High Conversion Th-U233 fuel assembly for current generation of PWRs

This paper presents a preliminary design of a high conversion Th-U233 fuel assembly applicable for current generation of Pressurized Water Reactor (PWRs). The considered fuel assembly has a typical 17×17 PWR lattice. However in order to increase the conversion of Th232 to U233, the assembly was subdivided into the two regions called seed and blanket. The central seed region has a higher than blanket U233 content and acts as a neutron source for the peripheral blanket region. The latest acts as a U233 breeder. While the seed fuel pins have a standard dimensions the blanket fuel radius was increased in order to reduce the moderation and to facilitate the resonance neutron absorption in blanket Th232. The U233 content in the seed and blanket regions was optimized to achieve maximal initial to discharged fissile inventory ratio (FIR) taking into account the following constrains: 1) target fuel cycle length of 18 months assuming 3-batch reloading scheme, 2) during the fuel cycle the core k-eff should be higher or equal to 1. In this study the neutronic calculations were performed on the fuel assembly level using the Helios deterministic lattice transport code. The fuel cycle length and the core k-eff were estimated with the help of Non Linear Reactivity Model. The applicability of the Helios code for the analysis of the Th-based high conversion designs was confirmed with the help of continuous-energy Monte-Carlo code Serpent. The results of optimization studies show that for the heterogeneous seed and blanket (SB) fuel assembly the FIR of about 0.94 can be achieved.