Assessment of the neutron and gamma sources of the spent BWR fuel: Interim report on Task FIN JNT A 1071 of the Finnish support programme to IAEA Safeguards

The neutron and gamma sources of the spent nuclear fuel were assessed by calculating the nuclide inventory of the spent fuel using SAS2H/ORIGEN-S code. The calculations were performed by VTT Energy under Task JNT A 1071 FIN Partial Defect Test on Spent Fuel LWRs. According to the calculation results the neutron emission of the spent LWR fuel is dominantly from spontaneous fission of Cm-244 at cooling times longer than two years. The parameters that affect the Cm-244 concentration at discharge are fuel bundle type, burnup, and in BWR also void fraction. The power history has only a slight influence on buildup of Cm-244. Also the axial distribution of the neutron source can be determined. Spent LWR fuel contains a variety of gamma emitting nuclides. However, only Cs-137 and Cs-134 gamma peaks stand out from the gamma spectrum. Cs-137 is a better indicator of the burnup than Cs-134, but the ratio between the amount of Cs-134 and Cs-137 can be used to estimate the cooling time.