Sol-Gel Microsphere Pelletization Process for Fabrication of High-Density ThO₂—2% UO₂ Fuel for Advanced Pressurized Heavy Water Reactors

High-density ThO₂-2% UO₂ (²³³U) pellets are a strong candidate fuel for advanced pressurized heavy water reactors. A process flow sheet based on sol-gel microsphere pelletization has been developed for the first time for fabrication of high-density mixed-oxide pellets at relatively low compaction pressures (350 MPa) and low sintering temperatures (1773 K). The process avoids handling or generation of radioactive dusts and is suitable for remote fabrication of highly radiotoxic ²³³U-bearing oxide fuels. The external gelation of thorium process of the Jülich Nuclear Research Center has been used with three major modifications for preparation of ThO₂− UO₂ sol-gel microspheres, suitable for pelletization and sintering. First, a feed solution of lower molarity is used. Second, ∼1 wt% calcium nitrate is added in the heavy metal nitrate feed solution in order to have ∼0.4% CaO as a “sintering aid” in the subsequent mixed-oxide microspheres. Third, ∼30 g/ℓ carbon black are added in the sol prior to gelation. The pores formed in the sol-gel microspheres after burning off the carbon black particles reduce the crushing strength of the microspheres and facilitate pelletization. The sintered pellets thus prepared have high densities and uniformly distributed pores between 2 and 3 µm in size.