Burnable Poison optimization for Seed-Blanket Cores

The main objective of the Seed and Blanket Units (SBU) core designs is to reduce production of Pu and long-term toxicity of the spent PWR fuel. The SBU concept assumes a heterogeneous seed-blanket fuel assembly, with spatial separation of the U and Th parts of the fuel. In the SBU assembly the Seed fuel is an alloy of 20% enriched metallic Uranium and zircaloy, the Blanket fuel is ThO2 mixed with about 13% of 12.2% enriched UO2. The Uranium is included in the mix in order to increase the BOL power in the Th pins and dilute the bred U233 isotope to avoid proliferation concerns. The 108 Seed fuel rods are located in the central region of the assembly and surrounded by 156 blanket rods. The use of metallic fuel in the Seed enables high density of fissile material. The U-Zr alloy also has a higher thermal conductivity than UO2, although this advantage is partially offset by the low melting temperature of metallic fuels. More importantly, compared with oxide fuel, the radiation induced creep and swelling phenomena are more pronounced in metallic fuel at elevated temperatures due to the loss of its crystallographic state. This loss occurs at a rather low temperature of 6600 C, and in U-Zr alloys at 6160 C. For this reason, reduction of the local pin power peaking in the assembly is particularly important