Annual outages and modernizations

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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 for the Olkiluoto power plants have been announced up until 2027. The goal of the annual maintenance activities is to ensure the continuous, uninterrupted electricity production of the plant units. Factors influencing the timing of maintenance include workforce availability and the refuelling cycle. The aim is to have only one plant unit under maintenance at a time. Traditionally, the annual maintenance of Olkiluoto 1 and Olkiluoto 2 has been carried out consecutively in the spring.

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 this year was about 74 days.

During the first years of operation, Olkiluoto 3 will undergo a service outage every year, but later the plant unit can be operated also in longer operating cycles of 18–24 months. 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.

Frequently asked questions about 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 were 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 took part in the annual outage works, in addition to TVO’s own personnel.

How many of the outage staff came from abroad?
In 2024 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 had about 550 specialists from abroad.

Which factors determine the timing of the annual outages?
Annual outages are scheduled for several years ahead.

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 was 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 were 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. Annual Outage of OL3 had more work activities and work phases than OL1 and OL2.

Why was the OL3 service schedule prolonged by a month?

There are several reasons for this. This was 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.

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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 – 16 June (duration 35 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)

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