Microstructure-property relation for thoria-urania SIMFUEL

The average composition of the proposed fuel for an Advanced Heavy Water Reactor (AHWR) is ThO2-3.45%UO2. Studies on simulated high burn-up nuclear fuels (SIMFUEL) attempts to replicate the chemical state of the solid fission products generated in irradiated fuel. SIMFUEL corresponding to average burn up of 28000 and 43000 MWd/Te was fabricated by powder metallurgical route by adding high purity (>99.9%) fission products like SrO, Y2O3, ZrO2, MoO3, BaCO3, La2O3, CeO2, Nd2O3, RuO2, Rh2O3, Pd, Cs2CO3, Pr6O11 and Sm2O3 in ThO2 and UO2 powders based on the ORIGEN-2 CODE. Pellets were pressed and sintered at 1650 deg C for 4 hours in Ar+8% H2 atmosphere. For microstructural analysis, sintered pellet samples were metallographically polished and subsequently thermally etched at 1500 deg C for 4 hours. Microstructural characterization was carried out using a optical microscope and scanning electron microscope. It was observed that simulated fuel has smaller grain size compared to grain size of ThO2+3.45% UO2. Some important thermophysical properties like, thermal diffusivity, thermal conductivity and electrical resistivity as a function of temperature were experimentally measured. It was observed that SIMFUEL has lower thermal diffusivity, thermal conductivity and electrical resistivity as compared to ThO2+3.45% UO2. Degradation of these properties of SIMFUEL has been explained on the basis of their microstructure. (author)