Higher than expected vibration of the pressuriser surge line belonging to the reactor cooling circuit at the Olkiluoto-2 nuclear power plant under construction in Finland remains “the biggest open issue” arising from hot functional tests carried out in 2018, the regulator Stuk has said.

In its annual report for 2019 Stuk said the cause of the vibration is still unknown. A possible cause is a hydraulic initiator of the primary circuit causing the pipe to vibrate at the natural frequency of the surge line.

To suppress vibrations, plant supplier Areva-Simens will install two dampers in the line.

Stuk has approved the plan for principles for the dampers and inspected a part of the more detailed plans and analyses regarding the design of the dampers. These must be processed by Stuk before starting the operation of the EPR plant, the report said.