Finnish utility Teollisuuden Voima Oyj (TVO) said it had detected and subsequently repaired defects and deficiencies in the seals of the connectors of unit 3 at the Olkiluoto nuclear power plant. The EPR entered commercial operation on 1 May.

Olkiluoto 3 (Image: TVO)

The company said: "During the years 2022-2023, several cases of signal failures in safety-classified temperature measurements have been detected. In February, it was found that some connectors of temperature measurements were missing either one or both of the required seals".

A total of 108 connectors were inspected, with 29 of them found to be missing seals, TVO said. "All the inspected connectors have now been provided with new seals".

TVO noted the connectors with missing seals will work under normal operating conditions, but do not necessarily fulfil the requirements specified for accident conditions. "Their operability cannot be guaranteed in loss of coolant accidents," it added.

As the incident was accompanied by inadequate guidance and the defect was detected in several locations, TVO concluded that the incident falls under category one on the International Nuclear and Radiological Event Scale (INES). According to the INES, the incident was classified as "an anomaly".

On 17 April, TVO submitted a special report of the event to Finland's Radiation and Nuclear Safety Authority (STUK) for approval. STUK approved the special report and TVO's assessment of the INES rating on 10 May.

"The absence of a single seal as such is not a safety issue, but the recurrence of the error increased the significance of the incident on the assessment scale," STUK noted.

OL3 attained first criticality on 21 December 2021 and was connected to the grid on 12 March 2022. The EPR, a 1600 MWe pressurised water reactor, then entered a phase of test production during which some 3300 tests were conducted and more than 9000 test reports collated.

On 20 April, TVO submitted the Provisional Takeover Certificate to the Areva-Siemens consortium, the OL3 plant supplier. The final acceptance of the plant unit will take place upon termination of the two-year warranty period.