Critical Experiments Using an Axially Heterogeneous Assembly and Their Analysis

An experimental study of the heterogeneous liquid-metal fast breeder reactor core has been made using FCA VII-3 assemblies of an axially heterogeneous configuration. The internal blankets inserted in these assemblies differed in composition, arrangement, and thickness. The analysis was carried out using the Japan Atomic Energy Research Institute Fast Set Version II and the diffusion code in R-Z geometry. The S₄ calculation was made to evaluate the transport effects. Comparison between the calculated and experimental results reveals that the eigenvalue and the plutonium sample worth in the core are underestimated for the heterogeneous assembly, although they are well represented for the homogeneous assembly. The ²³⁸U capture rate is underestimated in the internal blanket relative to the core of the heterogeneous assembly. Further study is needed to solve the inconsistent prediction of the sodium-void worth observed between the heterogeneous and homogeneous assemblies.