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 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 57 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.

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Annual outages 2024

OL3 – Service outage

2 March – 28 April (duration 57 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

OL2: Refuelling outage

28 April – 6 May (duration 8 days)

Most important work activities:

• Refuelling
• Measurement of steam separator
• 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)

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