A new experimental version, RELAP/SCDAPSIM/MOD3.6, developed by Innovative Systems Software to support the analysis of Pressurized Heavy Water Reactors (PHWRs) under severe accident conditions allows the user to develop a very detailed model of the CANDU core. The modeling improvements treat the unique features of fuel channel reactor designs such as the radial and axial power distribution in the core, radial heat transfer between the fuel channels, and the behavior of the fuel channels during the heat-up and boildown of the liquid in the calandria vessel. Up until this version of RELAP/SCDAPSIM severe accident computer codes were not able to model a CANDU Reactor in detail. The detailed core model is based on grouping the 380 fuel channels in a CANDU core, taking into account the four core pass channels in the core, to allow for the flow of the coolant in both directions. The new representation of the core also allows a very detailed modeling of the high power and low power feeders connecting the fuel channels and a more accurate configuration of the feeders as they are connected to the headers. This paper will present 1) a brief description of the model improvements in RELAP/SCDAPSIM that allow the user to more accurately model a CANDU reactor 2) a detailed description the input model, 3) preliminary analytical results using the new model and finally 4) some suggestions for additional improvements and testing.
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