Malfunction triggers emergency shutdown at Finnish reactor

11 December 2020

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.

Units 1 and 2 of the Olkiluoto plant (Image: TVO)

TVO said on 13 December that a planned failure inspection of the shutdown cooling system was underway before the disturbance occurred. When the pump of the cooling system was stopped, one of the valves was broken, it said. The repair work took about two hours to complete. During the repair work, hot process water was able to enter backwards longer than planned into the filters of the cleaning system of the reactor due to a difference in pressure. After the completion of the repair work, the cooling system was restarted, including the reactor water clean-up system. The filter of the cleaning system is designed to withstand water of about 70°C, but due to the time to complete the repair work, water of about 100°C was momentarily able to flow into it. This hot water dissolved substances from the filter into the reactor water, which were then activated as they passed through the reactor core. Consequently, the radiation levels of the steam passing through the main steam lines momentarily rose to about 3 to 4 times higher compared with normal levels. This launched safety systems as planned, and led 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 on 10 December 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.

On 13 December, TVO said the incident had been preliminary rated zero on the International Nuclear Event Scale, that is one with no safety significance. The utility also said it expected the unit to be reconnected to the Finnish grid in about two weeks' time.

Olkiluoto 2 is an 890 MWe boiling water reactor which began commercial operation in July 1982.

Researched and written by World Nuclear News