ATBR—A Thorium Breeder Reactor Concept for Early Induction of Thorium in an Enriched Uranium Reactor

Abstract

A new reactor concept has been proposed for induction of thorium in an enriched uranium reactor. The neutronic characteristics of the fissile and fertile materials have been exploited to arrive at optimal fuel assembly and core configurations. Each fuel assembly consists of an enriched uranium seed zone and a thoria blanket zone. They are in the form of ring-type fuel clusters. The fuel is contained in vertical pressure tubes placed in a hexagonal lattice array in a D2O moderator. Boiling H2O coolant is used. The 235U enrichment is ~5.4%. The thoria rods contain the 233U bred in situ by irradiation of one batch load of mere thoria clusters (without the seed zone) for one fuel cycle in the same reactor. There is no need for external feed enrichment in thoria rods. Additionally, some moveable thoria clusters are used for the purpose of xenon override. The fissile production rate from the fertile material and the consumption rate of fissile inventory is judiciously balanced by the choice of U/Th fuel rod diameter and the number and location of thoria rods in the fuel assembly and in the core. During steady-state operation at rated power level, there is no need for any conventional control maneuvers such as change in soluble boron concentration or control rod movement as a function of burnup. Burnable poison rods are also not required. A very small reactivity fluctuation of ±2 mk in 300 effective full-power days of operation is achieved and can be nearly met by coolant inlet enthalpy changes or moveable thoria clusters. Control is required only for cold shutdown of the reactor. The uranium as well as thoria rods achieve a fairly high burnup of 30 to 35 GWd/tonne at the time of discharge. Since the excess reactivity for hot-full-power operation is nearly zero at all times during the fuel cycle and since the coefficients of reactivity due to temperature and density variations of coolant are nearly zero by design, there is hardly any possibility of severe accidents involving large reactivity excursions.