Assessment of utilization of thorium in BWRs

Performance characteristics and trends which could affect the incentives for and feasibility of use of thorium-based fuels in BWRs were investigated using preliminary scoping techniques. Benchmark comparisons with selected critical experiments and Monte Carlo calculations were made for simple geometries. Several fuel compositions, including uranium fissile isotopes diluted (''denatured'') with U-238 to reduce fissile enrichment below that suitable for weapon purposes, were evaluated. Emphasis was focused on design approaches which utilize unique BWR characteristics such as increased coolant boiling and nonuniform lattices. Thorium-based fuel material utilization efficiency in the BWR, without reprocessing and uranium recycle, shows no improvement in resource utilization relative to the UO/sub 2/ stowaway recycle. If plutonium recycle were restricted, then a major resource utilization incentive would exist for thorium use in LWRs with denatured uranium recycle and plutonium use in secured energy centers. The characteristics of hypothesized symbiotic systems involving secured plutonium burning sites are illustrated. However, the significant quantity of plutonium produced when ''denatured'' uranium is used raises question as to the nonproliferation effectiveness of the system. A distinctive incentive for thorium in the BWR appears to be its potential for favorable effects on power distribution, reactivity control, and margins for core dynamic response. The most promising designs are those which selectively locate thorium-bearing rods to most effectively utilize the unique heterogeneity of the BWR lattice, which is composed of individually channeled fuel assemblies separated by water spaces. Mixed lattice designs with only a few ThO/sub 2/ rods were found to significantly improve dynamic and control characteristics.