Production of some important nuclides in spent UO2 fuel

The amount of Spent Nuclear Fuel (SNF) generated in the world is increasing over time. Moreover, it is expected to continue to increase in the future. Most of the reactors in operation nowadays are PWRs. These reactors use enriched uranium dioxide (UO2) as fuel. During reactor operation, neutron-induced reactions occur in the fuel, i.e., most importantly fission and capture. Due to the presence of radionuclides a SNF assembly needs to be characterised for its decay heat and neutron and -ray emission for a safe, secure, ecological and economical transport, intermediate storage and final disposal. To avoid too conservative loading schemes, the inventory of strong neutron absorbing nuclides, including Fission Products (FP) and actinides, is also required. In addition, the amounts of some specific nuclides such as 148Nd and 149Sm are important to verify the irradiation history of the assembly. Some of the observables, i.e. decay heat and neutron and -ray emission rate, can be determined by non-destructive analysis methods. However, the decay heat measurements are too long for routine operations. In addition, to determine the inventory of specific nuclides for criticality safety assessments and for verification of the fuel history, theoretical calculations are required. Therefore, a full characterisation of a SNF assembly will rely on theoretical calculations combined with results of NDA measurements. The calculations involve a neutron transport code combined with a nuclide creation and fuel depletion code. The results of such calculations strongly depend on nuclear data, fuel fabrication data and reactor operation and irradiation conditions. However, build-up of some important nuclides can be estimated using analytical approximations. The accuracy of this approach might not be sufficient for reliable SNF characterisation, however it is useful for a qualitative sensitivity analysis. The nuclides studied are the following: • 134Cs which is important for characterisation of SNF due to a significant contribution to decay heat and γ-ray emission for cooling times up to 10 a and 30 a, respectively. • 137Cs which is important for characterisation of SNF due to a significant contribution to decay heat for cooling times up to 100 a and γ-ray emission for cooling times up to 300 a. • 90Sr which is important for characterisation of SNF due to a significant contribution to decay heat for cooling times up to 100 a. • 148Nd which is important for characterisation of SNF as a burnup indicator. In this master's thesis, some components which have an impact on the observables of interest are described. For each nuclide, the analysis includes an outline of its production path, an overview of existing experimental and evaluated nuclear data, and sensitivities of its production to nuclear data and irradiation history. The sensitivity analysis to nuclear data includes cross section data, fission yields and decay data. The sensitivity analysis to irradiation history includes cooling time, burnup and initial 235U enrichment.