Modifications in SCDAP code for early phase degradation in a CANDU fuel channel

The existence of horizontal fuel channels surrounded by moderator in a CANada Deuterium Uranium (CANDU) reactor type constitutes the major feature which prevents the direct application to this reactor design of the severe accident physical models developed for PWR/BWRs. During a large loss of coolant accident (LLOCA) with a coincidence of a loss of emergency core cooling (LOECC), particular deformation phenomena take place inside a CANDU fuel channel. Another peculiarity appears at melt relocation in horizontal geometry. Brief discussion of accident phenomena is included.SCDAP/RELAP5 is a widespread and detailed computer code for severe accident analysis that can be adapted to benchmark the CANDU dedicated tools, MAAP4-CANDU and ISAAC. The paper summarizes the applications to this reactor type previously performed by the authors with the standard (not modified) SCDAP/RELAP5 (RELAP/SCDAPSIM/MOD3.4). To obtain a code version able to simulate a CANDU channel during the early phase of a severe accident, two new models are developed inside the SCDAP routines structure. These models concern the metallic melt relocation inside the horizontal fuel channel and fuel bundle slumping.Major features of the original LIQSOL (Liquefaction-flow-Solidification) model in SCDAP are described, together with the assumptions based on experimental evidence that led to the development of the modified model. Also, two sub-models that form the basis of fuel bundle slumping treatment are presented: (a) in-built resizing of hydrodynamic sub-channels inside the fuel channel, and (b) heat transfer at direct contact, fuel to fuel and fuel to shroud. The effects of the new models on the overall performance of the code are tested in several different cases and against the results of the standard RELAP/SCDAPSIM/MOD3.4(bi7). The plans to validate the new models are discussed, as well as some general adaptation directions to different phases of a severe accident in CANDUs, foreseen for SCDAP.