Measurement of Gas Permeability of Irradiated and Virgin Graphite Specimens

Gas permeability describes the rate at which gases can pass through a material under pressure-driven flow. It is a property of interest for graphite components within gas-cooled reactors, indicating the level of permeation of cooling gases within the components and hence the rate of reaction between coolant gases and the graphite. These gas-graphite reactions are responsible for radiolytic oxidation in reactors in the United Kingdom and have the potential for thermal oxidation in high-temperature reactors (HTRs), which can limit reactor lifetimes. It is also important for understanding vulnerabilities in reactors (e.g., the extent of thermal oxidation in HTRs during air-ingress fault scenarios). At the time of writing no specific ASTM standard exists for the measurement of permeability or diffusivity on graphite, but ASTM C781, Standard Practices for Testing Graphite Materials for Gas-Cooled Nuclear Reactor Components, recommends using ASTM C577, Standard Test Method for Permeability of Refractories, with a transport gas of helium for the measurement of permeability. The National Nuclear Laboratory (NNL) has conducted permeability measurements on graphite samples trepanned from both advanced gas-cooled reactor (AGR) and Magnox cores over the past three decades as part of reactor monitoring campaigns. This paper reviews permeability measurements on irradiated and virgin AGR graphite conducted using the “vacuum leak” methodology and a novel method currently in development at the NNL that does not impose the same stringent sealing requirements on samples. The results are used to examine the size dependence of samples, virgin material variation, and the evolution of permeability with radiolytic oxidation.