Annual outages and modernizations projects

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Annual outages are necessary to carry out refuelling and specific maintenance and service activities. At the same time, they are part of the sustained lifecycle management of the plant units.

Annual outages are planned years in advance. The annual outage time points of all our plant units have been announced to the market up till 2029.

The purpose of annual outages is to ensure the continuous uninterrupted electricity production of the plant units.

The scheduling of the annual outages is affected by, among others, the availability of personnel and the refuelling cycle. The objective is to only have one unit at a time undergoing an annual service outage. The annual outages of Olkiluoto 1 and Olkiluoto 2 have traditionally been carried out one after the other in the spring.

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Annual outage at two plant units in spring and at third unit in autumn

In 2026, annual outages will be carried out in Olkiluoto both in the spring and in the autumn. The annual outage programme will start on 6 April, when the annual outage of Olkiluoto 2 begins. This year, the service outage at OL2 will take about 10 days.

The next unit to undergo the annual outage is Olkiluoto 1 where the outage begins on 19 April and due to the extent of work, goes on for about 55 days.

The annual outage of Olkiluoto 3 will be carried out in the autumn. The service outage with a planned duration of about 50 days commences on 10 September.

Every year, annual outages cover the replacement of the uranium fuel, all required fault rectification activities as well as any modification work. Any preparations needed for the following year’s annual outage are also carried out in connection with the outage.

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Annual outages 2026 – time plan and most important work activities

OL2: Refuelling outage
6 April – 16 April (duration abt. 10 days)
Most important work activities:

• refuelling
• Inspection of turbine, condenser, and generator
• annual tests on reactor protection systems
• replacement of crane in turbine hall
• maintenance on main steam line valves
• periodic inspections and maintenance of electrical systems
• modernisation work on reactor level measurement

OL1: Service outage
19 April – 13 June (duration abt. 55 days)
Most important work activities:

• refuelling
• extensive periodic inspections and maintenance of electrical systems
• pressure test in primary circuit
• containment leak-tightness test and replacement of penetrations
• replacement of generator switch
• recoating of storage tanks for demineralised water
• replacement of reactor I&C
• repair work on front water box of turbine condenser
• repair work on service water channels
• maintenance on high-pressure turbine
• repair work on high-pressure preheaters for feedwater system
• draining and repair work on condensation zone
• renewal of equipment for compressed air and nitrogen systems
• annual tests on reactor protection systems
• modernisation work on reactor level measurement
• testing of the new fuel transfer machine during the annual outage

OL3: Service outage
10 September – 30 October (duration abt. 50 days)
Most important work activities:

• refuelling and fuel inspections
• maintenance and inspections of reactor coolant pumps
• steam generator washing and inspections
• maintenance work on electrical switchgear
• turbine and generator inspections
• reheater inspections

‍Annual outages – Frequently Asked Questions

Why are annual outages at the plants so important?

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.

Also, during every annual outage, part of the fuel, about one quarter, is replaced. This ensures that production will be as efficient and stable as possible.

Which factors determine the timing of the annual outages?

Annual outages are scheduled for several years ahead. For example, we have informed the electricity market (Nord Pool) of the time points of the year 2029 annual outages already in early 2026.

The service outages of the Olkiluoto nuclear power plants are scheduled to take place outside the winter season, with only one unit at a time undergoing an annual outage.

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.

How many people will be involved in the annual outages in the spring of 2026?

In addition to TVO’s own personnel, this year about a 1,400 subcontractors will take part in the annual outage works of OL1 and OL2.

Why will annual outage not start at OL3 until autumn?

The start time of the annual outage at OL3 is based on its operating cycle which was extended to 18 months. The previous annual outage at OL3 took place in the spring of 2025 and in that connection, it was refuelled for 18 months of operation. The upcoming fuel plan is also based on 18 months, which means that no annual outage will be carried out at OL3 in 2027. Instead, the next service outage after this year will take place in the spring of 2028.

Why does the annual outage at OL3 usually take longer than the outages at the OL1 and OL2 plant units?

It is true that annual outages tend to take longer at OL3 than at OL1 and OL2. However, in 2026, with 55 days, the longest annual outage will be that at OL1. The long duration of the OL1 outage is the result of the large number of extensive and time-consuming work packages to be implemented this year.

Annual outages generally take longer at a pressurised water plant (OL3) compared to boiling water plants (OL1 and OL2) because both the shutdown and cooling phase and the start-up and heating phase are longer.

Moreover, being the largest nuclear power plant unit in Europe, the number of equipment and components to be serviced is higher at OL3 than at our other two plant units.

Why is annual outage this year so long at Olkiluoto 1?

The long duration of the annual outage at OL1 is the result of the large number of extensive and time-consuming work packages and plant modifications to be implemented this year. Such extensive packages include, for example, repair work on the front water box of the turbine condenser, the replacement of reactor I&C, maintenance of the high-pressure turbine as well as the replacement of the containment penetrations, and a pressure test of the primary circuit.