Uranium-238 to Uranium-235 Fission-Ratio Distribution in Spherical Lithium-Metal Assemblies With and Without a Graphite Reflector

Spherical lithium-metal assemblies with and without a graphite reflector were prepared by stacking lithium arid graphite blocks to investigate the neu-tronics in the fusion reactor blanket. The ratio of ²³⁸ U-to-²⁸⁵U fission rates was measured by micro -fission chambers. The fission ratio was calculated by the one-dimensional transport code ANISN with the Pg-Sg approximation using a 42-group neutron cross-section set. The fission ratios measured in the lithium-metal assembly without a graphite reflector agree well with the calculated ones. In the assembly with a graphite reflector, however, there is a large discrepancy between experiment and calculation.