(Th-U)O₂ MOX Fuel Fabrication and Dry Recycling of the Sintered Rejects

Abstract

Thorium is an important nuclear material. Due to its abundance in large quantity in India, our preceptors have envisaged three-stage nuclear power program. Thoria-based fuel is planned to be used in the third stage of nuclear power program. (Th-LEU)O₂ mixed oxide (MOX) is the proposed fuel for AHWR-LEU-300 with UO₂ content varying from 13 to 30 w%. Fabrication of ThO₂-NU (natural uranium) fuel of similar composition with UO₂ varying from 13 to 30% has been carried out in kg scale at Radiometallurgy Division. Conventional powder metallurgical processes like mixing, cold compaction, and sintering were employed for fabrication of ThO₂-UO₂ fuel pellets. Sintered density up to 93–94% TD could be achieved. XRD result on sintered pellet showed single-phase formation. Fuel fabrication process is always associated with generation of green/sintered rejects. Recycling of the sintered reject is important to judiciously utilize the feed material and avoid the accumulation of the waste. As thorium dioxide is a stable compound, recycling of process rejects of thoria-based fuels either by dry or wet route is difficult unlike UO₂-based fuels. The stable oxidation state of thorium oxide limits oxidative-reductive processing of sinter rejects. Dissolution of thoria fuels in nitric acid is also difficult. The presence of UO₂ (13–30 w%) in the ThO₂-UO₂ fuel pellets makes it amenable to oxidative-reductive processing by modifying the fabrication parameters. A process for recycling of the sintered reject has been successfully worked out. This paper discusses the fabrication aspects of Th-NU MOX fuel and dry recycling of process rejects.