STUK supervised the first annual maintenance of unit 3 of the Olkiluoto nuclear power plant

About 40 experts from the Radiation and Nuclear Safety Authority (STUK) supervised the first annual maintenance of unit 3 of Teollisuuden Voima Oyj (TVO)'s Olkiluoto nuclear power plant. According to STUK's findings, the annual maintenance was carried out safely. TVO prioritized safety in its decision-making throughout the maintenance, and the longer-than-planned duration of the annual maintenance did not affect this.

TVO carried out the annual maintenance of unit 3 of the nuclear power plant from 2 March to 16 May 2024. STUK’s task is to ensure that nuclear and radiation safety is not compromised during the annual maintenance and that the start of the plant and the resumption of electricity production can be performed safely. TVO is responsible for the safety of the maintenance and operation of the plant.

Since this was the first annual maintenance of the unit, STUK specifically monitored TVO's preparations for the annual maintenance and ensured that the maintenance was carried out safely. STUK also examined the reports submitted to it by TVO and supervised the inspections, testing and maintenance work carried out on site. During the first annual maintenance, safety-relevant work included inspections of the nuclear fuel and the reactor’s primary circuit and its most important components.

TVO organised the maintenance work well and the activities were well thought-out and systematic. Unexpected failures and other setbacks were resolved without compromising safety requirements. STUK found that TVO allocated the necessary amount of time for the implementation and planning of the annual maintenance and prioritised safety. It is important for TVO to take into account the lessons learned during the first annual service and develop its activities accordingly. STUK will monitor the implementation of lessons learned before the next annual maintenance.

Foreign objects in fuel

During the maintenance, TVO transferred all the nuclear fuel from the reactor to a separate fuel building and inspected the fuel rod clusters. During the inspections, TVO detected foreign objects in several fuel rod clusters and submitted a report on this to STUK. On the basis of the investigation, it can be established that the foreign objects entered the fuel during the fuel transfers and were not in the reactor when it was in operation. The origin of the foreign objects is still being investigated. TVO removed the foreign objects and inspected each fuel rod cluster to be reloaded into the reactor to prevent any foreign objects from entering the reactor and posing a safety risk. STUK requires TVO to submit a report on further investigations of the foreign objects and possible measures before the next annual maintenance.

During the annual maintenance, TVO also continued to inspect the connectors that are important for measurements needed in emergencies. In 2023, the company detected defects in the connectors. TVO provided a report to STUK on the inspections and a safety assessment, on the basis of which STUK concluded that the connectors important for safety are in working order.

STUK also monitors the radiation doses of employees during the annual maintenance. Olkiluoto 3 is a new plant unit and the radiation levels at the plant have been low. Despite the long duration of the annual maintenance, the radiation doses of employees during the annual maintenance of Olkiluoto 3 were significantly lower than the average doses in annual maintenance of nuclear power plants. STUK found that the plant could be started safely in the start-up preparedness inspection carried out at the end of the annual maintenance on 13 May 2024. The annual maintenance procedure ended when TVO connected the plant to the grid on 16 May 2024.