The distribution and selective decontamination of carbon-14 from nuclear graphite

Highlights: • A detailed ¹⁴C release profile from irradiated graphite is constructed. • A reducing gradient of ¹⁴C from gas-facing surfaces is observed and discussed. • The ¹⁴C-enriched surface layer persists deeper than any carbonaceous deposit. • The ¹⁴C release profile is repeatable for a given graphite source and location. This work investigates ¹⁴C release behaviour from UK Magnox reactor sourced irradiated graphite under relatively low temperature oxidation conditions: a 1% oxygen in argon atmosphere at temperatures ranging from 600°C to 700°C, and durations from 4 to 120 hours. A method is used for construction of a detailed ¹⁴C release profile with mass loss. The ¹⁴C release profile is predictable between samples, and an empirical release profile is derived to predict releases to higher mass losses than in this work. An accelerated ¹⁴C release rate at small mass losses is observed, indicative of a ¹⁴C-enriched surface region with a depleting concentration gradient into the material, and the source terms and complexities associated with predicting where the ¹⁴C arose from are discussed. Selective decontamination of a fraction of the ¹⁴C is possible, with the limitation of a reducing efficiency with mass loss and time. This work demonstrates a method for determining the distribution of ¹⁴C between near-surface regions and the bulk material in differing sources of irradiated graphite, and provides data in support of making an informed assessment for the adoption of low temperature thermal treatment and isotopic reduction of irradiated graphite wastes.