TVO to restart Olkiluoto 2 following cooling water incident

16 December 2020

Finland’s Teollisuuden Voima Oyj (TVO) expects unit 2 of its Olkiluoto nuclear plant to be reconnected to the Finnish grid later this week, after receiving restart approval from the country's nuclear regulator.

The 890MWe Olkiluoto 2 boiling water reactor (BWR) automatically shut down on 10 December due to a fault in the purification system for the reactor cooling water, which led to a temporary increase in radiation levels in the circuit.

TVO said the incident was preliminary rated zero on the International Nuclear and Radiological Event Scale (INES).

TVO explained on 13 December that the incident occurred during a planned failure inspection of the shutdown cooling system. When the pump of the cooling system was stopped, one of the valves broke and the repair work took about two hours to complete. During that time, hot process water was able to enter backwards longer than planned into the filters of the reactor cleaning system due to a difference in pressure.

When the cooling system was restarted the incident was triggered. The filter of the cleaning system is designed to withstand water of about 70°C, but because of the time taken for the repair work, water of about 100°C was momentarily able to flow into it, dissolving substances from the filter into the reactor water. These were then activated as they passed through the reactor core and the radiation levels of the steam passing through the main steam lines momentarily rose to about 3 to 4 times normal levels. This triggered the safety systems and the reactor automatically shut down.

STUK issued a statement saying, "no radioactivity was released that would have had any impact on people or environment”. It said the disturbance caused no damage to the fuel and, as a result, posed no risk of harmful radioactive release.

On Monday, 14 December, TVO submitted an application for start-up permission to STUK. STUK reviewed the reports submitted by TVO and supervised the inspections, tests and maintenance activities conducted on the plant site and has today granted start-up permission for Olkiluoto 2.

The inspections and clarifications "did not bring up any obstacles to the start-up of the plant unit in terms of safety or the availability of systems or components," TVO said.

TVO will finalise the last required inspections, tests and maintenance activities before the start-up of the plant unit.
The latest schedule estimates the reconnection of the plant unit to the national grid to take place during the end of the week, it added.

Finland has four operating nuclear power units – two 890MWe BWRs at Olkiluoto and two Russian supplied VVER-440s at Loviisa - which together produce 30% of the country’s electricity. OL2, supplied by the Swedish company Asea-Atom, has been in operation since 1982. A third unit, an EPR supplied by Areva-Siemens – is under construction at Olkiluoto and is scheduled for start up in 2022 after a decade of delays. Finland’s sixth unit, a VVER-1200 supplied by Rosatom is planned for construction in the northern Finnish municipality of Pyhäjoki.

The emergency shutdown of OL2 last week momentarily raised the price of electricity in Finland to a record level of €3500 on 10 December, up from €139 the day before.

Photo: Olkiluoto 1&2 (Credit: TVO)

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