Feedback effects of deformations of fuel temperature during degraded cooling accidents in CANDUs

During postulated degraded cooling accidents in CANDU reactors, some fuel channels may receive only single-phase steam. The amount of this steam flow is governed by the pressure differential across the fuel channel, as well as the pressure-loss characteristics of the channel flow path. Any deformation of the bundle and the fuel channel components, due to heat-up resulting from inadequate steam cooling, will alter the pressure-loss characteristics. This in turn will affect the subsequent steam flow and, hence, the deformation behaviour of the fuel. Deformations will also affect the normal heat transfer paths available in the fuel channels by establishing contacts among the channel components. They will also affect the fuel temperatures by altering the coolant flow pattern through the fuel bundle. In a deformed bundle, the subchannel flow areas can be significantly reduced, limiting the access of steam to the bundle interior. This paper describes the computer model CHAN-II (MOD6) which was developed to analyse the feedback effects of deformations on fuel temperatures in CANDU fuel channels. Sample results are presented and they show that deformations have the effect of lowering the average fuel temperature in the fuel channel during degraded cooling accidents. (author)