Olkiluoto 3

In January, the Olkiluoto 3 (OL3) plant started a planned repair and maintenance shutdown to check the feedwater pumps. Following these inspections, all four feedwater pumps were replaced with new, more durable impellers. During the repair shutdown, TVO and the plant supplier also carried out other necessary repair, maintenance and inspection work. After the shutdown, during the ramp-up of the plant, normal operational tests were carried out on the safety valve stations of the pressure vessel. The tests did not achieve acceptable results and TVO decided to service the valve stations. For the maintenance, TVO decided to drive the plant back to cold mode on 11 February 2023. The function of the pressure relief valves is to provide overpressure protection for the primary reactor circuit during operation and in the event of malfunctions and accidents. It is not safe to operate the plant if the valve stations are not in working order.

STUK monitored the activities at Olkiluoto during the maintenance shutdown and carried out inspections, for example, at the pressure vessel safety valve stations. STUK required TVO to report on the pressure vessel safety valves before starting up the plant. In addition, STUK required a report on the operational condition of some of the connectors for safety critical measurements. This was required because TVO had found deficiencies in the connectors during its own inspections. Based on these findings, TVO had carried out further inspections and corrective measures to ensure the operational condition. STUK had no objections to the reports.

The pressure regulator relief valve stations were serviced in February and March, and during the plant start-up they were functionally tested to ensure that the valves were functioning properly. The operational tests were carried out satisfactorily and the commissioning tests could be continued. The commissioning tests included a performance test as part of the complementary power tests and test runs by the plant supplier and TVO to ensure a trouble-free run and power control tests.

The tests were successfully completed and the commissioning ended on 16 April, after which OL3 started commercial operation, i.e. regular electricity production. As a result, the overall responsibility for the operation and maintenance of OL3 was transferred from the plant supplier to TVO. Overall, the commissioning tests carried out at the plant were successful from the point of view of nuclear safety.

TVO did not need a special licence from STUK to start commercial operation, but STUK monitored the preparation for commercial operation as part of its supervision and carried out a separate inspection to review readiness in March 2023.As a conclusion of the inspection, STUK required TVO to submit a proposal for procedures under warranty to ensure that during future operating cycles any defects affecting nuclear safety can be corrected without additional delays. TVO presented the guarantee procedures to STUK in April and responded or presented measures to STUK's earlier open requirements. 

According to the information received from the plant supplier, some fatigue analyses related to the plant's primary circuit components will have to be re-evaluated. Based on strength calculations for the second EPR unit, anomalies have been found in the fatigue analyses of the welded joints of the pressure vessel supports calculated for OL3, which significantly reduce the calculated lifetime of the supports. STUK has accepted the report and did not see any obstacle to the operation of the plant, but in its decision it requested measures to ensure that the lifetime of the pressure vessel complies with the design criteria. In addition, STUK required a study of the effects of the deviation for the steam generators. In addition to the above-mentioned studies, STUK has required an assessment of the effects of stress corrosion cracking phenomena observed at the French plants on the OL3 unit. An extension until June 2023 has been granted for this study. Although the studies have not yet been received, it is not considered that this will have an immediate impact on the commercial operation of the plant. 

From now on, STUK will oversee OL3 in accordance with the regulatory oversight procedures for operating nuclear power plants, and in the future the oversight will be reported as part of STUK's annual reporting.

In December–January, Teollisuuden Voima and the plant provider continued to commission OL3 with full power tests. Prior to this, there was a maintenance and repair outage, where the main focus was on the main feed water pumps. After the commissioning period, the plant went through another repair and maintenance outage, in which the feed water pumps are subjected to additional inspections as planned.

In connection with the inspections of the main feed water pumps during the previous outage, cracks were found in all the impellers of these pumps. Based on the clarification, TVO and the plant provider decided to continue the commissioning tests by leaving cracked impellers in two feed water pumps. The loads on the pump impellers were determined by technical flow calculations and the progression of the crack by fatigue analyses and crack growth calculations. Based on the analyses, the growth of cracks does not progress to such an extent that severe damage to the pumps would occur. The pumps will be re-checked during the current outage to ensure that the assessment is correct. From the point of view of the usability of the plant and electricity production, the operation of feed water pumps is necessary, but they are not significant from the nuclear safety point of view.

