A Validation of the CATHENA Fuel Channel Model for a Post Blowdown Analysis against a High Temperature Thermal-Chemical Experiment for a Ballooned CANDU Fuel Channel

To form a licensing bases for the new methodology of a fuel channel safety analysis code system for CANDU- 6, a CATHENA model of the post-blowdown fuel channel analysis for an aged fuel channel with a crept pressure tube has been developed, and tested for a high temperature thermal-chemical experiment CS28-2. Pursuant to the objective of this study the current study has focused on understanding the involved phenomena such as the radiation and convection heat transfer and high temperature metal-water reaction of the 28- element cluster type fuel bundle in the crept pressure tube, their interrelations, and how the treatment of the psudo-subchannels in the 1-D thermalhydraulic code can affect the prediction with an attempt to properly account for the important physics of the involved phenomena, and how it relates to the experimental results. The transient simulation results for the Fuel Element Simulators (FES) of three fuel rings and the pressure tube were quite encouraging provided some adjustment of the fuel channel annulus gas thermal conductance is used. However this raises a question on how the authors can justify using the adjusted thermal conductance for the CO2 gas gap. Various possible arguments for justifying the obtained results based on an adjusted gap thermal resistance were proposed and discussed. In spite of these difficulties, through this study, it was found that the radiation heat transfer model of CATHENA among FES of three rings and the pressure tube as well as the exothermic metal-water reaction model based on the Urbanic-Heidrick correlation are quite accurate and sound even for the offset cluster fuel bundle of an aged fuel channel