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

(U0.45Pu0.55)N and (U0.8Pu0.2)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 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. (orig.)