SOL-GEL MICROSPHERE PELLETIZATION PROCESS FOR FABRICATION OF (U,PU)O-2, (U,PU)C AND (U,PU)N FUEL PELLETS FOR THE PROTOTYPE FAST BREEDER REACTOR IN INDIA

A ''dust-free'' sol-gel microsphere pelletisation (SGMP) process has b een developed for fabrication of (U,Pu)O-2, (U,Pu)C and (U,Pu)N fuel p ellets containing around 15% plutonium for the forthcoming prototype f ast breeder reactor (PFBR) in India. The objective: was to produce hom ogeneous sintered pellets of similar to 85% T.D. with a predominantly open-pore structure. Hydrated gel-microspheres of UO3 + PuO2 and UO3 PuO2 + C have been prepared from nitrate solutions of uranium and plu tonium by the ''ammonia internal gelation'' process, using hexamethyle ne tetramine (HMTA) as an ammonia generator and silicone oil at 90 +/- 1 degrees C as gelation bath. For oxide fuel pellets, the hydrated UO 3 + PuO2 gel-microspheres were calcined at around 700 degrees C in Ar + 8% H-2 atmosphere to produce ''non-porous'', ''free-flowing'' and co arse (around 400 micron) microspheres which could be directly pelletis ed at 550 MPa to green pellets. The mixed oxide pellets were subjected either to low temperature (similar to 1100 degrees C) oxidative sinte ring (LTS) in N-2 + air containing similar to 1500 ppm O-2 or to high temperature (1650 degrees C) sintering (HTS) in Ar + 8% H-2 For monoca rbide and mononitride pellets, hydrated gel-microspheres of UO3 + PuO2 + C were subjected to carbothermic synthesis in vacuum (similar to 1 Pa) and flowing nitrogen (flow rate: 1.2 m(3)/h) in the temperature ra nge of 1450-1550 degrees C respectively. The monocarbide and mononitri de microspheres thus produced were relatively hard and required higher compaction pressure (similar to 1200 MPa) for making green pellets wh ich could be sintered to 85% TD in Ar + 8% H-2 at 1700 degrees C. The sintered oxide, monocarbide and mononitride pellets had a ''blackberry '' ''open'' pore microstructure with fine grain size. The microspheres retained their individual identity in the sintered pellets because du ring sintering densification took place mainly within and not between the microspheres.