Modeling of 14C Vertical Distribution in Bottom Sediments of the Ignalina Nuclear Power Plant Cooling Reservoir

Abstract

The Ignalina Nuclear Power Plant (INPP) in Lithuania is a rare case when lake water is used instead of river or sea water for cooling. Lake Drūkšiai with water residence time of 3–4 year and undisturbed sediment layers is a unique system to assess the impact of a nuclear facility on the aquatic ecosystem with a sufficiently high temporal resolution. We constructed a model of radiocarbon cycling processes in lake ecosystem which evaluates the 14C specific activity vertical distribution in two organic sediment fractions: alkali-soluble and alkali-insoluble. Model calculations proved that during the first 15 years of operation since 1983, 14C annual aqueous releases from the INPP were in water dissolved inorganic carbon form and varied in the range of 2.4 ÷ 3.7 × 108 Bq/year. The results of the modeling of hypothetic scenarios also showed that there was the only one episode of elevated releases from the INPP in 2000–2001, which changed the interaction between the two organic sediment fractions for the period of 2000–2006. It was caused most probably by released chemicals from INPP but not by 14C contamination. Interaction processes between both sediment fractions recovered to its original state after 2006, indicating that the released additional chemical compounds lake ecosystem have been cleaned-up.

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

“…The sediment core was taken in 2013, and covered only a four-years period after the closure of the INPP. During this period, the estimated 14 C excess in lake sediment organic matter [6] was a few times higher than that recorded during the first 14 years of the INPP operation. Furthermore, our previous studies on 14 C airborne releases from INPP, when radiocarbon measurements were performed in tree rings of pine trees sampled at a distance of 6.6 km from the nuclear power plant, showed up to 5-6% higher 14 C concentrations (compared to samples from the background site) during 4-5 years after plant closure.…”

“…As can be seen in Fig 4, until the period of increased 14 C pollution in 2000 and 1995 as revealed by the cores sampled in 2013 [6] and 2019, respectively, up to 0.37 �0.6 × 10 9 Bq/y of 14 C was introduced into Lake Drūks ˇiai. During the periods 1995-2000, 2001-2003, 2004-2007, core 2019No2 showed a 14 C release rate from the INPP to water up to 1.31×10 9 Bq/y, 1.57×10 9 Bq/y, and 1.43×10 9 Bq/y, respectively, while 14 C releases seen in the organic fractions of the core 2019No3 were lower up to 1.24×10 9 Bq/y, and 0.94×10 9 Bq/y for the periods [1995][1996][1997][1998][1999][2000][2001][2002][2003][2004][2005][2006][2007], respectively.…”

“…The sample site was chosen at the same place (within ~50-100 m) where the sediment core was taken in 2013 (55˚38 0 49 00 N; 26˚35 0 07 00 E) [5,6]. This sampling station is located at the farthest distance from the INPP, in the area of the deepest depression (30-33 m) of Lake Drūks ˇiai (Fig 1).…”

“…The hypothetical radiocarbon specific activity values in alkali -soluble and -insoluble sediment organic fractions that would have occurred without any impact of the INPP were calculated using the results of our previous measurements in a sediment core taken in 2013 (S4 and S5 Figs), where it was determined [6] that the RRA values of both fractions returned their former pre-bomb values at the beginning of the INPP operation, approximately in 1986. Thus, hypothetical 14 C specific activity values for the organic fractions of both cores for the period of the INPP operation were determined from Eq 2, using a constant RRA values of 1986.…”

“…In our previous work [5,6], we evaluated 14 C liquid discharges from INPP over its entire operation period by analyzing radiocarbon concentration variation in two organic sediment fractions: alkali-soluble and -insoluble. The sediment core was taken in 2013, and covered only a four-years period after the closure of the INPP.…”