LATTICE CALCULATIONS AND THREE-DIMENSIONAL EFFECTS IN A LASER FUSION-FISSION REACTOR.

Three-dimensional neutronics models of the SOLASE-H fusion-fission reactor have been analyzed by Monte Carlo. In this design, light water reactor (LWR) fertile ThO//2 fuel bundles are enriched in the fissile isotope **2**3**3U and then shipped for burning in the LWRs. A concept where the fertile fuel bundles constitute a lattice configuration with the moderator-multiplier material is investigated. Parametric lattice calculations as a function of the neutron moderator-multiplier to fuel volume ratio ( nu //m/ nu //f) in the lattice show that it is possible in such a concept to enhance the fissile nuclei production density in the fertile fuel, compared to cases where a lattice configuration is not used. This leads to shorter times to attain projected average fissile enrichments, using substantially smaller fuel inventories. Surrounding the whole reactor cavity with the neutron multiplier is found to enhance the fissile breeding in the radial blanket. Severe asymmetries in the spatial distribution of the fissile enrichment are detected and suggest the necessity of elaborate fuel irradiation and management programs in the case of a laser-driven system. The concept of a lattice configuration and the use of the whole solid angle surrounding the fusion source for neutron multiplication are recommended for adoption in future magnetic and inertial confinement fusion-fission hybrid reactor designs.