An automatic shutdown yesterday of unit 2 at the Olkiluoto nuclear power plant in Finland was caused by a fault in the purification system for the reactor cooling water, which led to a temporary increase in radiation levels in the circuit, Finnish utility Teollisuuden Voima (TVO) said. The emergency standby resulting from the incident has now ended and repair work is under way to bring the unit back online.
TVO said the failure occurred when warmer water than usual entered the cleaning system of unit 2's reactor water as a result of a failure in the purfication system. This released filter material into the reactor pressure vessel, in which it was activated. The radiation level in the steam line increased, which caused a momentary peak in the steam line's radiation measurement. This launched safety systems as planned, and led to the reactor automatically shutting down.
"The fuel was not damaged, consequently there was no risk of harmful radioactive release," TVO said.
Emergency preparedness actions were initiated at the Olkiluoto plant immediately after the failure occurred at 12.22pm yesterday and ended at 4.50pm "when the reason for the failure became clear and the plant unit's risk level went back to normal", TVO said. The plant is now "working according to standard procedures".
The country's Radiation and Nuclear Safety Authority (STUK) said it continued the oversight of the situation during the night. It noted the radiation situation at the plant and in the environment is normal. No radioactivity was released during the incident that would have had any impact on people or the environment, it noted, adding that no workers at the plant had been exposed to radiation.
TVO said today that Olkiluoto 2 is being prepared for a cold shutdown state, "which means that the plant unit's reactor will be driven down and cooled off. In this case, there is no pressure in the reactor and the temperature is below 100 degrees".
The immediate repair work has progressed as planned, it added. "The plant unit is not producing electricity for the time being and information about the plant rejoining the grid will be given separately at a later time," it said.
Olkiluoto 2 is an 890 MWe boiling water reactor which began commercial operation in July 1982.
