Annual maintenance in the coming years


OL1 4.5.-11.5.2025 (7 days)
OL2 18.5.-5.6.2025 (18 days)
OL3 1.3.-7.4.2025 (37 days)


OL1 15.3.-4.5.2026 (50 days)
OL2 1.3.-8.3.2026 (7 days)
OL3 9.5.-8.6.2026 (30 days)


OL1 4.4.-11.4.2027 (7 days)
OL2 18.4.-7.6.2027 (50 days)
OL3 18.9.-30.10.2027 (42 days)

Annual outages and modernizations

The plant units in Olkiluoto undergo maintenance every year and are upgraded on a regular basis and methodically throughout their service life. We modernise our plant units systematically. Solutions based on the most advanced technology are introduced throughout the operation of the plants in order to improve availability, productivity, and safety. We have invested every year tens of millions of euros in maintenance and modernisation efforts at the plant units.

Annual outages are necessary to carry out refuelling and specific maintenance and service activities. At the same time, annual outages play a key role in the long-term lifecycle management of our plant units.

Annual outages are a means to ensure that the Olkiluoto nuclear power plant remains in good condition at all times. In practice, we have until now carried out the outages alternately as a shorter refuelling outage and a longer service outage which also includes refuelling.

Annual outages are planned years in advance. The outage times of the Olkiluoto power plants have been scheduled up till the year 2027. The purpose of annual outages is to ensure the continuous uninterrupted electricity production of the plant units. The timing of the annual outages is affected by factors such as the availability of manpower and the 12-month fuel cycle. Just one unit at a time is undergoing a service outage. The annual outages of Olkiluoto 1 and Olkiluoto 2 have traditionally been carried out one after the other in the spring when the price of electricity from secondary sources is the lowest.

Annual outages at all three plant units

Full power production started at Olkiluoto 3 in March 2023 and the first service outage of the unit started on 2 March 2024. The duration of the outage will be about 74 days this year.

During the first years of operation, Olkiluoto 3 will undergo a service outage every year, but after that, the plant unit can be operated also in somewhat longer operating cycles. At Olkiluoto 1 and Olkiluoto 2, annual outages are implemented every year.

The refuelling outage covers the replacement of the uranium fuel, the required fault repair and maintenance activities, as well as any planned modifications. We also carry out preparations as necessary for activities planned for the following year’s service outage.

All in all, the maintenance plan for this year’s annual outage at OL3 includes some 1,900 work activities with about 6,500 work phases. The work list for the annual outage of OL2 shows about 550 activities with a total of about 1,200 work phases. And the plan for the OL1 is to carry out some 1,200 work activities with about 3,200 work phases.

Frequently asked questions about upcoming annual outages

Why are annual outages at the plants so important?
During every annual outage, part of the fuel, about one quarter, is replaced. At nuclear power plants, a significant part of equipment maintenance and repair activities can be carried out with the plant in power operation, but on some of the equipment maintenance is only possible when the plant is in a shutdown condition. All major modernisation projects are also implemented during annual outages.

How many people will be involved in the annual outages in the spring of 2024? How much of the manpower for annual outages comes from outside TVO?
This year, about 1,400 employees of subcontractors take part in the annual outage works, in addition to TVO’s own personnel.

How many of the outage staff come from abroad?
Some 150 professionals with special skills working in the annual outages of OL1 and OL2 come from abroad. For the annual outage of OL3, we will see some 550 specialists from abroad.

Which factors determine the timing of the annual outages?
Annual outages are scheduled for several years ahead. The electricity market (Nord Pool) was informed about the times of the year 2027 annual outages, for example, in January 2024.

The service outages of the Olkiluoto nuclear power plants are scheduled to take place outside the winter season, with only one unit out of operation at a time. The same group of skilled subcontractors can participate in each of the annual outages when they take place one after the other. The timing is also affected by the need to replace part of the uranium fuel and to carry out necessary periodic maintenance activities.

Why did the OL3 annual outage start already in March? Would it not be more sensible to schedule it for the summer?
The annual outage is to be carried out in March because OL3 will then have been in production for one year. The fuel plan has been prepared for an operating cycle of one year. Furthermore, some of the periodic maintenance activities scheduled for the first annual outage of OL3 are of a nature that they cannot be postponed.

