Cold is no enemy in Olkiluoto – cold seawater actually improves efficiency of nuclear power plants
The winter season or outdoor temperatures which this year have dropped to quite freezing figures at times are not a source of concern to the Olkiluoto nuclear power plants – quite the opposite, actually. In fact, a low seawater temperature improves the efficiency of the Olkiluoto energy mills. And this is more than welcome in a country such as Finland where the nation’s electricity demand reaches peak heights in cold weather.
Let us first address the most experienced ones of the plant units on the Olkiluoto Island, Olkiluoto 1 and Olkiluoto 2. The 890 megawatt nominal output rating of these plants is achieved when the temperature of the seawater used for cooling is about nine degrees Celsius. The colder the seawater is, the more power the plants produce.
Temperature remains below nine degrees for a large part of the year
Off the Island of Olkiluoto, the temperature of seawater is at or below nine degrees for most of the year. This is shown by long-term statistics. In these parts of the coastline, the mean seawater temperature typically falls below nine degrees at the turn of September to October and only rises above it again sometime in May-June. So it stays below nine degrees for well over half of the year, and mostly for even much longer.
When the seawater gets colder, the production output of OL1 and OL2 increases to the extent that at zero degrees, the electricity output is about eight megawatts above the nominal rating. Not a huge change, then, but in the right direction in any case. After all, electricity consumption in Finland follows the thermometer quite closely, reaching maximum levels during the harshest winter periods as we have witnessed several times this year.
And the opposite is true in the summer
The logic applies also in reverse. When the seawater is warm, say +21 degrees which would be near optimal swimming conditions, the electricity output of the OL1 and OL2 plant units drops by about 28 megawatts from the nominal rating to a production level of about 862 megawatts. This means that the plant appears to have a natural tendency to follow the rules of demand and supply, since the electricity consumption is at its lowest in Finland in the summer.
The phenomenon is exactly the same at the newest of Olkiluoto’s plant units, Olkiluoto 3. It also improves its production efficiency at cold temperatures. In other words, OL3 also supplies electricity to the national grid the most when the need for electricity is the highest.
It is easy to deduce from what is said above that the rather cold beginning to 2026 has been an efficient one in Olkiluoto. Regardless of the freezing temperatures – or rather, thanks to them – the combined electricity production of the plants has been at an very high level.
Year starts with impressive figures
The total amount of electricity produced in Olkiluoto and supplied to the national grid has been a respectable 3,206 megawatts on average per each and every hour during the early part of the year. That corresponds to about 28.4 percent of all the electricity produced in Finland and about 24.7 percent of the country’s electricity consumption.
To sum up, electricity produced by nuclear power is stable and predictable – both in freezing cold and at the heat of the summer – 24/7/365.
Thankfully, we have Olkiluoto.
Text: Ville Kulmala and Tapani Karjanlahti
Photos: Tapani Karjanlahti