Assessment of airborne 14CO2 pollution from a nuclear power plant with RBMK-type reactors by transfer-factor modelling and tree-ring measurements.

The article provides results of the study on retrospective evaluation of 14C pollution from Ignalina Nuclear Power Plant (INPP). Transfer factor approach was applied coupled with the radiocarbon measurements in Scots pine tree rings of extended network of 9 sampling sites around the INPP within the distance of 1.8 - 6.6 km from the INPP ventilation stacks. The measured 14C activity in tree rings suggested the annual airborne release estimate from the INPP to be in the range 0.43 - 11 TBq depending on the lifetime period of INPP which is in good agreement with other studies. The estimated average annual emission factor ranged from about 1 TBq/GWa in early operational years up to about 3 TBq/GWa after 10 years of operation. The most probable reason for this increase is the accumulation and aging of the graphite stack of INPP RBMK reactors. The last operational decade (1999 - 2009) is characterised by large fluctuations of releases which may be caused by the maintenance and waste management activities at INPP. The trend of last decade of our observations (2009 - 2019) is a continuous decrease of emissions up to 0.5 TBq per year after final shutdown of the reactors in 2009. The study shows high potential of accumulated 14C activity release due to dismantling and decontamination activities during the decommissioning phase of the NPP. The extended sampling site network effectively functioned as an aerial averaging system, reducing the influence of local peculiarities on sampling and modelling. As a result, the overall radiocarbon emissions reconstruction model uncertainty was estimated at approximately of 24%.