Coupling RELAP5 and Pressure Tube Deformation Models to Analyze the Thermal-Mechanical Phenomena in CANDU Severe Accidents

Abstract

During the channel heat-up phase of postulated severe accidents in CANDU reactors,the pressure tube may balloon or sag into contact with its calandria tube depending on the internal pressure. If the channel is surrounded by the moderator, this contact will establish an effective heat sinkto the moderator.If the channel is uncovered, the fuel channel assemblywill sag or disassemble. These phenomena, which have significant effects on accident progression, however, are not mechanisticallymodeled by most existing severe accident codes. Three channel deformation modelsbased on existing phenomena in literature have been coupled to RELAP5 to provide more robust treatment of the deformation phases of severe accidents. Two coupling methods are used: one usesa Python script to externally couple these models with RELAP5/Mod3.3;in the other method, the models are compiled into RELAP/SCDAPSIM/Mod3.6 as new SCDAP subroutines. The main objectives of this paper areto introduce these models and the coupling methods, and to benchmark them against several PT deformation experiments.