Evolution of irradiated Pile Grade A graphite microstructure under novel molten salt decontamination conditions

Abstract

The irradiated graphite waste stream represents a significant challenge for nuclear power plant decommissioning in the UK, with an estimated 96,000 tonnes of graphite waste arising from the shutdown of the UK's gas-cooled reactors. The removal of activated impurities and fission products from irradiated graphite has been successfully performed previously [1] using an electrochemical molten salt decontamination approach. In this study, the material behaviour and structural changes of the treated nuclear graphite under molten salt treatment conditions have been assessed using multi-technique characterisation (Brunauer–Emmett–Teller surface area, Scanning Electron Microscopy, X-ray Photoelectron Spectroscopy and X-ray Diffraction). This novel research highlights that molten salt treatment leads to changes to the binder and impregnated phases while leaving the filler particles intact under the researched treatment conditions. Significant differences in atomic concentrations of C 1s deconvoluted peaks were observed, suggesting that the mechanism involved both diffusion of pre-adsorbed oxygen and limited chlorination of the surface during the decontamination process. The stability of the lattice parameters and minimal change in crystalline dimensions in molten salt-treated graphite material combined with limited mass loss provides a first-of-a-kind insight into the mechanisms behind graphite decontamination using the electrochemical molten salt approach. The findings support the future potential for wide-scale irradiated graphite treatment to achieve waste volume reduction, minimisation and re-categorisation in line with the waste hierarchy.