OL3, the newest nuclear power plant unit of Teollisuuden Voima (TVO), has been in commercial operation and produced electricity for almost a year, and the first period of operation before service has now ended. The plant's first annual maintenance began on 2 March. 

For the Radiation and Nuclear Safety Authority (STUK), the first operating year of OL3 has been a busy one. STUK's regulatory control did not end with the completion of the plant project, although people sometimes ask me why we are still supervising the plant, even though it should be ready.

I could explain that, if you have a car, simply getting a driver's licence and having your car pass the motor vehicle inspection won't be enough; the authorities monitor traffic as well to ensure that cars are driven safely and in compliance with traffic rules.

In practice, STUK's supervision means that we are present at the nuclear power plant, collecting observations, monitoring the operations and, if necessary, intervening in them. We also review the plans and reports related to OL3's practices and have regular discussions with TVO about the plant's situation, observations and deviations.

In addition to being a radiation and nuclear safety authority, we also acts as a pressure equipment authority at the nuclear power plant, carrying out inspections related to pressure equipment.

First year of OL3

During the first operating period, the use of OL3 has been safe and TVO's actions have mainly been in accordance with the requirements.

There have been some instances of deviations from the requirements. The instances have been both technical and operational, but they have not posed an immediate threat to safety. It is only human that mistakes sometimes happen. The power plant's operating methods and processes must be built to enable the detection of errors and the limitation of their consequences.

Significant incidents are always thoroughly investigated. The aim is not to find the culprits but to identify the causes of errors so that they can be avoided in the future. Incidents are always a learning experience, resulting in corrective actions. This has also been the approach at OL3. Continuous learning and improvement of practices, as well as a strong understanding of and commitment to the importance of nuclear safety, form the basis of the operations.

In addition to monitoring the technical condition of the plant, we also monitor TVO's proper attitude towards nuclear safety.

The task of the nuclear power plant is to produce electricity, but trust in the practices of TVO and OL3 is built on demonstrating that nuclear safety is the number one priority. 

During the first year of operation, STUK has, for justified reasons, approved TVO's proposal to continue the plant's power operation in individual situations where it has not been possible to fully comply with the plant's technical operating conditions in terms of safety. The situations have been short-term and TVO has been able to reliably justify how nuclear safety has been ensured during the unusual situation.

TVO's practices have been careful and safe, but it is also evident that the plant is new and the operating methods are still being developed. In other words, the new plant does not yet have such strong and well-established operating methods as the nuclear power plant units that have been in operation for a longer time. This is natural but has also resulted in some busy work for STUK. Some situations have come up unexpectedly.

For example, a partial failure of the emergency cooling system of the air conditioning in the main control room in October 2023 caused some haste. It is only required under certain emergencies to ensure that the conditions in the main control room remain within acceptable parameters. STUK approved the deviation and the additional measures taken to ensure safety for the duration of correcting the defect.

The goal is for the foundations of safety to be further strengthened as the practices become more systematic and predictable. When viewed as a whole, OL3 and TVO have operated safely.

First annual maintenance of OL3

It is now time for the first annual maintenance of the new nuclear power plant. Let's once again use a car as an example of annual maintenance: you take the car to the workshop and disassemble, inspect and service the car, repair anything that's broken, and finally put everything back together.

The maintenance of a nuclear power plant also includes filling the tank with some superfuel to last the next year and, finally, testing the system to ensure that everything works. After an inspection the journey continues, always in line with traffic rules.

During the first operating period, the OL3 has been serviced as planned, but not everything can be serviced and inspected during power operation, in other words when the plant generates electricity.

For this reason, the planned first annual maintenance outage started in March. During the outage, the reactor pressure vessel will be opened and all nuclear fuels will be transferred out of the reactor for inspections, among other things. After the annual maintenance, some of the spent fuel will remain in the water basin to wait for final disposal and the corresponding amount of new fuel will be added to the reactor. 

Having reviewed TVO's safety-related plans in advance, STUK will monitor the operations at the plant during the annual maintenance. When servicing the system, the cooling of nuclear fuels must be ensured as the fuels continue to produce afterheat even after shutdown and removal from the reactor.

Another key issue is radiation protection measures to keep occupational doses within safe limits and as low as possible during the annual maintenance work. This must always be carefully taken into account at the nuclear power plant, and is the main characteristic that makes the service work of a nuclear power plant different from the service work of other types of power plants.

In annual maintenance, various mechanical inspections are carried out at the plant to ensure that the equipment and components are intact and in good working order. STUK also participates in these inspections.

After the annual maintenance, TVO will be able to restart the plant when STUK grants a start-up permit. The conditions for starting the OL3's second period of use will also be verified at that point. After the annual maintenance, STUK will prepare a summary of the supervision and publish it on STUK's website.

Essi Vanhanen is Oversight Manager of Olkiluoto 3 at STUK