Mapping of the distribution of 137Cs in Irish Sea sediments

The Sellafield nuclear fuel reprocessing plant released a total of 41 PBq of ¹³⁷Cs into the Irish Sea in the period 1952–1998. The present work integrates a series of surveys of the ¹³⁷Cs distribution in sediment offshore Sellafield which was aimed at obtaining a detailed picture of the patterns of sediment contamination and their time evolution, and an estimate of the ¹³⁷Cs inventory in sediment. The authors applied NaI(Tl)-based underwater -ray spectrometry to record a series of 1800 one-minute spectra along 14 transects totaling 160 Nm. The dominant feature of the contamination pattern obtained from this survey remains a stripe extending northwestwards along the coast from the discharge point, as reported for previous surveys. Steep gradients are observed perpendicular to the coast, with ¹³⁷Cs countrates decreasing to half the recorded maximum over a distance of less than 2000 m.