Before continuing the commissioning tests with the cracked impellers, STUK required TVO to provide a report in order to continue safe use (STUK 7/G42301/2022). Based on the clarifications, preparations have been made for damage caused in the event of a pump failure. From a nuclear safety point of view, the size of the loose parts reaching the steam generator is estimated to be so small that it is not considered to pose a significant risk to the integrity of the steam generator. Although the clarifications show that the impeller does not break, TVO and the plant provider have investigated the safety impact of a possible loose part on the plant and people and taken precautions in case of breakages. STUK considered that the clarification was appropriate, after which TVO was allowed to start the plant at its own discretion and to continue the commissioning tests. The commissioning continued from the end of December to the beginning of January.

The plant's response in possible operational occurrences has been tested in the completed commissioning tests. At the end of December, when the reactor power was at its full capacity, the behaviour of the plant was tested, among other things, in the event of a turbine trip and shutdown of the main reactor coolant pump as well as the plant’s house load operation, where the OL3 was disconnected from the external power grid and the plant was left to feed electricity only to the plant's own consumers.STUK reviewed the plans and instructions of the power tests and observed the most significant tests in OL3’s control room. Reporting of the results of the main reactor coolant pump shutdown tests is still in progress at TVO, but during the test, the plant was found to behave safely. The commissioning tests carried out on the plant were successful as a whole from the point of view of nuclear safety.
Related to the commissioning tests Fingrid was unable to conduct a Fault Ride Through test at this stage, as problems arose during Fingrid's test arrangements. The Fault Ride Through test is not relevant from the point of view of nuclear safety, but it verifies the operation of the plant and its ability to support the electricity grid in the event of a failure in the electricity grid. Fingrid's press release on the test: https://www.fingrid.fi/en/news/news/2022/fingrid-to-test-the-functionality-of-the-electricity-system-with-fault-ride-through-of-olkiluoto-3/ 

In connection with the commissioning of OL3, there is still a one-month test run period before the start of regular electricity production. This will be carried out after the ongoing maintenance outage.

The electricity production of the plant will vary during the nuclear commissioning phase and regular electricity production and commercial operation will start after the commissioning phase. According to the schedule published by TVO, regular electricity production will start in March 2023.

Teollisuuden Voima and the plant supplier continued to commission OL3 in September–October through tests where the plant's capacity was at 60–100% of its full capacity. During the commissioning period, there have also been repair needs that have led to the plant and reactor being shut down. During the ongoing repair outage, the most significant work is the inspection and repair of the main feed water pumps, but other repair and maintenance work has also been carried out in the same time.

During inspections of the main feed water pumps, the pump impellers were found to be cracked in each of the four main feed water pumps. TVO, the plant provider and the equipment supplier have carried out a thorough investigation to find out the root cause of the matter and determine corrective actions. From the point of view of the usability of the plant and electricity production, the operation of feedwater pumps is necessary, but they are not significant from the nuclear safety point of view.

Before continuing the commissioning, STUK requires TVO to provide a clarification report in order to continue safe use (STUK 6/G42301/2022). After the clarification, TVO can make a decision on starting the nuclear power plant and continuing the commissioning. After that, if there is a significant deviation from the previously approved commissioning plan, TVO must submit a report to STUK on the changes and their safety effects.

The plant's response in possible operational occurrences has also been tested in the completed commissioning tests. With the reactor power at full capacity, the plant's behaviour in the event of the loss of the feed water pump was tested, among other things, a reactor trip test was carried out as planned. STUK reviewed the plans and instructions of the power tests and observed the most significant tests in Olkiluoto. In addition, STUK reviewed the results of the tests. The commissioning tests carried out on the plant were successful as a whole, and the plant behaved safely and according to the plans.

The commissioning tests carried out so far and the clarifications have shown that the plant is operating as planned. There are still some full power level tests according to the commissioning plan that need to be performed and they must be completed before commercial use. To operate the plant at full capacity, three main feed water pumps are required.

STUK observes the most significant tests on site and checks the results of the commissioning tests.

The electricity production of the plant will vary during the nuclear commissioning phase and regular electricity production and commercial operation will start after the commissioning phase. According to TVO's news 9 December, regular electricity production cold start earliast in February 2023.

