During recent decades, the thermal and radioactive discharges from nuclear power plants into the aquatic environment have become the subject of lively debate as an ecological concern. Heat as a separate pollutant was first brought into the public eye in the UK a good 50 years ago; very few research programmes dealing specifically with the effects of thermal discharges were originated anywhere in the world before the early 1950s. By the mid-1960s there were many research projects concerned with thermal discharges in the UK, USA, USSR and Europe, and the term ‘thermal pollution’ was taken into general use. From 1960 to 1970 the literature concerned with pollution by heat grew to several hundred per year (Langford 1990). In 1974, the book ‘Kylvatten – effekter på miljön’ (Cooling water – effects on the environment) was published in Sweden (SNV 1974), and already in the 1960s discussion about the thermal effects of the planned nuclear power plants had started in Finland.
In 1974, the International Atomic Energy Agency (IAEA) organized a Symposium on ‘Environmental Effects of Cooling Systems at Nuclear Power Plants’ in Oslo (IAEA 1975a), and in 1975 a Symposium on ‘Combined Effects of Radioactive, Chemical and Thermal Releases to the Environment’ in Stockholm (IAEA 1975b). In 1980, the IAEA organized a Symposium on ‘Impacts of Radioactive Releases into the Marine Environment’ in Vienna (IAEA 1981), and since then the results of monitoring and studies carried out in the marine environments of nuclear power plants have been discussed in countless symposia and publications around the world.
Especially in the conditions specific to the northern Baltic Sea, where the biota is poor and adapted to relatively low temperatures and to seasonal variation with a cold ice-winter and a temperate summer, an increase of temperature may cause increased environmental stress to the organisms. Furthermore, owing to the brackish-water character of the Baltic Sea, many organisms exist near the limit of their physiological tolerance and have poor resistance to additional stresses (Dybern and Fonselius 1981). The effects of heated effluents are therefore of particular interest here. An increase of temperature stimulates the metabolism, increases the production of organic material and accelerates the growth of organisms; and consequently, it may lead to enhanced eutrophication (Autio et al. 1996). The distinction of the thermal effects from those caused by the increase of nutrients poses a challenge especially in the Gulf of Finland, where the levels of phosphorus and nitrogen have significantly increased during recent decades.
Linked with stimulated metabolism, a rise in temperature increases the uptake of several harmful substances (Grimås 1974) and most probably also
that of radionuclides in biota. The brackish-water character of the Baltic Sea, and especially the low salinity of water in the Finnish coastal areas, adds to the uptake of radionuclides, since the concentration factors of many radionuclides are much higher in low salinities than in real marine environments (cf. Hosseini et al. 2008). Even if the low salinity may make the life of many organisms of marine origin difficult, the warm water may on the other hand attract many immigrants to the discharge areas of cooling water. As another aspect, the effects of cooling water can be used to predict the possible biological changes in coastal waters caused, for example, by climatic change (Ilus and Keskitalo 2008).
There are four nuclear power plant (NPP) units in Finland: two pressurised water reactors at Loviisa (rated net electric power of each 488 MW), on the south coast, and two boiling water reactors at Olkiluoto (rated net electric power of each 840 MW) on the west coast of Finland (Fig. 1). The units at Loviisa were commissioned in 1977 and 1980, and those at Olkiluoto in 1978 and 1980.
Brackish sea water is used for cooling in the Finnish nuclear power plants. When running at full capacity, the power stations discharge 50 – 60 m3 s-1 cooling water into the sea, the discharged water being 10 – 13ºC warmer than the intake water.
Small planned and controlled discharges of radioactive substances (mainly of neutron activation products) are released into the recipient sea areas in the out-flowing cooling water. Extensive environmental monitoring and studies have been carried out in the sea areas surrounding the power stations since the Hästholmen and Olkiluoto islands were chosen as the sites for nuclear power plants in Finland. When the work was begun, the environmental effects of nuclear power were in general rather poorly known. In the course of more than 40 years, the extensive studies carried out have yielded a huge number of results to be utilized. During the last few years, the need for these results has increased with the adoption of the Environmental Impact Assessment procedures of the planned new nuclear power units in Finland.
The aim of this work was to compile the data and summarise the results in an extensive scientific publication, as a doctoral thesis and a legacy to posterity.
It should be emphasised that the work is mainly based on monitoring results, and that these monitoring programmes were not possible to establish in the same way as a research plan in basic research. The radioecological special projects initiated followed this procedure better. All the results are not included in this publication. The phytoplankton results from Loviisa were earlier published elsewhere (Bagge and Niemi 1971, Ilus and Keskitalo 1987 and 2008) and were left out of this consideration. Fish and fishery research have never been included in the water quality and biological studies carried out by STUK, but have been conducted by consultants of the branch in question.
Fig. 1. Location of the Loviisa and Olkiluoto nuclear power stations.