Energy source and fluid representation in a structural response code-STRAW

In evaluating the transient response of reactor components such as fuel subassemblies, the interaction of these components with their environment plays a critical role. Preliminary studies indicate that the surrounding coolant and the adjacent subassemblies interact quite strongly with the accident subassembly, so that the response predicted in studies on an isolated accident subassembly is considerably greater than what would actually occur in the environment of the reactor, particularly for loads with rise times of 1 msec or less. Therefore, unless these surroundings are included, in design or safety evaluations, the conclusions drawn are unduly conservative. This paper describes three additional developments in the STRAW (Structural Transient Response of Assembly Wrappers) code for including the effects of surroundings: (1) hydrodynamic finite elements, which are used to represent the plane motion of the fluid surrounding and within the subassemblies; (2) a superimposed superelement which represents the effects of axial flow; (3) an energy source model identical to that used in REXCO which can be used to model molten fuel coolant interactions (MFCI) or other envisioned energy release sources such as cladding-coolant interaction. (Auth.)