Teollisuuden Voima (TVO) and the plant provider have continued to commission OL3 through tests carried out at 60% of its full capacity. This was preceded by a repair outage, as detached parts of steam guide plates were found in the turbine’s steam reheater in May. At the end of the repair, TVO provided STUK with a clarification and justifications according to which the loose part findings have no impact on nuclear safety.  In connection with the power increase after the repair outage, TVO and the plant provider also discovered the need to update the operating automation software. TVO provided STUK with a clarification and, after the update, the commissioning tests continued as planned.

The plant response in possible operational occurrences has also been tested in the commissioning tests. With a reactor power of approximately 60%, the plant was tested as planned for the switch to house load operation, where the OL3 was disconnected from the external power grid and the plant was left to feed electricity only to the plant's own consumers. The test was passed.

Prior to the repair outage, the plant underwent the planned reactor trip test. In the reactor trip test, the resulting launch of the turbine trip took longer than expected. TVO provided STUK with a modification plan and a safety assessment, on the basis of which the maximum delay between the reactor and turbine trips used in the safety analyses was extended. In the safety assessment, TVO analysed the impact of the turbine trip delay on the plant behaviour in different situations. STUK assessed the impact of the change as minor and approved the modification related to the initial values and acceptance criteria of the analyses.

STUK reviewed the plans and instructions of the power tests and observed the most significant tests in Olkiluoto. In addition, STUK reviewed the results of the tests. In general, the commissioning tests carried out on the plant were successful, and the facility behaved safely and according to the plans.

The commissioning tests carried out so far and the evaluations have shown that the plant is operating as planned and that it is safe to increase the plant's power in accordance with the commissioning testing plan.

TVO submitted an application for licence to increase the power of the reactor to more than 60% of the full 4300 MW thermal capacity to STUK on 5 September 2022. STUK approved the application with its decision STUK 3/G44401/2022, 6 September 2022.

Next, the nuclear commissioning of OL3 will continue with a step-by-step increase in power from 60% to 100%. During this process, the plant vendor and TVO will ensure that the plant functions at these power levels as planned, and the plant behaviour during different operational occurrences is also tested. The tests are carried out in accordance with the previously prepared plans and guidelines which are approved by STUK. STUK observes the most significant tests on site and reviews the results of the commissioning tests. If there are no significant defects in the use and behaviour of the plant and no significant deviations during the tests from the safety point of view occurs, a separate permission to continue is no longer needed from STUK during the nuclear commissioning phase.

The electricity production of the plant will vary during the nuclear commissioning phase and regular electricity production and commercial operation will start after the commissioning phase. According to the schedule published by TVO, regular electricity production will start in December 2022.

Last May, detached parts of steam guide plates were found in the turbine’s steam reheater of OL3, after which a repair outage started in the plant. At the end of the repair, TVO provided STUK with a clarification and justifications according to which the foreign material findings have no impact on nuclear safety. TVO and the plant supplier then continued to commission the plant and STUK supervised the commissioning.

 In connection with the power increase after the repair outage, it became clear to TVO and the plant supplier that the turbine-side automation systems of the nuclear power plant need to be updated. TVO and the plant supplier will continue to carry out the 60% plant tests after the update. In the subsequent commissioning phase, the plant’s power will be increased above 60% and eventually to its full capacity. Before increasing the power, TVO needs STUK’s permission.

The commissioning of OL3 has continued with tests at 30-60% of full reactor power, the implementation of the necessary modifications and the definition of the correct settings for the turbine island. During commissioning, TVO and the plant supplier have also carried out repair and inspection work on the turbine island, which required the plant to be run in cold shutdown mode. The maintenance shutdown is still ongoing. STUK has required TVO to clarify that the loose particles found in the turbine island, i.e. the secondary containment, do not have an impact on nuclear safety before the plant is started up.

Before the maintenance shutdown, TVO and the plant supplier carried out commissioning tests as planned. The commissioning tests also included testing the plant's response to expected operational failures, such as reactor and turbine shutdowns at 60% power level. A separate turbine trip test was successfully performed. In the reactor shutdown test, the time taken to start the turbine shutdown following the event was longer than expected. An assessment of this and a modification of the acceptance criterion are still under consideration by STUK, after which an assessment of the acceptability of the test can be made.

STUK checked the plans and instructions for the power tests and monitored the most important tests at Olkiluoto. In addition, STUK reviewed the results of the tests carried out. Based on the measurements, the reactor core and primary circuit have behaved largely as expected during the tests. There have been small differences in the power distribution of the reactor core compared to the preliminary calculations, but these are not relevant for the safe operation of the reactor. Overall, the commissioning tests on the plant have been mainly successful and the plant has behaved safely.

