Characterisation of airborne uranium and thorium contamination in NorthernEngland through measurement of U, Th and U-235/U-238 in tree bark

Samples of tree bark were collected from four locations in Northern England (a typical rural site, a coal-fired power station, a uranium (isotopic) en richment plant and a nuclear fuel fabrication facility), to assess the natu re and extent of airborne uranium and thorium contamination. The U and Th c oncentrations of bark were determined by inductively coupled plasma mass sp ectrometry after conventional nebulisation of bark digests, whilst measurem ent of U-235/U-238 isotopic ratio utilised high efficiency nebulisation. Ur anium concentrations varied between and within the sites (range, 0.01-12 mu g g(-1)), with maximum values recorded within 1 km of the nuclear fuel fabr ication plant (Springfields). In comparison, the concentration of Th in bar k was low (mean, 0.018 mug g(-1)) at all sites with the exception of the ar ea affected by coal combustion (0.2-0.8 mug g(-1)). The U/Th ratio varied f rom 0.5 to 3900 compared with the average crustal ratio of 0.3. Low values (<2) were recorded at the 'coal' and 'rural' sites whilst Capenhurst and Sp ringfields showed high values indicating the relative magnitude of uranium elevation. Significant enrichment of the natural U-235/U-238 ratio (0.00725 ) was observed near the nuclear installations, in particular, the enrichmen t plant (Capenhurst).