Physics studies of small, fast reactors using lead-based coolants

Several recent energy production planning studies have identified a potential market for small nuclear power systems for international deployment. For this application, proliferation and diversion resistance are key reactor design concerns. It is desirable, for example, to make access to the fresh and spent reactor fuel extremely difficult. A variety of design features that support this goal can be envisioned based on long-lived core designs with infrequent refueling. Ideally, the core lifetime could be extended to the reactor lifetime, and the system could be completely sealed. Liquid-metal-cooled fast reactors (LMRs) have several advantages for such applications; for example, the high-power density of LMRs facilitates development of compact reactor configurations that are more easily transported, and the inherent reactivity feedback behavior of small LMRs facilitates a high degree of passive safety. Significant extension of the fuel lifetime requires core power density reduction. Operation at reduced power density facilitates the use of heavy liquid metals as the coolant; at high-power density, the pumping power requirements become excessive. One option explored in recent design studies is the use of lead-based alloys. A variety of low-power-rating [100-MW(electric)] fast reactor core designs were studied to move systematically from conventional compact (high-power density), sodium-cooled designs to derated, long-lived, single-batch, lead-alloy-cooled natural-circulation designs. Based on these trade studies, a derated configuration with a core diameter of ≅2.5 m and an active core height of 2 m was developed. Performance characteristics are compared in Table 1 for compact and derated 100-MW(electric) LMR core designs utilizing both sodium and LBE coolant. Compact core designs for these small power ratings can be quite small: approximately880 ell total active volume. However, to achieve the LBE natural-circulation design with a fuel lifetime of 15 yr with no refueling requires the power density be reduced by a factor of approximately8 (a core volume of 6,815 ell). The use of LBE coolant reduces the core leakage fraction by 2 to 3%, with a corresponding decrease in the required enrichment. The reduced enrichment allows the inclusion of additional fertile material, which significantly reduces the burnup reactivity loss in the long-lived design. The reduced enrichment also implies a higher flux level for the same power density, and therefore, the discharge fluence levels are higher for LBE-cooled designs