Olkiluoto Unit 2 Going Back Online After Shut Down Incident

Finland’s Olkiluoto Unit 2 reactor can re-connect to the electricity grid after an automatic shutdown incident on December 10 that was graded zero in the grading system of nuclear power plant incidents.

Olkiluoto NPPThe country’s nuclear power oversight agency STUK (the Radiation and Nuclear Authority) has granted permission for a restart of commercial operations, which is expected to take place by the end of the week.

The plant shut down “due to automatic scram,” said plant operator Teollisuuden Voioma (TVO). Even with permission granted, prior to start up “thorough inspections, maintenance and repair measures are carried out,” according to a company statement.

The incident began with a planned failure inspection of the shutdown cooling system. "Then the pump of the cooling system was stopped, one of the valves was broken." In the time it took to repair the pump -- about two hours -- hot process water entered backward into the filters of the reactor's cleaning system. A difference in pressures caused the process water to migrate backwards.

After the completion of the repair work, the shutdown cooling system was started, including the reactor water clean-up system. Filters in that system are designed to operate with the water at 70 degrees. However, the incident allowed water at 100 degrees to momentarily flow through the filters. This caused dissolved substances from the filters to flow into the reactor water and these substances were activated as they passed through the reactor core.

In turn, radiation levels of the steam moving through the main steam lines rose momentarily to 3 and 4 times normal levels.

Such incidents as these cause concerns among community residents, even as they prove that the safety systems are functioning as planned. "This is one of the safety functions of the plant and it's job is to isolate the reactor's containment ... so that no radiation can reach the environment," TVO said.

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