Preparation, characterisation and out-of-pile property evaluation of (U,Pu)N fuel pellets

Abstract

(U_0.45Pu_0.55)N and (U_0.8Pu_0.2)N are being considered in India as advanced alternative fuels for the operating fast breeder test reactor (FBTR) and the forthcoming prototype fast breeder reactor (PFBR). Mixed nitride fuel pellets containing <0.1 wt% each of oxygen and carbon impurities were fabricated by the conventional "powder-pellet" (POP) and the advanced "sol-gel microsphere pelletisation" (SGMP) processes, involving two major steps. First, carbothermic reduction of an oxide-graphite powder mixture (in the form of tablets) or gel-microspheres at 1773-1823 K in N₂ followed by N₂ + H₂ and Ar+H₂ atmospheres. The nitride microspheres could be directly pelletised and sintered to pellets of relatively low density (≤ 85% TD) with an "open" pore structure desirable for LMFBR application. Thermal conductivity and hot hardness of nitride pellets were evaluated up to 1800 and 1500 K respectively. The out-of-pile chemical compatibility experiments of mixed nitride fuel pellets for FBTR with SS 316 cladding at 973 K for 1000 h did not reveal any significant fuel-cladding chemical interaction.