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The Olkiluoto site in Finland. (Kallerna/Wikimedia Commons)
A nuclear reactor, Olkiluoto 2, in Finland is expected to operate at 735 MW---155 MW below its full capacity until the end of next year. There were issues with its generator rotor that caused Teollisuuden Voima (TVO) to cut back on its output. The reactor is operating at this reduced output until the new rotor is installed
A boiling-water reactor (BWR), Olkiluoto 2 has a thermal output of 2,500 megawatts along with a total electrical capacity of 890 MW. The turbine train is rated at 3,000 RPM and features a high-pressure turbine and four low-pressure turbines.
OL2 first started experiencing issues two years ago. On September 9th, 2024, TVO detected higher-than-normal moisture levels before the rotors stopped working. The unit was then disconnected from Finland’s power grid. While the company determined the generator’s rotor caused the failure, they couldn’t figure out why it happened. One month later, the reactor connected to the grid again after they installed a backup rotor in place of the faulty one.
Since the spare turned out to be the last one in the inventory, TVO kept the unit at a reduced output. It generated 725 MW after the October 2024 replacement. Afterward, TVO increased the output to 735 MW and kept it there. TVO says they set the initial limitation to lower the chances of another rotor failure.
In April 2025, another spare rotor arrived. This time, TVO started boosting the output to its 890 MW capacity. Power production increased on April 14th, with full power production five days later. However, on April 15th, TVO detected more moisture in the generator, pausing power generation. Nuclear safety wasn’t affected. There are still uncertainties about the cause of the moisture issue. Johanna Aho, TVO communications manager, said it was too early to determine what happened.
TVO installed a refurbished, spare rotor that arrived in April, replacing the failed one. On May 6th, 2025, the OL2 unit generated power again at 735 MW. On July 15th, 2026, the Olkiluoto site received a refurbished spare rotor. This isn’t expected to be installed right away and will be kept as a backup. According to TVO, a new, technically improved rotor will arrive late next year.
The restriction lasted longer than anticipated. The company was set to raise the output limit during the 2026 annual outage. It then extended the timeline to 2027 instead. TVO says the reactor is staying at 735 MW until it installs the new rotor. At 100% capacity, OL2 supplies 8% of Finland’s energy usage.
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