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

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 Druksiai 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 C-14 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, C-14 annual aqueous releases from the INPP were in water dissolved inorganic carbon form and varied in the range of 2.4 divided by 3.7 x 10(8) 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 C-14 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.