During the commissioning tests of this phase, one unforeseen event has occurred during the reactor quick shutdown test. The fast shutdown was tested as planned, but at the same time there was an unscheduled safety function activation, where the emergency booster pumps started unexpectedly. The safety initiation ensured the safe status of the plant by injecting additional boron into the reactor to stop the chain reaction, and the event did not pose a hazard to humans or the environment. The start-up occurred due to an erroneously triggered signal from the automation system. As a corrective measure, a change in the automation system will be made to prevent a similar unexpected event. The initial INES category of the event was 0. The INES category is a classification scale for nuclear and radiological incidents. A category 0 event is an event of such minor safety significance that it cannot be placed on the proper scale.

In the past, higher than expected neutron flux oscillations have been observed in other reactors of the same type. This oscillation has been found to be caused by the lateral movement of the fuel assemblies due to oscillations in the reactor coolant flow, which in turn contributes to the deceleration of neutrons in the reactor. In this respect, TVO submitted an application to STUK for modification work on OL3. STUK approved the modification work, which includes changes to the operation of the automation and safety analyses to demonstrate safe operation.

After the ongoing repair shutdown, the nuclear commissioning of OL3 will continue at the 60% reactor power level, including a test of the transition to self-sustained operation. During this test, the plant will remain in operation to supply electricity only for the plant's own needs. After that, TVO will need STUK's approval to increase the reactor's power to more than 60% of full power.

In the past few months, the commissioning of OL3 has proceeded primarily by tests conducted at 25–30% of reactor power, including the implementation of any necessary modifications and configuration of the settings of the turbine island.

The plant was synchronized with the grid in March when the reactor power was at 25% of its full capacity. The commissioning tests have also included testing the plant's response in anticipated operational occurrences. With the reactor power at approximately 30%, the tests included a turbine trip, the switchover from a 400 kV grid supply to a 110 kV grid supply and the response to the loss of offsite power.

STUK reviewed the plans and instructions of the power tests and observed the most significant tests in Olkiluoto. In addition, STUK reviewed the results of the tests. The measurements show that the reactor core and primary circuit behaved as expected during the tests. Most of the commissioning tests carried out on the plant were successful and the plant unit behaved safely.

Since the February review, one unplanned reactor trip has occurred in the commissioning tests. A reactor trip is a normal safety protection function that shuts down the plant by using control rods and adding boron in the cooling water. A reactor trip is automatic and is typically triggered when the process parameters deviate from the expected state. Safety functions like this ensure that safety is not compromised.

The reactor trip occurred on 27 February 2022 due to high steam pressure in the secondary circuit. The event originated from the condenser system in the secondary circuit, which, through a chain of events, led to the closing of the turbine bypass system. When heat could not be transferred through the bypass system to the condenser, the pressure increased, resulting in a reactor trip and the opening of the steam venting station in the secondary circuit. The incident did not endanger safety and the safety systems functioned as designed during the reactor trip.
The INES rating of the incident was 0. INES rating is the rating scale of nuclear and radiological events. Rating 0 refers to an event whose safety significance is so limited that it cannot be placed on the actual scale. An example of such an event is a fast shutdown of the reactor (a reactor trip) when all the plant's systems are operating as planned during the situation.

The commissioning tests carried out have shown that the plant is operating as planned and that it is safe to increase the plant's power to the level required for the next commissioning phase.

TVO submitted an application for licence to increase the power of the reactor to more than 30% of the full 4300 MW thermal capacity to STUK on 29 March 2022. STUK approved the application with its decision STUK 30 March 2022.

Next, the nuclear commissioning of OL3 will continue with a step-by-step increase in power from 30% to 60%. During this process, the plant provider and TVO will ensure that the plant functions at these power levels as planned, and the plant behaviour during operational occurrences is also tested. The tests are carried out in accordance with the previously prepared plans and guidelines which are approved by STUK. STUK observes the most significant tests on site and checks the results of the commissioning tests.

Next, TVO will need an authorization granted by STUK to the OL3 unit when the commissioning tests are to be carried out at more than 60% of power.

The electricity production of the plant will vary during the nuclear commissioning phase and regular electricity production and commercial operation will only start after the commissioning phase.

