Hunting the missing plutonium: intertidal radionuclide inventories in the NE Irish Sea, UK

Only 60-70% of the Pu and Am discharged from the Sellafield into the Irish Sea is accounted for by current inventory estimates. At least some of the remaining 30-40% is related to underestimates in intertidal sediment inventories. In previous work we produced improved estimates of intertidal radionuclide inventories in the NE Irish Sea to a depth of 50 cm; the average depth possible using simple hand coring to obtain sediment samples. New sampling methods, devised to extend inventories to greater depth, provided cores 2-3 times longer than those obtained previously. This was achieved with a 3m vibro-corer deployed from a small all-terrain vehicle. Over 50 cores were obtained from the Solway Firth and Morecambe Bay. All the cores were logged on the BGS gamma core logger. A basal sample from each core was analysed by low background HpGe gamma spectrometry to establish whether contamination was present at depth. Selected sub-samples were also analysed to verify the core logger data. The logger results showed much greater variation than had been seen in previous shorter sediment cores. No detectable 137Cs or 241Am was seen in the basal samples from 7 Solway cores and 12 from Morecambe Bay. The cores suggest that the thickness of sediment contaminated by the Sellafield discharges is quite variable in detail, with significant differences between different areas. In certain relatively near shore settings less than a metre of contaminated sediment is present, while further from the shore there is at least 2-3m. We do not yet have sufficient evidence to define the depth of the contaminated zone in detail The data increase the overall estimate of the known 137Cs and actinide inventories in the tidal flat sediments by a factor of about four. Final inventory figures may still be higher, but await deeper sampling. (author)