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, annual outages play a key role in the long-term lifecycle management of our plant units.

Annual outages are planned years in advance. The outage times of the Olkiluoto power plants have been scheduled up till the year 2028.

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 a 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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For the second time, all three plant units to undergo annual outage

Annual outages will be carried out in Olkiluoto at all three plant units during the spring 2025. This will be the second time in Olkiluoto that service outages take place at three plants.

The annual outage process starts on 1 March with a service outage at the Olkiluoto 3 plant unit. The duration of the outage will be 63 days this year.

During the first years, Olkiluoto 3 will undergo a service outage every year, but later they can be carried out also with longer intervals. For example, the refuelling carried out during the service outage in the spring of 2025 is designed for an operating cycle of 1.5 years. At Olkiluoto 1 and Olkiluoto 2, annual outages are implemented every year.

Every year, one part of the outage is refuelling which covers the replacement of the uranium fuel, the required fault repair and maintenance activities, as well as any planned modifications. Preparations are also carried out as necessary for activities planned for the following year’s service outage.

All in all, the maintenance plan for the service outage at OL3 includes some 1,700 work activities with almost 5,000 work phases. At the moment, the work list for the annual outage of OL1 shows about 500 activities with a total of some 1,100 work phases. The plan for the OL2 is to carry out some 1,070 work activities with about 3,200 work phases.

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Annual outages 2025 – time plan and most important work activities

OL3 – Service outage
1 March – 2 May (duration 63 days)
Most important work activities:

• Refuelling and fuel inspections
• Leak-tightness test of containment
• Leak-tightness tests on containment isolation valves
• Electrical maintenance
• Reactor pump work
• Steam generator washing and inspections
• Activities on plant supplier’s open work list

OL1: Refuelling outage
11 May – 22 May (11 days)
Most important work activities:

• Refuelling
• Measurements related to replacement of steam separator
• Leak-tightness tests on isolation valves
• Periodic inspections of electrical systems and maintenance on circuit breakers
• Annual maintenance and tests on I&C systems

OL2: Service outage
25 May – 15 June (21 days)
Most important work activities:

• Refuelling
• Tests included in the annual maintenance of the new loading machine.
• Replacement and testing of reactor service bridge
• Measurements related to replacement of steam separator
• Leak-tightness test of containment
• Leak-tightness tests on isolation valves
• Replacement of electrical bushing assemblies
• Periodic inspections of electrical systems and maintenance on circuit breakers
• Annual maintenance and tests on I&C systems

‍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 2028 annual outages already in early 2025.

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 does the OL3 annual outage start already in March? Would it not be more sensible to schedule is for the summer?

The annual outage is carried out in March because the fuel plan for OL3 has been prepared for an operating cycle of one year. Furthermore, some of the periodic maintenance activities scheduled for the annual outage of OL3 are of a nature that they cannot be postponed. The timing of OL3's annual maintenance is also significantly influenced by the fact that the warranty period for certain parts of OL3 is coming to an end during the spring. The service outages have been scheduled to take place before the end of the warranty period.

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

The annual maintenance of the pressurized water reactor (OL3) generally takes longer than that of the boiling water reactors (OL1 and OL2), as shutting down and cooling the plant, as well as starting it up and the heating phase, take more time. During the upcoming annual maintenance, a containment tightness test will be performed for the first time as part of the annual maintenance, which will also extend the maintenance by approximately one week.

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. This is illustrated by the fact that the maintenance plan for this year’s annual outage at OL3 includes some 1,635 work activities and 4,670 work phases. The work list for the upcoming outage at OL1, for example, comprises 497 activities.