Olkiluoto 3 maintenance began in Finland on Thursday, 10 September, taking the country’s largest nuclear reactor out of electricity production for about 50 days. Operator Teollisuuden Voima (TVO) expects the 1,600 MW unit to return to the grid on 30 October, after refuelling, inspections and repairs.
The shutdown follows the longest uninterrupted operating cycle recorded at the Olkiluoto nuclear power plant. According to TVO, Olkiluoto 3 had operated for 498 days since completing its previous maintenance period in spring 2025, producing around 17.2 terawatt-hours (TWh) of electricity.
Olkiluoto 3 maintenance follows an 18-month operating cycle
The timing of the maintenance reflects TVO’s move to an 18-month operating cycle for Olkiluoto 3, rather than carrying out maintenance every year at roughly the same time.
Around 2,000 maintenance activities, divided into approximately 6,000 work phases, are planned during the shutdown. Around 1,000 external workers will take part alongside TVO employees, with work continuing in several shifts around the clock.
The main tasks include replacing uranium fuel and carrying out related inspections. Workers will also inspect and repair the reactor coolant pumps, clean and inspect the steam generators and perform pressure tests on turbine systems.
Technical problems had already been detected in two of Olkiluoto 3’s main reactor coolant pumps during the summer. In August, TVO reduced output to around 1,300 MW, or 84 per cent of full capacity, while monitoring the problem. The company said the issue had no impact on nuclear safety.
A few hours before the planned shutdown on Thursday, the reactor was also automatically disconnected from electricity production because of a technical fault, according to Yle. The unit was already operating at reduced power and the incident did not pose a safety risk.
Finland will be without its largest power unit until late October
Olkiluoto 3 is the largest individual electricity-generating unit in Finland, meaning its absence removes a significant amount of domestic generation capacity from the electricity system for almost seven weeks.
The shutdown does not necessarily determine household electricity prices on its own. Prices in the Nordic electricity market depend on several factors, including weather, wind and hydropower production, electricity consumption and cross-border transmission capacity. However, periods in which a large generating unit is unavailable can reduce the supply available to the market and therefore contribute to price movements when other conditions are also tight.
The role of nuclear power in Finland’s electricity system has grown since Olkiluoto 3 entered regular production in 2023. According to Statistics Finland, nuclear power accounted for 39 per cent of Finnish electricity production in 2024, making it the country’s largest source of electricity. Overall, 95 per cent of electricity generated in Finland that year came from fossil-free sources.
Olkiluoto 1 and Olkiluoto 2 remain in normal operation during the OL3 maintenance period. Both units underwent their scheduled maintenance earlier in 2026.
The next Olkiluoto 3 shutdown is planned for 2028
The extended operating cycle also means that Olkiluoto 3 is not scheduled for another planned maintenance shutdown immediately next year. TVO says the next service outage is due in spring 2028.
The current shutdown will therefore be an important test of the longer maintenance cycle introduced for Finland’s newest reactor. After the work is completed, Olkiluoto 3 is expected to begin another 18-month operating period, returning a major source of domestic and fossil-free electricity to the Finnish and wider Nordic power system.
