Validation of neutronic codes for distorted core configurations with the Sneak - 12 critical assemblies

The reactivity effects of material rearrangements, simulating conditions in a postulated liquid metal fast breeder reactor accident, were measured in three different critical assemblies: SNEAK 12A, a single-zone core, fueled with enriched uranium metal plates; SNEAK 12B, which had a central test zone fueled with Pu02U02 rod bundles surrounded by a buffer and a driver zone and finally, SNEAK 12C, which had nearly the same integral compositions as SNEAK 12B but was loaded totally with plates. The reactivity effects were calculated using current Kernforschungszentrum Karlsruhe methods and data and, in part, also using the corresponding modules of the SIMMER-II accident analysis system. For some configurations also a comparison of measured and calculated fission rate distributions was performed. The evaluation yielded similar results for the three assemblies: For most cases investigated satisfactory aggreement between theory and experiment was reached when two-dimensional transport eigenvalue calculations or exact transport perturbation methods were used. As far as larger deviations occurred, transport results generally were on the conservative side. First order transport perturbation theory only worked well in a limited number of cases. Diffusion calculations often led to large discrepancies, particularly when the experiments involved fuel dilution