Authors: P. Araya and M. Greiner
2008 ASME Pressure Vessels and Piping Division Conference
Abstract
Spent nuclear fuel assemblies are subjected to vacuum drying operation, when removed from cooling pools. The pressure of the cover gas in casks that contain assemblies is lowered to levels of rarefaction in order to remove any residual water by vaporization. Numerical simulations of an experimental apparatus
consisting of an 8x8 square array of heater rods within an aluminum enclosure are used to model a Boiling Water Reactor (BWR) fuel assembly during a drying operation. The total heat generation rate is varied from 100W to 500W and the enclosure wall temperature is set at 325°C. The void is filled with air and
low pressure simulations are run with three different models. Models that assume continuum and slip flow regimes are compared. To model temperature jump in a slip flow regime, a contact resistance is used on all inner surfaces of the model.
Simulations show that natural convection does not have significant effects on heat transfer when the pressure is lowered to 500 Pa. In general, radiative heat transfer dominates over the conduction in the cover gas at a wall temperature of 325°C. Results show that a model using a total vacuum with only radiative heat transfer at the wall, predicts maximum rod to wall temperature difference as much as 19.1% higher than a model at atmospheric pressure, whereas a model at 500 Pa, that considers a slip flow regime with temperature jump at the wall, predicts temperatures that are 7.0% higher. A low pressure model at 500 Pa without a temperature jump does not result in any significant
increase in temperatures. Temperatures from these simulations will be compared to future experiments at low pressures.
