The reactivity effects of material rearrangements, simulating conditions in a postulated liquid-metal fast breeder reactor accident, were measured in SNEAK-12A, a single-zone uranium-fueled critical assembly, and calculated using current Kernforschungszentrum Karlsruhe methods and data and, in part, also using the corresponding modules of the SIMMER-II accident analysis system. For all cases investigated, satisfactory agreement between theory and experiment was reached when two-dimensional transport eigenvalue calculations were used. The application of first-order perturbation theory or diffusion theory in a number of cases led to larger discrepancies, particularly when the experiments involved fuel compaction.