A physics study on alternative reflectors in a compact sodium-cooled breed-and-burn fast reactor

The breed-and-burn reactor (B&BR), also known as the Traveling Wave Reactor, is a unique fast reactor concept thatoffers attractive characteristics in terms of core performance, economics, and non-proliferation aspects. The B&BR has the ability tobreed its own fuel and use itin situ to achieve an extremely long life. A previous study of the neutronics of small and compact sodium-cooled B&BR to re-utilize the PWR spent fuel found that several concerns should be solved in order to increase the safety and performance of the B&BR. One of the most concerning safety issues is the relatively large sodium void reactivity, about 7~8. One approach to reduce the sodium void reactivity coefficient is by enhancing the leakagefrom the core. This can be done in two ways: axially or radially. Since the reactor height needs to be sufficiently high to achieve the breed and burn state, it is not an option to increase the axial leakage by reducing the core height. Instead, alternative radial reflector designs are investigated in order to improve the sodium void reactivity and density coefficient of the B&BR. In most of sodium-cooled fast reactors, a tight lattice of steel (HT-9) rods is used as the reflector assembly. In this study, we introduced a lead or Pb and Mg eutectic (LME) alloy as the radial reflector for an improved performance of the B&BR. The liquid Pb and LME are contained in HT-9 clad and a single reflector assembly consists of the Pb-or LME-containing rods. In addition, we also compare anMgO reflector with the Pb or LME reflectors. Impacts of the various reflector materials and reflector configurations are identified in terms of the reactor performance and safety parameters includingthe sodium void reactivity. The calculations were done using the Monte Carlo code McCARD.