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Surveillance news

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STUK supervises the commissioning of the third unit of the Olkiluoto nuclear power plant. TVO started loading fuel into the plant's reactor in March 2021 and received a permit from STUK to start up the nuclear reactor on 16 December.

These writings describe the start-up of a new nuclear power plant from the perspective of the supervisory authority.

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STUK's OL3 supervision during power tests (25–30%)

30 March 2022

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. According to the schedule published by TVO, regular electricity production will start in July 2022.

 

 

STUK's OL3 supervision during power tests (5–30%)

7.2. 2022

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's OL3 supervision during the first criticality and low power tests

4.1.2022

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. According to the timetable published by TVO, this is scheduled to start in June 2022.
 

STUK's OL3 supervision from fuel loading to the beginning of criticality

​​​​​​​16.12.2021

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%.

The regular electricity generation and commercial operation of the plant will start after the nuclear commissioning phase. According to the timetable published by TVO, this is scheduled to start in June 2022.