Electricity production has resumed at Olkiluoto NPP unit 3 (OL3) after its annual service outage, Finnish power company Teollisuuden Voima Oyj (TVO) reported. About 2,000 different work activities and about 6,000 work phases were carried out during the outage. Apart from refuelling, the most important works 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. This was the first outage following an operating cycle of 18 months. The design basis refuelling cycle of OL3 is 12–24 months.
The annual outage 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,” TVO said.
“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, said OL3’s Plant Manager Heikki Lukkari. “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.”
There were some areas in need of improvement that were 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,” TVO noted.
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.”
