Annual outage at Olkiluoto 3 completed after smooth implementation – electricity production resumed

As the largest nuclear power plant unit in Europe, the amount of equipment and components requiring service is extremely high at OL3. All in all, about 2,000 different work activities and about 6,000 work phases were carried out during OL3’s service outage. Apart from refueling, the most important works carried out included the containment leak-tightness test, work on reactor coolant system pumps, washing and inspection of steam generators, and warranty work conducted by the plant supplier.

In addition to TVO's own personnel, this year about 1,000 employees of subcontractors took part in the annual outage works at OL3, about half of them arriving to Olkiluoto from abroad. The plant unit now underwent a service outage for the first time for an operating cycle of 18 months. The design basis refueling cycle of OL3 is 12–24 months.

Useful lessons learned during first annual outage

The annual outage at OL3 did not take as long as originally planned. The outage was scheduled to take 63 days and continue until 2 May. The actual duration was 59 days and 10 hours.
The shorter outage period was enabled by the experiences from and lessons learned during the first annual outage as well as the fact that the schedule also provided for unanticipated failure repairs. This year, such surprises affecting the schedule were not encountered to any great degree in OL3’s annual outage.

-At Olkiluoto 3, the fuel assemblies are removed from the reactor at the start of the annual outage into fuel pools located in a separate fuel building. They undergo a meticulous inspection before a large part of them is transferred back into the reactor together with fresh fuel. We managed to complete this process much more smoothly than last year. We also brought the containment leak test, now carried out for the first time during the annual outage, to a conclusion ahead of the schedule, says OL3’s Plant Manager Heikki Lukkari.

Areas in need of improvement also identified during annual outages

At the initial stage of the annual outage, water used at OL3 as reactor coolant gained access into some containment rooms and into the containment floor drain system in connection with the filling of the reactor pool. The necessary action has been taken to avoid similar incidents in the future.

Problems with seals in the connectors of certain measurement instruments were also detected during the annual outage. TVO has checked and sorted similar connectors in measurement instruments already in 2023 and 2024. The connectors with defective seals function under normal operating conditions, but do not necessarily fulfil the requirements specified for accident conditions. The Radiation and Nuclear Safety Authority of Finland has provisionally rated the event to level 1* on the international INES scale. The additional inspections carried out by TVO on the connectors of the measurement instruments verified the conformity of the connectors with requirements.

Fuel inspections revealed some very small and fragile pieces of foreign material in some of the fuel assemblies. These findings were dealt with appropriately. TVO continues to carry out investigations and take action regarding the foreign material detected in the fuel of OL3. The detected foreign material does not prevent the safe and reliable operation of the fuel during the upcoming operating cycle.

Annual outage at Olkiluoto 1 starts next

The service outage at OL3 was the first of the annual outages of the three Olkiluoto plant units this year. 11. The refueling outage starting on 11 May at OL1 will take about 11 days. The last of the annual outages is carried out at OL2. The service outage of OL2 starts on 25 May and will take 21 days, ending on 15 June.

More detailed information about the schedule can be found in the Nordpool power exchange at: hhtps://umm.nordpoolgroup.com

For more information, please contact:
Johanna Aho, Communications Manager, tel. 040 729 0822

* The events occurring at a nuclear power plant are classified on the international INES scale according to their degree of severity. The INES scale has seven categories of severity. Category 4–7 events are classified as accidents, category 1–3 events as incidents or anomalies with a negative effect on safety, and category 0 events as deviations with no significance for safety.