Missing seals detected in connectors at OL3 plant unit in February 2023
During the years 2022-2023, several cases of signal failures in safety-classified temperature measurements have been detected at the Olkiluoto 3 plant unit. Due to these, a decision was made to carry out more extensive inspections on the temperature measurements. In February, it was found that some connectors of temperature measurements were missing either one or both of the required seals.
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. A total of 108 connectors were inspected and missing seals were detected in 29 of them. All the inspected connectors have now been provided with new seals.
The degree of severity of the event has been rated to level 1* on the international INES scale. Although the basic rating of the event is 0, it was increased by one level, because deficiencies in procedures have, for their part, contributed to the occurrence of a common cause failure and in accident conditions, the detected failings could have caused safety functions not to operate as planned.
On 17 April 2023, TVO submitted to the Radiation and Nuclear Safety Authority of Finland (STUK) for approval a special report of the event. STUK approved the special report and TVO's assessment of the INES rating on 10 May 2023.
* The events occurring at a nuclear power plant are classified on the international INES scale according to their degree of severity. The INES scale has seven categories of severity. Category 4-7 events are classified as accidents, category 1-3 events as incidents or anomalies with a negative effect on safety, and category 0 events as deviations with no significance for safety.
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