Structural response of fast-reactor core subassemblies to thermal loading

The purpose of this analysis is to predict thermal stresses during a subassembly-to-subassembly failure propagation event. During this event the accident subassembly would deposit molten fuel on an adjacent subassembly duct which would set up high thermal stresses along a portion of one flat. In order to determine the structural behavior of a thermally loaded hexcan, the stiffness matrix method of structural mechanics is incorporated into SADCAT, a finite element code developed at Argonne National Laboratory. The thermal loads are determined in the following manner. First, the thermal stresses in each element are computed using the element nodal temperatures and material properties. Next, the local thermal internal nodal loads are determined from the thermal stresses. Then the local thermal loads are assembled into global thermal loads which are then treated as external nodal loads. Since geometric nonlinearity and material nonlinearity (elasto-plastic behavior) are accounted for, the computational procedure followed was an incremental linearization technique with energy balance checks. Results for hexcan displacements and internal stress distributions are presented for both unirradiated and irradiated duct material. A comparison is made between duct wall response as predicted from a plane stress and plane strain two-dimensional analysis and the behavior as predicted by a three-dimensional analysis. It is found that for a given reference temperature and yield strength there is a limit to the maximum temperature difference through the duct wall which can be tolerated by the heated zone before buckling or wrinkling occurred