The release of carbon-14 from irradiated PGA graphite by thermal treatment in air

Abstract

The United Kingdom Magnox reactors have all ceased generation and await decommissioning. This fleet of reactors contains 50,000–60,000 tonnes of irradiated graphite categorised as Intermediate Level Waste, which will require long-term storage/disposal either in a proposed Geological Disposal Facility (England and Wales) or in an on-site near surface storage facility (Scotland). Carbon-14 (C-14) is one of the principal long-lived radionuclides in the graphite that determines this waste categorisation. Graphite from some Magnox reactors is known to contain carbonaceous surface deposits. Recent studies have shown that the C-14 activities (expressed as Bq per gramme of material) in these deposits are significantly higher compared to those in the underlying graphite. The mechanism for this phenomenon has not yet been explained but the adsorption of the C-14 pre-cursor nitrogen points to a possible production pathway. Such C-14-rich deposits could potentially lead to some alleviation on waste categorisation through their removal by thermal treatment in air. Historical aqueous leaching studies have further shown that there is a small but relatively rapid initial release of C-14 from irradiated graphite, leaving a much more significant non-leachable fraction. If this mobile C-14 fraction were associated with carbonaceous deposits or even graphite surfaces that had been exposed to nitrogen, thermal treatment in air could render the material more radiologically inert even if removal provided no alleviation on waste categorisation, thereby benefiting packaging and storage options. The study presented here broadens previous thermal treatment investigations of C-14 in carbonaceous deposits to a larger number of core samples from two Magnox reactors with differing operational histories. Thermal treatment in air has also been used to investigate C-14 distributions in the underlying graphite. In addition, a test experiment is presented to investigate any possible link between the mobile fraction of C-14 observed in leaching studies and C-14-rich carbonaceous deposits.