The neutronic analysis of heat pipe-cooled traveling wave reactors utilizing natural uranium

Abstract

The traveling wave reactor utilizing natural uranium fuel can achieve enhanced economic performance. This paper presents two novel designs for a heat pipe cooled traveling wave reactor (HPTWR) that incorporates natural uranium. In the initial HPTWR design, the reactor features an ignitor region containing high-assay low-enrichment uranium (HALEU) (20% and 10%) and a breeder region utilizing natural uranium fuel at 0.72% enrichment. The core is configured as a hexagonal prism and consists of 1242 individual UN fuel elements. Each fuel element is a smaller hexagonal prism with a heat pipe channel at its center. The current design employs a lithium and molybdenum/rhenium (Mo/Re) heat pipe. Simulation results indicate that at the beginning of the cycle (BOC), the core reactivity is substantial, with a K_eff value reaching 1.36. The power profile is asymmetric, peaking in the ignition region. After 175 years of operation, when the traveling wave propagates into the breeding region, the power profile assumes a relatively symmetric form. An improved HPTWR design is then proposed, in which the fuel concentration and enrichment of ²³⁵U are adjusted axially in the extended ignition region. This adjustment results in a more symmetric power profile and a significantly lower K_eff of 1.055 at BOC. The initial HPTWR design can achieve a refueling cycle of 350 years with a thermal power output of 100 MW, while the improved HPTWR design can attain a refueling cycle of 335 years with the same breeder region and power level.