Understanding the formation and behaviour of C-14 in irradiated Magnox graphite

Nuclear power generation in the United Kingdom is based principally on graphite-moderated gas-cooled reactors. One of the principal long-lived radionuclides produced during graphite irradiation is radiocarbon (C-14). Graphite from some Magnox reactors is known to contain carbonaceous surface deposits, which have been shown to contain significantly higher C-14 activity compared to the underlying graphite. This work is part of a multi-year programme designed to inform decisions on graphite waste management by further scientific understanding of C-14 formation and behaviour in core graphite. The aim of this paper was to investigate the effect of the reactor operating conditions on the formation of C-14 in the graphite and carbonaceous deposits. A comparison of C-14 specific activities in regions of graphite core components directly exposed to and shielded from coolant gas indicates that exposure to the coolant does not have a significant effect on the C-14 activity of the graphite and the deposits. C-14 in the graphite appears to be linearly related to irradiation dose. Comparison with activation model predictions shows good agreement with the C-14 activity in the graphite in the fuel channels based upon an assumed nitrogen impurity.