IRRADIATION BEHAVIOR OF HIGH BURNUP ThO$sub 2$-4.45 percent UO$sub 2$ FUEL RODS

A series of fuel rods, Th02-4.45$>UO2 pellets clad with type 304 stainless steel, has been successfully irradiated in MTR or ETR process water.The fuel rods achieved burnups that ranged from 37,000 to 96,000 Mwd/MT(U+Th) at linear heat ratings up to 48,000 Btu/hr-ft.Three fuel rods were selected from this series of irradiation tests and given a thorough postirradiation examination.The results of this examination showed that no appreciable swelling occurred in any of the fuel rods examined.The fission-gas release was 14$ for the rod that experienced the highest burnup, which indicates that fission-gas retention in the Th02-U02 system is comparable to other fueled-oxide systems irradiated under similar conditions.No major changes were observed in the microstructural details of the fuel pellets or the cladding after irradiation.Analysis of the fuel byx-ray diffraction showed evidence of lattice strain and that a relatively crystalline structure was maintained.This series of experiments is additional evidence of the stability of Th02-U02 fuels when irradiated to high burnups.