DOE OSTI2019
The fast reactor (FR) fuels typically require higher fissile content than the thermal reactor fuels because the cross sections of the fissile isotopes are smaller relative to the fertile isotopes and higher neutron leakage from the reactor core. The medium-sized high-burnup sodium-cooled fast reactors (SFRs) in the framework of uranium-plutonium/transuranic (U/Pu or U/TRU) fuel-cycle can be optimized to operate with less than 17 % of Pu/TRU content in the driver fuel while having higher TRU conversion ratios (CR~1.0) [1]. However, possible shortage or inaccessibility to TRU based fuel could be accommodated by the use of uranium-based fuels on demand such as low enriched uranium (LEU) having significantly higher 235U content than used in the commercial light water reactors (LWRs), which is defined as high-assay LEU. The reactor systems having low TRU conversion ratio are commonly categorized as burner reactors, which include LWRs and even can be achieved by FRs with designed to utilize higher TRU fractions (e.g., increased neutron leakage and lower fuel volume fractions). Such reactor systems need to be initially loaded with high excess reactivity, which decreases toward the end of cycle, in order to achieve target burnup conditions. Therefore, in order to offset the excess reactivity, neutron absorbing materials need to be inserted into the core to compensate (e.g., control rods). By reducing the neutron leakage (e.g., increasing the fuel volume fraction or thicker blankets), the TRU conversion ratio in the FR can be increased. If the TRU conversion ratio becomes 1.0 (technically slightly greater than 1.0 to account for process losses during recycle), the reactor is in the condition known as “breakeven”, in which fuel contains the same amount of fissile material after recycling that was used to fuel the reactor. In such a condition, combined with the fact that fission products do not have strong poisoning effects in FRs, the reactivity swing becomes smaller than that of burner type reactors. For breeders, the reactivity can increase toward the end of the cycle because more fissile material is produced than consumed. In this regard, the breakeven FR core is implicitly optimized in a sense by eliminating the large insertion of control elements to offset the reactivity during the normal operation. However, the breakeven condition here is assumed to be achieved only by the TRU based fuel. Possible lack of TRU based fuel needs to be compensated by available LEU resources containing equivalent amount of fissile (i.e., 235U). The use of LEU will have some impact on important safety parameters that need to be evaluated. U-235 has a higher effective delayed neutron fraction than Pu-239. Also, the number of neutrons produced by fissions per neutron absorption (i.e. ?) of 235U in the fast energy range (>1e4 eV) is lower than that of 239Pu, which may lead to the poor fissile conversion when LEU is used as a high-conversion FR fuel. The full-paper presents the study of the impact of partially loaded LEU fuels within TRU fueled breakeven SFR on the transition of nuclide evolutions and core characteristics particularly focusing on the burnup reactivity swing. The full paper analyzes how the large reactivity swing occurs during the LEU loading and addresses possible options to reduce it to the level equivalent to that of the TRU fueled SFR core.
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