The warming up of seawater, caused by the cooling water's thermal load, is the most important environmental impact of the Olkiluoto nuclear power plant. The total amount of seawater used for the cooling of the OL1 and OL2 plant units is approximately 76 m3/s.
In 2018, 2,200 million cubic meters of seawater was used for cooling, and the resulting thermal load on the sea was 25.8 TWh. Seawater temperatures are monitored as required by the environmental permit. One of the environmental permit regulations is that the seawater temperature does not exceed the target value of 30°C when measured as a weekly average at a distance of 500 meters from the cooling water discharge channel. Limit values have also been specified for the amount of cooling water (max. 4,415 million m³) and the thermal load (max. 56.9 TWh) in the environmental permit. None of the permit limits were exceeded in 2018.
As the cooling water passes through a plant unit, its temperature increases by approximately 10°C, after which it mixes with seawater. The cooling water does not come into direct contact with the power plant’s circulating water. Throughout the operation of the power plant, TVO has monitored the impacts of cooling water and conducted related surveys. The cooling water accumulates in an extensive sea area in the surface layer from where part of the heat transfers into the air. Depending on the weather conditions, an increase in temperature can be observed at an approximate distance of 3–5 kilometres from the cooling water discharge location. The cooling water also causes changes in the ice conditions, as the cooling water discharge area remains unfrozen throughout the winter. The size of the unfrozen and weak ice area varies depending on the winter weather, being at a maximum of around 7 km2. TVO issues warnings about the unfrozen area to the local residents in newspapers and on ice warning boards. The warm cooling water extends the growth period in the unfrozen sea area and increases its overall biological production. Other biological effects caused by the cooling water are minor.
| OL1 | 1,058 | 1,134 | 1,114 | 1,164 | 1,140 |
| OL2 | 1,142 | 976 | 1,147 | 1,075 | 1,172 |
Total
| 2,200 | 2,109 | 2,261 | 2,238 | 2,312 |
| OL1 | 12,380 | 13,263 | 13,098 | 13,721 | 13,481 |
| OL2 | 13,401 | 11,423 | 13,474 | 12,677 | 13,871 |
Total
| 25,781 | 24,686 | 26,572 | 26,398 | 27,352 |
