Sol Gel Microsphere Pelletisation of UO2 and UO2-PuO2 Pellets of PHWR Fuel Specifications Using Internal Gelation Process

Abstract

Sol-gel microsphere pelletisation (SGMP) process, adapting the ammonia internal gelation route for preparation of gel microspheres from heavy metal nitrate solution, has been developed for fabrication of high density (≥96% T.D.) UO₂and UO₂-≤4%PuO₂ pellets of PHWR fuel specifications. "Porous" microspheres of UO₂ and (U,Pu)O₂ were found to be suitable for direct pelletisation. These were prepared by admixing carbon black "pore former" to the broth prior to gelation and later removing the same by controlled air-calcination. "Porous" microspheres crushed easily and lost their individual identity during pelletisation, sintered to high density pellets and led to uniformly distributed "closed" pores in the diameter range of 2-5 micron in sintered pellets which contributed to improved thermal conductivity. TiO₂ dopant in oxide microspheres led to sintered pellets of large grains (~50 μm).