document: The Magnox reactors represent the first generation of gas-cooled reactors in the UK that used carbon dioxide (CO2) as the primary coolant and a honeycomb network of graphite bricks to provide neutron moderation. During reactor operation significant amounts of carbon monoxide (CO) was produced from the CO2 coolant. This CO in turn can be radiolytically polymerised to form a carbonaceous deposit on free surfaces [12]. This non-graphitic carbon deposit is significantly more chemically reactive to air than the underlying graphite [12,13]. During the lifetime of some Magnox reactors, small quantities of methane gas were injected into the coolant gas to inhibit weight loss of the graphite core due to radiolytic oxidation [14]. Methane (CH4) is a precursor for carbonaceous deposits that form a sacrificial layer protecting the underlying graphite from excessive weight loss [15] and reduction in mechanical strength [16]. It is assumed nitrogen incorporation during deposit formation is the subsequent production route for the high 14C levels observed.