Quantification of excess Carbon-14 specific activity in terrestrial biota in the off-site locations of the PHWR nuclear power plant at Kaiga, India

This is the first detailed study on ¹⁴C activity in the environment surrounding a nuclear facility in India. Samples of food matrices and wild plants from the off-site locations of the PHWR nuclear power plant (NPP) at Kaiga were analysed by liquid scintillation spectrometry, results were validated by accelerator mass spectrometry, and an extensive database (N = 142) was established. The stable isotope ratio of carbon (δ¹³C) in terrestrial plants varied from -33.5 to -23.3 ‰. The maximum excess ¹⁴C activity recorded in terrestrial biota was 44 Bq kg^-1C (19 pMC). About 75 % of the samples exhibited specific activity in the range 228-249 Bq kg^-1C (101-110 pMC). Statistical tests on the ¹⁴C specific activity dataset for 2.3-5, 5-10, and 10-20 km radial zones confirmed that the impact of the operation of the NPP on the environment beyond 5 km is minimal. The study suggests that the ¹⁴C activity released through gaseous effluents from Kaiga NPP is transported to greater distances along the axis of the valley than that predicted by the Gaussian plume model and those reported for other NPP sites worldwide. This is due to the unique topography of the Kaiga valley in which wind flow channelling, strong winds in the valley mouth, and calm wind within the valley due to the blocking effect by hills for the south-westerly wind regime play dominant roles in the transport of gaseous effluents. The ¹⁴C specific activity values at upwind monitoring stations located at >5 km distance from the NPP during the south-westerly wind regime were higher than those observed during the north-easterly wind regime when the same monitoring stations were located on the downwind side. The ingestion dose to the population in the 2.3-5 km radius zone, attributable to the release of ¹⁴C from the NPP, was 0.75 μSv y^-1. This is a negligibly small fraction of the ICRP recommended dose limit of 1000 μSv y^-1 for the public from other than natural sources. The dose due to the natural ¹⁴C activity in the Kaiga region was 12 µSv y^-1, corresponding to the ambient natural activity of 230 Bq kg^-1 C.