Three-Region Core Design for 200-MW(electric) Molten-Salt Reactor with Thorium-Uranium Fuel

In this paper, an improved design for a small molten-salt reactor (MSR) that uses neutron flux flattening, which is referred to as FUJI-U3, is proposed. This reactor is a 200-MW(electric) power reactor, and its core contains graphite (as the moderator) and fuel salt. The fuel salt is composed of ThF₄ as the fertile material, ²³³UF₄ as the fissile material, and LiF-BeF₂ as both the solvent and heat transfer medium. A basic improvement in FUJI-U3 is the introduction of the design concept of a three-region core in order to avoid the replacement of graphite, which is achieved by reducing the maximum neutron flux. Since there is a limit for irradiation growth in graphite, this reduction in the maximum neutron flux contributes to a longer lifetime of the graphite. Based on calculations using the nuclear analysis code SRAC95 and the burnup analysis code ORIGEN2, it is concluded that there is no need to replace the graphite moderator of FUJI-U3 for 30 yr. Further, the chemical-processing interval of the fuel salt is studied for 7.5, 15, and 30 yr. An increase in this time interval will also contribute to reduce maintenance and cost.