Missing seals detected at unit 3 of Finland’s Olkiluoto NPP

17 May 2023

According to Finland’s Teollisuuden Voima (TVO), several cases of signal failures in safety-classified temperature measurements had been detected the EPR reactor at unit 3 of the Olkiluoto NPP (OL3) in 2022-2023. As a result, 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, TVO said.

OL3 began regular electricity production in April following the completion of trial operation. Construction of OL3 began in 2005 and various setbacks and delays mean the plant is some 14 years behind the original schedule and significantly over budget. OL3’s final price tag is put at some $11bn ($12bn), some three times what was initially estimated. OL3 attained first criticality in December 2021 and was connected to the grid on in March 2022. The 1600 MWe EPR was operated at full capacity for the first time in late September 2022. However, cracks were identified in the impellers of the feedwater pumps located in the turbine island, causing further delays.

TVO said the connectors with the seals missing 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 at level 1 on the International Nuclear & Radiological Event Scale (INES). Although the basic rating of the event is 0, it was increased to 1, because deficiencies in procedures have contributed to the occurrence of a common cause failure. TVO said that, in accident conditions, the detected failings could have caused safety functions not to operate as planned.

In April, TVO submitted a special report of the event to the Radiation & Nuclear Safety Authority (STUK), which has now approved the report and TVO's assessment of the INES rating. INES 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.

In a recent article on TVO’s website entitled “Reasons why Olkiluoto 3 is so significant”, TVO completion of OL3 is highly important for Finland. “Finishing this project means that we have an additional 1,600 MW of urgently needed fossil-free energy for domestic consumption. The new reactor also received an enthusiastic reception from the nuclear industry, as it has been a while since one has been constructed in Europe. The project has also provided Finland with a massive amount of fresh commissioning know-how in the nuclear energy field.”

Image: Olkiluoto 3 (courtesy of TVO)

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