Enrichment of C-14 on Surface Deposits of Oldbury Reactor Graphite Investigated with the Use of Magnetic Sector Secondary Ion Mass Spectrometry

Large quantities of irradiated graphite containing C-14 will arise from the decommissioning of the UK’s Magnox power stations. Magnetic sector secondary ion mass spectrometry (MS-SIMS) has been used to investigate the distribution of this radioisotope within a number of trepanned graphite samples from a variety of radial and axial positions within reactor one at Oldbury, a Magnox nuclear reactor. The methodology used for these determinations ensured that possible mass interferences between C-14 species and oxygen-bearing species were eliminated from the analysis. This work indicates that a carbonaceous deposit found on exposed channel wall face samples has a relative C-14 enrichment compared to the underlying graphite, with concentrations calculated between approximately 5 and 55 ppm. Samples without this deposit gave C-14 concentrations below the limits of detection of the instrument, verifying that the enrichment is due to the deposit. Variations in C-14 concentration with sample height have been shown, with samples lower in the core having an increased C-14 concentration compared with those higher up.