STUK authorised start-up of Olkiluoto 1

STUK made its decision on Sunday evening at 23.00 hours based on TVO’s application for the start- up of Unit  1.

The start-up of Unit 1 on Friday after a maintenance outage failed due to an electrical fault.

TVO has reported the event to STUK. Prior to the restart of Unit 1, TVO is to modify generator protections that can help prevent recurrence.

STUK considers that TVO’s both Olkiluoto reactors can be operated safely at the reduced, approx. 80% power. Prior to the raising of reactor power to 100%, the overvoltage endurance of primary coolant pumps and components operated under their control must be improved at both plant units.

The safety of the plant or its environment was not compromised.

The event was classified Level 1 (lowest classified event) on INES, the 7- level International Nuclear Event Scale.

Two other INES events occurred at Olkiluoto

During the annual maintenance of Unit 1 of Olkiluoto nuclear power plant, two other events occurred that were given an INES classification.

The Technical Specifications were not complied with during annual maintenance, on 19 May, when a control rod was withdrawn from the reactor core without a written permission. The plant unit’s safety was not compromised but the event was classified INES Level 1 because the Technical Specifications were not complied with.

During the plant’s start-up, problems were detected in its standby electric supply. An emergency diesel generator failed to start during testing because starter motors had failed. TVO inspected all the other emergency diesel generators and found corresponding faults in three of them, and repaired them without delay. Emergency diesel generators supply electrical power to all safety systems during a transient. The INES classification of the event is Level 1.