Abstract
Pile Grade A graphite was used as a moderator and reflector material in the first generation of UK Magnox nuclear power reactors. As all of these reactors are now shut down there is a need to examine the concentration and distribution of long lived radioisotopes, such as 14C, to aid in understanding their behaviour in a geological disposal facility. A selection of irradiated graphite samples from Oldbury reactor one were examined where it was observed that Raman spectroscopy can distinguish between underlying graphite and a surface deposit found on exposed channel wall surfaces. The concentration of 14C in this deposit was examined by sequentially oxidising the graphite samples in air at low temperatures (450°C and 600°C) to remove the deposit and then the underlying graphite. The gases produced were captured in a series of bubbler solutions that were analysed using liquid scintillation counting. It was observed that the surface deposit was relatively enriched with 14C, with samples originating lower in the reactor exhibiting a higher concentration of 14C. Oxidation at 600°C showed that the remaining graphite material consisted of two fractions of 14C, a surface associated fraction and a graphite lattice associated fraction. The results presented correlate well with previous studies on irradiated graphite that suggest there are up to three fractions of 14C; a readily releasable fraction (corresponding to that removed by oxidation at 450°C in this study), a slowly releasable fraction (removed early at 600°C in this study), and an unreleasable fraction (removed later at 600°C in this study).
Highlights
Graphite was used as a moderator and reflector material in the UK Magnox gas-cooled reactors due to its low capture and high scattering cross sections [1, 2]
Fast neutron bombardment during operational lifetime resulted in the formation of several activation products [3] and a suitable radioactive waste management strategy is required for the safe disposal of graphite waste [1]
Raman spectroscopy has been widely used to examine the properties of nuclear graphite [30, 31]
Summary
Graphite was used as a moderator and reflector material in the UK Magnox gas-cooled reactors due to its low capture and high scattering cross sections [1, 2]. Fast neutron bombardment during operational lifetime resulted in the formation of several activation products [3] and a suitable radioactive waste management strategy is required for the safe disposal of graphite waste [1]. In the United Kingdom approximately 45,000 m3 of Pile Grade A (PGA) graphite waste will be generated during the decommissioning of the first generation of gascooled, graphite-moderated Magnox reactors that requires disposal [4, 5]. Examination of Oldbury Reactor Graphite to Determine the Concentration and Distribution of 14C
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