Improved PWR Core Characteristics with Thorium-containing Fuel

Doctoral thesis, 2014

Thorium is an element for possible use as nuclear fuel, because of the fertile property of Th-232, which could convert into U-233 after neutron capture and beta decays. However, thorium based fuels have not been successfully implemented, because of the high cost and the higher complexity compared with once-through uranium fuel. Therefore, the focus of this thesis is on thorium for use in traditional light water reactors for improving core characteristics by: achieving a more homogenous power distribution to provide larger thermal margins, high plutonium incineration efficiency to reduce the plutonium stockpile, and improved load-following capacity. The chosen methodology is similar to the one commonly used in the industry. The model is based on Ringhals-3 PWR, and Studsvik Scandpower Ltd. codes are used to calculate the neutronics of the nuclear fuel and the reactor cores. The thesis is divided into three parts. The first part addresses the fertile absorber concept, which is achieved by adding small amounts of thorium in traditional uranium fuel pellets to improve the thermal margins. The benefits are a reduced need of traditional burnable absorbers, and a slower decrease of reactivity as function of burnup. The second part describes the use of thorium-plutonium mixed oxide fuel to reduce the plutonium stockpile, since thorium barely generates any new plutonium compared with uranium-plutonium mixed oxide fuel. However, the thorium-plutonium mixed oxide cores alter the core behavior compared with the traditional uranium core such as stronger Doppler feedback and lower fraction of delayed neutrons, which require boron (B-10) enrichment, and increased number of control rods with improved efficiency. The third part of the thesis is focusing on improved load-following capacity by thorium-containing fuel. Th-232 has about three times higher thermal neutron absorption cross section compared with U-238, which hardens the neutron spectrum. This leads to less sensitivity to neutron spectrum changes, lower xenon poisoning, and lower axial power fluctuations upon load changes. Therefore, thorium-containing fuel improve the load-following capacity, and could be a positive complement to intermittent power sources, such as solar or wind power.