Preliminary research on thorium-uranium fuel cycle characteristic in PWR

Physical analysis program, WIMSD5-SN2-CYCLE3D and CASMO3-CYCLE, are established for study on thorium-uranium fuel cycle. Unit one of Daya Bay PWR is used as referenced reactor model. New assembly including 4.2% enrichment U-235 elements and 36 thorium dioxide elements is designed without change of physical dimension and structural materials of assembly and core. Neutronic calculation is done on thorium-based assembly and uranium-based assembly with 3.2% U-235 enrichments. The process of initial fuel cycle to the equilibrium uranium-plutonium fuel cycle on Daya Bay PWR core with 12 months refueling period is simulated. Then with initial condition of equilibrium uranium-plutonium fuel cycle, thorium-based assemblies are placed in the core to replace uranium-based assemblies in batches, and the refueling process from the equilibrium uranium-plutonium fuel cycle to the equilibrium thorium-uranium fuel cycle is simulated and analyzed. The calculation results show that total fissile nuclides' consumption every day of the equilibrium thorium-uranium cycle is decreased by 18.4% than that of the equilibrium uranium-plutonium cycle, and long-life radioactive nuclear waste is reduced. Meanwhile, the cycle length of the equilibrium thorium-uranium cycle is reduced by about 90 EFPD than that of the equilibrium uranium-plutonium cycle, which will shorten the refueling period and increase operating cost. Moreover, it will bring great difficulty of fuel management, safety control and disposal of spent fuel. U235 enrichment is proposed to improve for utilization of thorium in PWR. (author)