STUK granted Teollisuuden Voima (TVO) permission to increase the power of the OL3 reactor above 5% of its full capacity on 4 January 2022. Subsequently, the commissioning of the plant continued with a gradual increase in power towards 30%. During the power increase, it is ensured that the plant functions as planned.

STUK monitors the commissioning on site in Olkiluoto. When the power was increased from 5% to 30%, the plant supplier and the licence holder carried out measurements and calibrations (power distribution, neutron flux, core instrumentation, adjustment functions, vibration levels), periodic tests and tested the plant's response to various disturbances.

STUK reviewed the plans and instructions of the power tests and observed the most significant tests in Olkiluoto. In addition, STUK has reviewed the results of the tests performed. Most of the tests were successful and the facility behaved safely and according to the plans. The most significant exceptions at the time of commissioning are discussed below.

During the commissioning tests, two reactor trips, which were not part of the tests, occurred. A reactor trip is a normal protective function that stops the reactor quickly and ensures that safety is not compromised. In the reactor trip, the plant is shut down by means of control rods, and this is ensured by adding boron to the cooling water. The automatic protection function is typically triggered when the process parameters deviate from the expected state.

The first reactor trip occurred on 14 January 2022 when the plant tested the steam boiler's low surface limiting function. During the test, the surface of one steam boiler rose above the reactor trip limit. As a result of the event, the plant's automation functions will be changed. During the commissioning phase, it is typical that the adjustment functions are tuned and the need for changes may still occur.

The second reactor trip occurred on 29 January 2022 when the generator was being synchronised with the 400 kV power grid. During the synchronisation test, the average temperature in the primary circuit of the reactor decreased and, as a result, the power of the reactor increased from 25% to 35%. At this stage of commissioning, the reactor has been set to a reactor trip limit of 35%, which resulted in a reactor trip triggered by the power increase protection function. The plant will continue in operation once the necessary changes have been made.

The above events did not compromise safety. The initial INES rating for both events was 0. INES is the severity scale of nuclear and radiological events. Rating INES 0 includes an event whose safety significance is so limited that it cannot be placed on the actual scale. Rating 0 includes, for example, a reactor trip when all the plant's systems are operating as planned during the situation.

The commissioning tests have shown, so far, that the plant functions safely. When the commissioning tests have shown that something needs to be changed, it is ensured that the plant is in a safe state to make the changes, and the plant will be run down if necessary.
STUK monitors the implementation of the most significant changes and checks the related documents. The operations of the plant according to the plan will be verified by new tests after the modifications. After this, the plant will continue to be commissioned in accordance with the previously prepared commissioning plans and guidelines which are approved by STUK. The next time TVO will need STUK's permission is when it will increase the power of the reactor above 30% of its full capacity.

The electricity production of the plant will vary during the nuclear commissioning phase and regular electricity production and commercial operation will only start after the commissioning phase. According to the timetable published by TVO, electricity production will start at the end of February and regular electricity production in July 2022.

STUK granted OL3 the criticality and low power test permission on 16 December 2021, and verified the start-up preparedness in Olkiluoto on 18 December 2021. Subsequently, preparation actions related to the first criticality, i.e. the start-up of the reactor, were made at the plant. The plant can be deemed to have been started on 21 December 2021, when the reactor core was made critical for the first time.

STUK followed the start-up on-site in Olkiluoto. OL3 was started by diluting the boron concentration of the water in the reactor and controlling the control rods. The operational staff of Teollisuuden voima (TVO) performed the planned measures in a controlled and safely. TVO's responsibility for the situation and the plant was highlighted and clear. The plant provider (consortium made up of AREVA GmbH, AREVA NP SAS and Siemens AG) is involved during the commissioning, but TVO is responsible for the nuclear and radiation safety. Only TVO's licensed nuclear power plant controllers, who are approved by STUK, are allowed to control the plant from its control room.

After the plant was started, TVO and the plant providers carried out low  power tests.  The low power tests confirmed that the reactor core behaves as expected and that the core fulfils the assumptions of the safety analyses. Low power tests were carried out in several control rod positions and measured quantities related to reactor physics (measurements of critical boron concentration, isothermal temperature coefficient and reactivity effect of the control rods).
STUK reviewed the low power test plans and instructions and observed the tests in Olkiluoto. In addition, STUK reviewed the results of the tests. The tests were carried out successfully, the reactor behaved very accurately according to the preliminary calculations and the pre-defined safety criteria were met.

