Design and Operation of Used Nuclear Fuel Drying System - 23421

Used nuclear fuel has been stored in in-ground storage structures since the early 1960's at Canadian Nuclear Laboratories in Chalk River, Ontario, Canada. One storage location has been susceptible to surface water ingress over an extended period of time. The fuel, natural uranium metal clad in aluminum, is susceptible to corrosion in this environment, and has led to a significant degradation of the fuel to the point where approximately 70% of the uranium metal fuel has oxidized. This has resulted in the accumulation of fuel debris within the storage structure along with a large quantity of contaminated water. Water has been pumped out on two previous occasions however a small amount of water remains at the bottom of the storage structure. To mitigate any potential environmental release of radioactive contaminants from the storage structure, a closed loop evaporative drying system using argon as the carrier gas was developed. The system was put into operation on 2022 July 25 and has successfully removed the bulk water from the tile hole with no release of contamination to the environment. The drying system consists of a closed loop argon recirculation system employing a positive displacement blower for process flow, a dehumidifier to condense and remove moisture, High Efficiency Particulate Air (HEPA) filters to remove any contaminants that become entrained with the carrier gas, compressed argon supply, and an oxygen monitoring system to measure the oxygen concentration in the argon carrier gas. The system is connected to the in-ground storage structure through a custom interface and the system is leak checked to ensure minimal argon leakage. Electrical resistance heaters heat the argon gas prior to entering the in-ground storage structure. The heaters were set to 95 deg. C, as this was found to provide an optimal evaporation rate. Argon was chosen to mitigate the risk of a hydrogen explosion. Oxygen sensors continuously monitor the oxygen concentration in the system, and will trigger shutdown of the system should the oxygen concentration exceed 4% in argon. Initial operation of the drying system consisted of daily startup and shut down. The typical volume of water collected was in the range of 300 to 500 mL per 10 hour day shift. To optimize the drying phase, the drying system was operated 24 hours per day on a 5 day schedule. Initial water removal rate was 1 to 1.25 L of condensate per 12 hour period. As the drying progressed, the rate of water removal dropped off to a steady state of 100 mL or less per 12 hour shift as the volume of water remaining decreased. (authors)