More information on the plant disturbance at Olkiluoto 2

Description of the disturbance

At noon on Thursday 10th December, 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. For this reason, the repair work took longer than planned, about two hours. During the prolonged 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 shutdown cooling system was started, including the reactor water clean-up system. The filter of the cleaning system is designed to withstand water of about 70 degrees, but due to the aforementioned reason, water of about 100 degrees was momentarily able to flow into it. In a boiling water reactor, the water circulates in a closed circuit, in which the water is constantly being cleaned with filters. In the disturbance situation which occurred at OL2, the heated up water dissolved substances from the filter into the reactor water. The substances were activated as they passed through the reactor core. Consequently, the activity levels (radiation levels) of the steam moving through the main steam lines momentarily rose to about 3 to 4 times higher compared with the normal activity level.

The safety systems operated as planned

The momentary increase of the activity level in the main steam lines caused the automatic isolation of the containment, the so-called i-isolation. This is one of the safety functions of the plant, and it's job is to isolate the reactor's containment and the reactor so that no radiation can reach the environment. The i-isolation is designed to operate in the case of an internal pipe leakage or significant fuel damage. In this case, the cause was neither. The i-isolation automatically initiates water spraying in the containment and a reactor trip, among other things. As a result of a reactor trip, all the control rods automatically go inside the reactor, and the chain reaction stops.

The plant and all of the safety systems worked as planned during the situation. The personnel operating the plant took action according to instructions. Similar events and their control have been practiced regularly with a simulator.

The aforementioned i-isolation is always categorized as a site area emergency, or an emergency preparedness situation. The emergency preparedness situation was initiated at the OL2 plant unit at 12.32 according to instructions. The emergency organization was summoned, and employees proceeded to assembly points, as has been practiced. In emergency preparedness actions at a nuclear power plant, the worst end result is always prepared for until proven otherwise. This was also the case during this situation.

The preliminary categorization of the event is INES 0

The disturbance did not cause danger to people or the environment. The preliminary categorization of the event on the INES scale is 0, which means that it did not have significance in relation to nuclear or radiation safety. The event did not have an influence on employees' radiation doses, since employees do not normally work in the area with the increased radiation level. Increased radiation levels were not detected in outdoor areas.

Restarting the plant requires careful inspection and testing

The use of a nuclear power plant is based on responsible and safe operations, which is why for instance after annual outages, restarting the plant requires careful inspection and testing.

Due to the water spraying of the containment, the equipment and systems inside it may be wet or may have been exposed to water. Therefore, the important equipment and systems inside the containment are inspected especially carefully. This means for instance the following inspection and testing actions:

• We will check electrical motors, sensors, connectors, transmitters, and limit switches.

• We will inspect insulation, penetrations, and the cleanliness of the water in the suppression pool.

• We will do operational tests on the valves of the containment, in which the valves are moved.

• We will test the maneuvering devices of the control rods and the reactor trip function.

Before restarting the plant unit, TVO will do a detailed investigation on the progression of events, as well as the necessary immediate measures to avoid a similar disturbance. The Radiation and Nuclear Safety Authority (STUK) will give the permit for restarting the plant unit. TVO is preparing the application for the restart permit for STUK. The application includes a comprehensive report of the actions required before the restart and a safety evaluation.

We informed on the schedule for the plant unit being back in operation in a news piece earlier today 13th December 2020 at 11:14.