The commissioning tests carried out have shown that the plant is operating as planned and that it is safe to increase the plant's power to the level required for the next commissioning phase.
TVO submitted an application for licence to increase the power of the reactor to more than 5% of the full 4300 MW thermal capacity to STUK on 3.1.2022. STUK approved the application with its decision STUK 1/G44401/2022, 4.1.2022.

Next, the nuclear commissioning of OL3 will continue with a step-by-step increase in power from 5% to 30%. During this process, it will be ensured that the plant functions in different power levels as planned, and the plant behaviour during operational occurrences is also tested. At this point, the plant's electric generator will also be synchronised with the national grid for the first time, and electricity production will begin. The tests are carried out in accordance with the previously prepared plans and guidelines which are approved by STUK. STUK observes the most significant tests on site and checks the results of the commissioning tests. STUK’s authorization is required next time the power output is more than 30% and then more than 60%.

The regular electricity generation and commercial operation of the plant will start after the nuclear commissioning phase. 

The loading of fuel to OL3 started at the end of March 2021. STUK observed the start and progress of the loading in Olkiluoto. The loading was carried out successfully, after which the pre-critical tests, i.e. hot functional tests, were performed at the OL3, in which the primary circuit was heated and pressurised to the operating conditions by using of the main circulation pumps. During the hot functional tests, the plant's systems and operation were tested as a whole for the parts that could not be tested without fuel in the reactor or that required re-testing after the modifications made.

STUK reviewed the plans and instructions of the tests and observed the most significant tests in Olkiluoto. In addition, the results of the tests were reviewed. During the commissioning tests, the activities of the licence holder and the plant provider, and in particular the maturity of the plant management, were also assessed.

The commissioning tests are used to demonstrate that the systems and plant are working as planned, and it is normal during the commissioning phase that a need for changes may still occur. The need for changes in the commissioning of OL3 caused delays in the commissioning, but the good performance of TVO and the plant provider ensured that these were properly addressed: the problems were resolved, the necessary changes and repairs were made and the functionality was verified by re-testing. STUK monitored the progress of the commissioning tests, the resolution of problems and checked the most significant modification work plans as well as carried out the necessary inspections in the repaired and modified sites. The need for modifications and scheduling delays caused by the turbine blockage during the hot functional tests made it possible to carry out modifications which were originally planned for later stages. For example, the structure of mechanical control valves of the pressuriser relief valves was improved by installing new parts whose material and design had been modified to avoid stress corrosion to the valves.

The commissioning tests carried out before the first criticality, including the re-tests following the modifications, have shown that the plant is now operating as planned. During the hot functional tests, it was also shown that the dampers, which added to the pressuriser surge line have sufficiently reduced the vibration of the line.

The commissioning is also a learning lesson for all the parties involved. TVO investigated the exceptional operational events that occurred during the commissioning tests, took immediate action based on them and carried out incident investigation on the most significant incidents. The incidents did not create nuclear or radiation safety hazards. In order to improve the safe use and operation of the plant, it was important that TVO systematically reviewed the incidents and made the necessary changes to the plant and the way it is used. Corrective measures included changes to the triggering limits of security functions in the plant's automation systems.

TVO submitted an application for a criticality permit to STUK on 8 December, which STUK approved by its decision on 16 December. The decision and the attached presentation memorandum discussed in more detail the time between fuel loading and first criticality, incidents and control measures as well as the fulfilment of the conditions of the criticality permit. STUK will still carry out a start-up preparedness verification before OL3's first criticality, in which context it will be verified that the open issues presented in the aforementioned decision have been fixed.

Next, the nuclear commissioning phase of OL3 will continue with the first criticality, i.e. the plant's reactor start-up, followed by low power tests. Low power tests ensure that the reactor core behaves as anticipated and that the plant and its systems are functioning as planned also when the reactor is working in power operation. After the low power tests, the power of the plant will be increased gradually. Power tests ensure that the plant functions in different power levels as planned, and the plant behaviour during operational occurrences is also tested. The tests are carried out in accordance with the previously prepared plans and guidelines, which STUK has inspected. STUK observes the most significant tests on site and checks the results of the commissioning tests. During the power tests, permits issued by STUK are required for power levels of 5%, 30% and 60%.