The annual maintenance of unit 1 and unit 2 of Teollisuuden Voima Oyj’s (TVO) Olkiluoto nuclear power plant is completed for this year. Around 30 experts from the Radiation and Nuclear Safety Authority were responsible for monitoring the maintenance.
Unit 2 of the Olkiluoto nuclear power plant underwent annual maintenance from 28 April to 9 May. Teollisuuden Voima Oy started the annual maintenance of unit 1 on 12 May. The maintenance shutdown of unit 1 took longer than TVO had planned, as a fault was detected in the power plant unit's generator during the maintenance and the investigation and correction of the fault took some time. The annual maintenance was completed on 16 June.
The generator and its operation do not affect nuclear safety, and work related to the generator is not supervised by the Radiation and Nuclear Safety Authority (STUK). Because of this, STUK already granted the reactor a start-up permit on 28 May 2024.
STUK’s task is to ensure that maintenance work related to nuclear safety is carried out without jeopardising nuclear and radiation safety and that the plants can be restarted for electricity production in a safe manner. The licensee, in other words TVO, is responsible for the safety of maintenance and operation of the plant.
Mikko Heinonen, Oversight Manager at STUK, explains that this year’s annual maintenance mainly involved regular maintenance and inspection work and the replacement of spent nuclear fuel with fresh fuel.
The changes to the backup diesel generators in both plant units were important for the safety of the plants. The changes will improve the units’ ability to detect any undervoltage in the external power grid and automatically start the backup diesel generators to secure power supply to safety-critical equipment.
In addition, TVO installed a new reactor surface monitoring system for trial use on unit 1. It will improve preparedness for rare failures where the normal water level measurement system in the reactor does not work. The trial use of the system is planned to last at least two years. STUK considers it important that the project has progressed to the trial phase after a long period of planning. After trial use, the new surface monitoring system is intended to be integrated into the plant’s safety automation system.
Before the annual maintenance of units 1 and 2 of the nuclear power plant, TVO carried out an annual maintenance of the latest, third unit. It took longer than TVO had planned, and the annual maintenance of OL2 started before the end of the OL3 maintenance. The situation put a strain on TVO’s staff and the subcontractors who carried out the maintenance work. TVO managed to handle the exceptional situation well, and the safety of the annual maintenance was not impaired by the overlapping maintenance.
According to STUK’s monitoring observations, the annual maintenance work was conducted safely and the small radiation doses of the employees were within safe limits. In places, the level of radiation at unit 2 was higher than expected during the annual maintenance. TVO will investigate the reason for this is and how the situation can be improved before the next annual maintenance.
