Production of LEU fully ceramic microencapsulated fuel for irradiation testing

The Advanced Fuels Campaign of the US DOE Fuel Cycle Research/Development program is currently engaged in production of FCM fuel for irradiation testing in the Advanced Test Reactor (ATR) at Idaho National Laboratory (INL). FCM fuel production is a serial process with 3 major steps namely kernel, TRISO, and compact production. The first step in the kernel process is production of air-dried UO₃ gel spheres with suspended carbon particles. These spheres, roughly 2 mm in size, are then calcined to produce a mixture of UO₂ and carbon. This calcination step may be performed in either argon, hydrogen, or mixtures of these gases. Once the calcination step is complete, temperature is increased (>1400 Celsius degrees) to convert this mixture to UO₂ + UC_x. Once dense oxide-carbide kernels are available, the inert environment is switched to a nitrogen containing one to facilitate carbothermic reduction of kernels and production of UN. Essentially, the oxygen and carbon in the oxide-carbide kernels react with one another to produce CO and are replaced with nitrogen inside the lattice. The TRISO step in fuel production is incorporation of a porous carbon buffer layer, a dense inner pyrolytic carbon layer (IPyC), a SiC layer, and finally and outer pyrolytic carbon layer (OPyC) onto the UN kernels. The coating layer deposition step is carried out inside a graphite furnace using a fluidized bed chemical vapor deposition (CVD) process. In essence, a mixture of reactant and inert gasses flow from the bottom through a bed of particles that are in turn fluidized by the gas stream. The particles undergo CVD and once a different coating layer is to be incorporated, the furnace temperature is adjusted and the reactant gases are changed. The final step in the series of fuel production activities is the consolidation of UN TRISO particles inside the SiC matrix to produce cylindrical pellets. It entails 3 critical areas: SiC powder mixing, over-coating of particles, and high-temperature densification. Optimal execution of the powder mixing step is vital to achieve a fully dense SiC matrix at the end of the process. The SiC matrix is consolidated using a special form of liquid phase sintering (LPS) involving nano-powder feedstock that is designated as the NITE process. The SiC nano-powder slurry is sprayed directly onto the particle surface using a piezoelectric nozzle. Once the over-coated particles are available, final consolidation step at high temperatures (1875 Celsius degrees) is carried out