Criticality and depletion analysis of reprocessed fuel spiked with thorium in a PWR core

Abstract

The aim of this work is to preliminarily evaluate the behaviour of a PWR core based on Brazilian Angra 2 when reprocessed fuel spiked with thorium (TRU) is inserted. The TRU fuel insertion was evaluated and compared with the standard UO2 to assess the possibility of its use in already established PWRs. For the core calculations, the neutronic parameters analysed were the infinite multiplication factor, the fuel evolution during the burnup, reactivity coefficients and the conversion ratio (CR). The core results indicated that the insertion of burnable poison rods is not necessary if reprocessed fuel spiked with thorium is used, because the TRU fuel behave like a neutron absorber. Besides, the conversion ratio obtained was more significant than the standard UO2 fuel, indicating the possibility of extending the burnup. The fuel assemblies without absorbers were then modelled and analysed, considering the kinf evolution and the delayed neutron fraction (DNF) during burnup. The assembly results show that the DNF of the assemblies using reprocessed fuel is smaller than the standard assemblies for all of them, which is due to the 239Pu presence and the 233U production that contribute to the low values obtained for delayed fission neutron fraction. These lower values of DNF suggest that an assembly fuelled with TRU fuel needs to be improved in order to raise the safety of the system. In general, the insertion of the TRU fuel in a PWR system presents improvements in terms of fuel economy and waste reduction, although it brings several neutronic alterations that need to be assessed in depth in order to ensure the safety of the reactor. The core model was based on the Final Safety Analysis Report(FSAR) of the Angra-2 reactor and simulated with SCALE6.0 nuclear code package.