EVALUATION OF LIQUID-METAL REACTOR DESIGN OPTIONS FOR REDUCTION OF SODIUM VOID WORTH

Systematic analyses of alternative methods for reducing the sodium void worth for plutonium-fueled liquid-metal reactors (LMRs) have been performed. The focus is on core designs of recent interest in the U.S. LMR program, i.e., designs in the 450- to 1200-MW(thermal) size range that make use of metal alloy fuel. The design alternatives encompass changes in composition and geometry. An internally consistent and comprehensive evaluation is made of the void worth reduction achievable by various methods and of the associated core physics performance trade-offs. The performance penalties (e.g., the reduced breeding efficiency and the increases in burnup reactivity loss and fissile mass requirement) caused by design changes that significantly reduce the void worth are quantified, and the relative merits of each design option are assessed. The results indicate that the penalties in burnup reactivity loss and fissile requirement can be minimized by use of a "tightly coupled" radially heterogeneous configuration of minimum volume consistent with fuel rating limits and by adjusting the core height-to-diameter ratio to a value sufficiently small to yield an acceptable void worth. The reactor breeding ratio penalty, however, is minimized by the use of loosely coupled heterogeneous cores or annular cores with a large central blanket zone. Penalties in core radius and volume can be minimized by core composition changes, specifically by replacing a fraction of the fuel (or steel) with sodium or a moderating material.