The 890-MW boiling water reactor unit, which began commercial operation in 1982, was disconnected from the national grid due to the incident on 10 December at 12:22 local time. Immediate actions required to prevent the recurrence of the event have been implemented, TVO said.
The company said it had submitted its restart application to Stuk on 14 December.
“The application provided a detailed clarification of the progression and causes of the event, a comprehensive report of actions to be carried out before startup and a safety assessment,” TVO said.
The failure occurred when warmer water entered the purification system as a result of the fault. This released contaminated filter material into the reactor pressure vessel and the radiation level in the steam line increased. This caused a momentary peak in the steam line’s radiation measurement which activated safety systems as planned and led to a reactor trip.
The incident was rated by Stuk as level 0 on the International Atomic Energy Agency’s Nuclear and Radiological Events Scale (INES) scale, which goes up to a maximum of 7. The lowest level is level 1 – an anomaly – which means the Olkiluoto incident was a below-scale event without any safety significance.
TVO, Stuk and the IAEA all said radiation levels around the plant were within the average range for background radiation. There was no fuel damage and all safety systems were fully functional.
The Olkiluoto nuclear station is on Olkiluoto Island about 220 km northwest of the capital, Helsinki. There are two boiling water reactor units at the station and one EPR under construction.