the possibility of simplifying the reactivity bias evaluation in bum up credit