Neutron Leakage Effects on the Fast Advantage Factor in a Seed-Blanket Assembly

Fast advantage factor measurements and calculations have been performed in a single module seed blanket assembly containing a ThO₂ blanket and a highly enriched uranium seed. Large variations were noted in the advantage factors, which were attributed to leakage effects due to the narrow seed region and the varying fast-neutron source distribution through the core. One-group, two- dimensional Monte Carlo calculations with the MARC program approximating the explicit core geometry are in agreement with the measured results. Calculations were also performed applying the principles of superposition and reciprocity to illustrate the dependence of the fast advantage factor on neutrons originating from various sections of a core.