The research, which we have carried out, is timely, because tritium is the major radioactive component of discharges from nuclear power plants (NPPs), equipped with BWR and PWR reactors and their analogies - WWER-440 and WWER-1000 (80% of Ukrainian NPPs are equipped with these reactors).

In accordance with the major objectives of the Ph.D. dissertation, the following works were completed:1) for the period from 1992 to 1995 we have studied tritium distribution in natural and artificial water bodies at the territory of Ukraine (these studies were focused mainly at the water bodies under NPPs impact, including the Dnieper river, its tributaries and water reservoirs);2) we have analysed the causes of increased tritium contents in the Dnieper cascade water reservoirs in 1994;3) using mathematical modelling methods, we have evaluated impacts of NPPs (equipped with WWER reactors) on tritium contents in nearby water bodies;4) we have evaluated tritium inflow into the Black Sea with the Dnieper water in 1992 - 1995;5) we have studied tritium distribution in waste bodies of the Chernobyl NPP "exclusion zone" within the period from 1993 to 1995;6) we have reviewed possible consequences of the Chernobyl disaster as for tritium contents in water bodies;7) we have experimentally studied changes in somatic and cytogenetic parameters of CYPRINUS CARPIO L., PLANOBRIS VORTEX L. and DAPHNIA MAGNA Str. under chronic impact of tritium;8) we have studied tritium effects in several generations of hydrobionts;9) we have studied stimulating effect of low-dose tritium activity on development of eggs and larvae of CYPRINUS CARPIO L.;10) we have studied combined impact of tritium and lead on viability of D. MAGNA Str;11) we have analysed probable mechanisms of protective and recovery capacity of hydrobionts, allowing them to recover from radiation-induced damage.

The most important results of the studies incorporate: (1) the impact assessment of Ukrainian NPPs, equipped with WWER reactors, as regional sources of tritium inflow into the water bodies, located nearby the NPPs sites; (2) the identification of toxic effects, that were caused by tritium impact on hydrobionts, at tritium levels registered in cooling ponds of Ukrainian NPPs.