The Finnish Radiation and Nuclear Safety Authority (STUK) has confirmed that an automatic emergency shutdown of the Olkiluoto nuclear power plant on December 10, 2020, was due to human error and not because of a technological fault or equipment breakdown.
In a statement on Wednesday, the STUK said that was the first emergency shutdown of a nuclear plant in the country, since the beginning of using nuclear energy in the national power grid in the mid-1970s, reports Xinhua news agency.
It explained that during maintenance a pump had been stopped, and when it was restarted improperly water had started going in the wrong direction, causing erroneous entry of hot water to the reactor cooling system.
STUK Deputy Director Tomi Routamo said that this error had resulted in the development of highly radiating isotope 16 in the system.
Then radiation levels in the circulation of steam increased three- or fourfold, triggering the emergency shutdown. Initially, a much more serious fuel leak was suspected.
Routamo added that the International Atomic Energy Agency (IAEA) had also reacted upon a Finnish notification while the seriousness of the situation was still uncertain.
Finland has also notified neighbouring countries.
Olkiluoto is operated by the Finnish energy company Teollisuuden Voima.
After the incident, STUK had admitted in a statement that informing the public about the shutdown was too slow.
The municipality of Eurajoki, in the immediate vicinity of the plant, was not informed until several hours after the incident, according to a local media report.
The relatively slow information about the incident was criticised at the time.
In a survey by the Finnish National Rescue Association published on Wednesday, half of the respondents were dissatisfied with the communications about the incident.