Design and performance of coated particle fuels for the thorium cycle high-temperature gas reactor

The reference fuel for the thorium cycle high-temperature gas reactor (HTGR) being developed commercially in the United States utilizes separate fissile and fertile coated particles, which are blended together and bonded into fuel rods by means of carbonaceous matrix. Fissile particles have TRISO coatings, while the fertile particles have BISO coatings. The design of both types of coated particles is based on the results of extensive irradiation tests and sophisticated analytical models. A high level of integrity of the coated particles under all normal operating conditions is ensured by careful design of the particles and control of reactor temperatures to minimize radiation-induced mechanical stresses in the particle coatings, and chemical interactions between fuel and fission products and the structural coating layers. The mechanical design of both BISO- and TRISO-coated particles is aided by analytical stress models using as input geometrical factors and basic properties and irradiation behaviour of the coating materials. To minimize the importance of uncertainties in the input data, model calculations are always normalized to the results of irradiation tests on fuel particles. The chemical performance of coated particle fuels is dominated by unidirectional migration of the fuel kernel into the coatings along the temperature gradient. The phenomenon is generally referred to as the amoeba effect. The amoeba effect in carbide fuels is shown to be controlled by thermal diffusion of carbon in the fuel phase, while in oxide fuels it is apparently controlled by gas-phase carbon transport in the accessible void volume within the coated particle. It is demonstrated that, with proper knowledge and control of the mechanical and chemical aspects of performance, a variety of coated particle fuel types may be designed to perform adequately under HTGR conditions. (author)