Why will the annual outage at OL3 take longer than the outages at the OL1 and OL2 plant units?
This will be the first annual outage carried out at Olkiluoto 3. As OL3 is the largest nuclear power plant unit in Europe, the number of components and equipment that need to be serviced is also large.

The maintenance plan for this year’s annual outage at OL3 includes some 1,900 work activities with almost 6,000 work phases. The work list for the upcoming outage at OL2, for example, comprises less than a thousand activities.

Why has the OL3 service schedule been prolonged by a month?

There are several reasons for this. This is the first service outage at the plant unit so it has now provided concrete information about the time that the different activities take.

Also, some technical problems have been encountered which have added extra days to the schedule.
We also detected indications of a risk of foreign material in the fuel elements causing us to inspect the elements with special attention before they were installed in the reactor. A total of 6,500 work phases are implemented during the service outage.

As with Olkiluoto 1 and Olkiluoto 2, the type, extent and sequence of the activities carried out during the annual outage will from now on be optimised also for OL3 in the outage planning process based on the experience gained.

The annual outage of Olkiluoto 3 has been prolonged by about a month. Would it have been possible to push back the outage of Olkiluoto 2 to prevent it from coinciding with the outage at OL3?

The dates of the annual outages, including any overlap, have been assessed in a thorough decision-making process.

We plan for the annual outages years ahead and detailed schedules are always prepared to ensure the timely execution of certain compulsory periodic maintenance activities on the equipment and systems. Reactor core and fuel design is also carried out specifically for each operating cycle (12 months). The implementation of the annual outages according to the plans is a prerequisite for the safe and reliable operation of the plant units. And besides, the more than 1,000 external employees who take part in the outages have also other worksites to manage. ¬

Is it possible that the annual outage of OL2 will also be extended?

Whenever maintenance activities are carried out at the plant unit in areas that are not accessible during normal operation, it is possible that unanticipated service needs or faults are identified. However, with more than 40 years of experience in the operation or Olkiluoto 1 and Olkiluoto 2, we are in a relatively good position to estimate the services schedules in advance and deal with unexpected situations in cooperation with out networks. Maintenance work is carried out 24/7 on an extremely intense schedule and we have even won awards for our efficient and diligent execution of the outages at these plant units.

Olkiluoto 1 is about to be shut down for annual outage on 12 May. Is it possible to push back the outage at OL1 if the current outage schedule of OL3 and/or OL2 is prolonged further?

This will be assessed separately should such a situation arise.

The extension of the OL3 maintenance schedule has resulted in two nuclear power plants undergoing outage at the same time. Is security of supply an aspect that is considered in case of a risk of prolonged service outages?

We always assess the risks, advantages, and disadvantages in great detail in such conditions.

Annual outages 2024

OL3 – Service outage

2 March – 16 May (duration 74 days)

Most important work activities:

  • Refuelling and visual inspection
  • Maintenance of switchgears in electrical division 3
  • Preventive maintenance on protection system
  • Software updates in I&C systems
  • Washing and inspection of secondary side tubes of steam generators
  • Maintenance on pressuriser safety relief valves
  • Leak-tightness tests on containment isolation valves
  • Adding of undervoltage criteria (83%) to automatic changeover of diesel generator bus
OL2: Refuelling outage

28 April – 9 May (duration 11 days)

Most important work activities:

  • Refuelling
  • Leak-tightness tests on isolation valves
  • Adding of undervoltage criteria (83%) to automatic changeover of diesel generator bus
  • Periodic inspections of electrical systems and maintenance on circuit breakers
  • Annual maintenance and tests on I&C systems
OL1: Service outage

12 May – 28 May (duration 16 days)

Most important work activities:

  • Refuelling
  • Leak-tightness tests on isolation valves
  • Measurement of steam separator
  • Replacement of control rod drive mechanisms (20 pcs)
  • Maintenance on main transformer and generator circuit breaker
  • Maintenance on feedwater and condensate pumps
  • Reactor automation replacement project
  • Maintenance on recirculation pumps (2 pcs)
  • Replacement of electrical feedthrough modules in containment (3 pcs)