Investigation of the Equilibrium Core Characteristics for the Ringhals-3 PWR with Improved Thermal Margins Using Uranium-thorium Fuel

Nowadays, PWR core thermal margins have decreased because of higher enrichment of U-235 in the fuel assemblies to achieve higher discharge burnup, higher core power to improve economical competiveness, and reduction of fast neutron leakage to prevent rapid ageing of the pressure vessel. Thus, a new uranium-thorium-based fuel assembly is proposed for improving the thermal margins. The proposed fuel is simulated for the Swedish Ringhals-3 PWR equilibrium core in a realistic demonstration. The difference between the uranium-thorium and traditional fuel assemblies is the fuel pellets composition. In the uranium-thorium fuel assembly, 7 weight percentage of thorium oxide is added to some of the fuel pellets. In those fuel pellets, the enrichment is kept below 5 % enrichment of U-235 in order to avoid extra certification on transport, management and storage.The two most significant results showed in this paper are the improved pin peak power, and the axial offset in the core. The pin peak power margin could be improved by 75 % by using the uranium-thorium-based fuel compared with the reference core containing traditional fuel assemblies. The maximum axial offset was decreased by 33 % using the uranium-thorium-based fuel compared with the reference core. The improved thermal margins in the core would allow more flexible core designs with less neutron leakage, or could be used for power uprates while guaranteeing sufficient safety margins