The use of mosses as indicators of airborne radionuclides near a major nuclear installation

Samples of naturally growing moss (mainly Pseudoscleropodium purum and Hypnum cupressiforme) were used to investigate the distribution of photon-emitting radionuclides, notably 129I, 137Cs, 144Ce and 241Am, at distances up to 12 km from the nuclear fuel reprocessing plant at Sellafield in West Cumbria. The relative activities of the naturally occurring radionuclides, 7Be and 210Pb, in moss and air indicated that the removal half-time of these radionuclides on moss is very long (best estimate of half-time, corrected for radioactive decay, was 240 days) and that the mosses are very efficient collectors of airborne particulates (product of deposition velocity and interception factor was 2.4 × 10 −2 m s −1 ). Hence the activities of artificially-produced radionuclides on the mosses present an effective record of local airborne radionuclide pollution integrated over a number of years. The activity of 210Pb in each sample was used to standardize the activities of artificial radionuclides in the samples. It was found that the activity ratio of 129I/ 210Pb, 137Cs/ 210Pb and 144Ce/ 210Pb declined with distance from Sellafield indicating that the major local source of these radionuclides has been material discharged to air from the site. The activity ratio of 241Am/ 210Pb declined with distance from the site and also with distance from the sea confirming that the resuspension of material originally discharged to sea is an important local source of 241Am.