Radiocarbon variations in tree rings since 1960 near the Tokai nuclear facility, Japan

Abstract

To evaluate the environmental impact of 14C discharged to the atmosphere from the Tokai nuclear facility during normal operational periods, cores were extracted from an 80-yr-old pine tree that grew close to the Tokai Nuclear Power Plants (Tokai NPP and Tokai-2 NPP) and the Tokai Reprocessing Plant (Tokai RP), and the 14C levels in the annual rings were measured using accelerator mass spectrometry (AMS). The 14C activity ranged from 232 to 403 Bq kg−1 C in the tree rings formed during the period 1959–2013. The activities in the periods 1959–1965 and 2011–2013 represent the background levels, and the activities recorded between 1966 and 2010 are significantly elevated compared with these background levels, indicating uptake by the tree of 14C discharged from the Tokai nuclear facility. The excess 14C activity values of 1.1–70 Bq kg−1 C for the period 1966–2010 are positively correlated with the estimated total annual 14C discharges of 0.019–2.0 TBq from the Tokai NPP, Tokai-2 NPP, and Tokai RP (R2 = 0.72; n = 40). This activity of <70 Bq kg−1 C corresponds to an additional annual effective dose to the local population, via the food ingestion pathway, of less than 3.9 µSv yr−1. These values are negligible compared with the average natural background radioactive dose in the world.