Analysis of Burnable Poison Effect on Combined (Th- U)O 2 Fuel Cycle Performance in 800 MWt Long-Life PWR

Thorium based fuel especially on thermal reactor has been a great future sight. To produce electricity, thorium should be combined by any fissile material. Many research has been conducted to examine feasibility of thorium fuel. In this present study, burnable poison feasibility on combined thorium-uranium based fuel for long life PWR at thermal power output 800 MWt has been conducted. Fuel of this reactor design has implemented ThO₂ combined by 40 – 60% enriched UO2 in an acceptable enrichment (13-20 % U-235). To reach long life operation time, fuel volume fraction examination varied for 40-60 %. Burnable poison on this design has been evaluated for 0.005-0.06 % Gd₂O₃, 0.5-2 % Pa-231, and 0.5-2 % Np- 237. Gd₂O₃ has very high thermal neutron absorption cross section, small amount of relatively sufficient to reduce excess reactivity, but the effect of this material only work in early burnup step. Np-237 has 190 barns thermal neutron capture cross section which can be used to minimize excess reactivity. Pa-231 has higher thermal neutron capture cross section (280 barns) rather than Np-237 (190 barns), hence the effect of Pa-231 stronger than Np-237 for minimize excess reactivity. In the other side, Pa-231 could compensate fissile isotope in the further burnup step. The optimized design of this study reached 10 years operation time with excess reactivity smaller than 2.5% dk/k.