Human error leads to water spill at Finnish EPR

Monday, 10 March 2025

Operator Teollisuuden Voima Oyj (TVO) said that on 7 March "a significant operating event occurred during the maintenance work when radioactive coolant leaked at the reactor plant into part of the rooms of the containment ... the significance of the event to radiation safety was low, in the end, owing to the safety actions taken".

In a pressurised water reactor, the reactor coolant is heated in the reactor pressure vessel of the nuclear power plant and then flows in the reactor coolant piping to the steam generator where a heat transfer to the secondary circuit occurs. The coolant is then pumped from the steam generators back to the pressure vessel.

TVO noted the reactor coolant that leaked into the containment flowed from the rooms into a drainage system that is designed to collect and drain any reactor coolant leakages. "The incident did not pose any risk to the personnel, the environment, or nuclear safety," TVO stressed.

"The radioactive wastewater will be handled in accordance with appropriate procedures utilising various systems," the company said. "Cleaning work was commenced quickly in the containment rooms and personnel instructed about necessary changes in accessible areas."

Finland's Radiation and Nuclear Safety Authority (STUK) said it is monitoring the annual maintenance at Olkiluoto 3 (OL3) and was informed of the leak immediately after it occurred. "TVO reacted quickly to the incident and started to remedy the situation immediately," it noted. "STUK monitored TVO's cleanup work and follow-up measures during the weekend."

STUK added: "TVO has started investigating the cause of the incident and will later submit a report to STUK on the results of the investigation, which will also specify measures to prevent a recurrence of the incident."

The annual outage started at the unit on 1 March and is planned to continue until early May. TVO said annual outage work continues at the unit as planned and the coolant leakage is not expected to impact the duration of the outage.

Apart from refuelling, the most important activities during the annual outage of OL3 include a containment leak-tightness test, work on the reactor pumps, warranty work carried out by the plant supplier as well as steam generator washes and inspections. In addition to TVO's own personnel, about 1000 employees of subcontractors are taking part in the current annual outage works at OL3, with about half of them coming from abroad.

The Olkiluoto 3 EPR began regular electricity production in April 2023 and entered commercial operation the following month. Although this is the 1600 MWe reactor's second annual outage, it is the first time it has undergone a service outage for an operating cycle of 18 months. The next annual outage at OL3 is planned in September 2026.