Measurement of the Thermal Accommodation Coefficient between Moist Helium and a Stainless Steel Surface

Nuclear fuel assemblies consist of zircaloy tubes filled with radioactive uranium-dioxide pellets that are held together in a uniform configuration. After discharge from a reactor, used nuclear fuel (UNF) assemblies are initially placed into a cooling pool to allow for a decrease in radioactivity and heat generation. They are then transferred to a cask that was lowered into the pool and loaded with the assemblies. The cask is then sealed, lifted out of the pool and drained. Some water may remain in the cask after the draining process. It is important to remove all remaining water to prevent surface corrosion and formation of flammable mixtures of hydrogen and oxygen. Vacuum drying is commonly used to remove remaining water. The low pressures during vacuum drying cause the cladding temperature to considerably increase, due to the temperature-jump effect at the gas-solid interfaces. The temperature-jump is characterized by the thermal accommodation coefficient, which is known for dry helium, but not for water vapor or moist helium. The objective of this work is to design and acquire data from a concentric cylinders experiment that will be used to benchmark computational fluid dynamics simulations of rarefied gas heat transfer through a mixture of water vapor and helium at low pressures during vacuum drying. The thermal accommodation coefficient of moist helium on a stainless steel surface is determined for various volume fractions of water. The experiment consists two long concentric cylinders spaced by a 2 mm gap. The inner cylinder consists of a heater rod inserted into a thick aluminum cylinder covered with a thin stainless steel sheath. The outer cylinder consists of a pressure vessel whose temperature is controlled using a water chiller. A water injection system is designed and constructed to precisely control the amount of water vapor to be injected into the experiment. The injection system allows pure water vapor into the experiment or mixtures of water vapor and helium. The thermal accommodation coefficient is determined by measuring the difference in temperature between the inner and out cylinder using thermocouples.