Fortum Power and Heat has completed this year’s annual outage of the Loviisa nuclear power plant. The Radiation and Nuclear Safety Authority supervised that the annual outage work was carried out safely and in accordance with provisions
The annual outage of Reactor 2 at the Loviisa nuclear power plant took place during the period 17 August–18 September while the annual outage of Reactor 1 was carried out during the period 6 September–3 October. Around 30 experts from the Radiation and Nuclear Safety Authority (STUK) took part in supervision of the annual outage. STUK inspected and approved the maintenance plans drawn up by the nuclear power company and supervised performance of the work.
This year, both reactors were scheduled to have a short annual outage, during which the power company would carry out fuel replacement and normal maintenance and inspection work, as well as some modifications and safety improvements. During the annual outage of Reactor 2, a leaking fuel assembly was removed. The fuel leak did not compromise radiation safety.
During Reactor 2’s fuel replacement, one of the control rods used to control the reactor’s power level became detached from the fuel charging machine’s clamp and fell into the pool. Fuel is stored in the pool during maintenance of the reactor and the replacement of fuel. The control rod was dented and could not be fed back into the reactor. The fuel stored in the spent fuel pool and the pool’s structures were not damaged.
The Radiation and Nuclear Safety Authority assessed the significance of the event to safety and concluded that it was an exceptional event affecting safety, meaning it is in class 1 on the International Nuclear Event Scale (INES). The event did not pose a risk to those working at the plant or to nuclear or radiation safety. As the event was similar to an event at the Loviisa power plant’s spent fuel storage last January where a fuel bundle fell, the INES class of the event was raised from 0 to 1 due to recurrence. Fortum will conduct an event investigation based on which it will determine the necessary corrective measures to prevent similar events.
The annual outage of Reactor 2 at the Loviisa nuclear power plant lasted considerably longer than originally planned. The outage took extra time in particular due to the repair of a back-up diesel generator that broke during the test run and the repair of faults detected in the seals of the reactor coolant pumps. These events did not affect safety.
The annual outage of Reactor 1 also took more time than Fortum had planned for. The reason for this was that boron dilute had leaked onto the platform during the final stages of the maintenance. The event in itself did not affect safety, but it took many days for Fortum to clean up the resulting mess. The leak resulted from the accumulator’s valve being opened incorrectly during maintenance.
The Loviisa nuclear power plant’s reactors are pressurised water reactors, in which boron-containing water is used both as normal reactor coolant and in accumulators, from which more water can be fed into the reactor if necessary. STUK required that Fortum provide an account of the measures taken after the leak was observed and the results of these measures before issuing a start-up permit for the reactor. From the point of view of plant safety, it is important that the boron dilute that leaked outside the reactor was thoroughly cleaned to prevent boron dilute residue from damaging the structures outside the reactor pressure vessel during the reactor’s operation.
On 16 September, STUK found in its start-up inspection that Loviisa nuclear power plant’s Reactor 2 was safe to start up. STUK came to the same conclusion during its start-up inspection for Reactor 1 on 30 September.
Radiation protection of workers
Special attention is paid to radiation safety and the radiation doses to workers during annual outages. STUK monitors the radiation doses to workers because maintenance work is carried out in places where people cannot stay during the normal operation of the plant due to high radiation dose rates.
The radiation doses to workers during the completed outages were significantly lower than the dose limits set. The radiation doses were approximately the same as during the 2023 annual outage, despite the extra work caused by unexpected events this year. Over the past few years, Fortum has succeeded in significantly reducing the radiation doses to workers during annual outages.
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