What cooling pond sediments can reveal about (14)C in nuclear power plant liquid effluents: Case study Lake Drūkšiai, Ignalina nuclear power plant cooling pond

The vertical distribution of radiocarbon ((14)C) was examined in two bottom sediment cores, taken from Lake Drūkšiai which had been used as a cooling pond for the Ignalina nuclear power plant (INPP) with two RBMK type reactors. The aim of this work was to reconstruct (14)C amounts in the lake ecosystem during an 8-year period after the INPP was closed, as any official monitoring of (14)C in liquid releases from the INPP was not performed. The possibility of comparing the variation of the (14)C specific activity in the corresponding layers of the same period of 3 different cores (one taken in 2013 and two in 2019) revealed the variability of the determined values of liquid radiocarbon discharges from the INPP into the lake. Cores taken in 2019 showed a permament(14)C release rate of 0.76±0.06 GBq/y all eight years after the closure of the INPP. The (14)C release rate established from radiocarbon measurements in both cores did not differ by more than 0.8 GBq/y. However, including data from the core taken several years ago, the estimated radiocarbon release rate values varied within 1.3 GBq/y.