Disassembly phase energetics: an examination of the impact of SIMMER models and assumptions

We present an assessment of several aspects of material motion as predicted by the SIMMER code during the disassembly and early transition phase of an unprotected loss of flow transient in a liquid metal cooled advanced reactor. We divide the study into the fuel expansion phase of an energetic dissassembly and into the early fuel motion stage of subassembly scale pin disruption under the forces of fission gas dispersal and sodium vapor flow. In the expansion phase we concentrate mainly on assessing the SIMMER estimates of energy partitioning of the expanding fuel bubble through the upper core and internal structures which may typically be present. Comparison with simple models leads us to conclude that the SIMMER formulation predicts a reasonable energy partition when heat transfer is not taken into account. The modeling of heat transfer is so complex that no general statement about the physical realizability of the SIMMER predicted energy partition due to the effect can now be made. This is particularly true when fuel to sodium heat transfer is assumed. We compared the subassembly scale early fuel motion with that given by the SLUMPY module in the SAS3A code. We find that SLUMPY may greatly overpredict the extent and magnitude of fuel motion in the time scales involved under LOF conditions using the nominal SLUMPY parameters