Commissioning EPR Olkiluoto 3 : first insights from the TVO team

The Olkiluoto-3 EPR reactor reached first criticality on December 21, 2021, and numerous physics tests and trials are currently being conducted. To learn more about the initial startup process, Sfen organized a webinar with the Finnish operator TVO which saw more than 700 participants in attendance.

Olkiluoto-3 (OL3) is the first EPR to reach criticality in Europe. OL3 is also the first reactor to be commissioned in Finland in 40 years. On December 21, 2021, less than a week after receiving authorization from the Finnish Nuclear Safety Authority (Stuk), the reactor reached first criticality and TVO is now conducting numerous tests at different power levels. Marjo Mustonen, Senior Vice President for Electricity Production, and Tommi Lamminpää, Principal Engineer, Reactor Supervision at TVO reviewed the steps they took to reach first criticality for the EPR OL3.

First, they discussed the importance of an external source of neutrons for the initial start-up and detailed the two mechanisms that allowed the reactor to reach criticality.

Contribution of Primary Neutron Sources

One of the points presented by TVO during the webinar is the importance of inserting a neutron source into the core (in the form of source rods that are installed in some fuel assemblies) to generate a sufficient neutron flux during start-up. This allows reliable measurement of the neutron flux and any fluctuations caused by the instrumentation (especially during the subcritical state in order to determine the precise moment of first criticality), which is an essential function for safety.

The First Nuclear Reaction

First criticality is the start of the chain reaction process in a nuclear reactor. Two mechanisms are used:
• Decreased boron concentration in the primary circuit: boron is a “nuclear poison (or neutron poison)” due to its capacity for absorbing neutrons. The initial boron concentration ahead of the first criticality phase prevents the nuclear reaction from starting.

• Removal of control rods: the control rods, which also absorb neutrons, make it possible to control the reactor by varying the power levels or stopping it quickly.

Almost all the rods are removed from the core and boron is gradually diluted in the primary circuit to approach criticality. This is then achieved by extracting the last control rods: the production of neutrons by fissions becomes greater than their absorption via boron and control rods. Since it is necessary to approach criticality very gradually, this operation must be repeated several times (dilution should not cause criticality but rather approach it just enough so that the removal of the final rods is sufficient).

Once criticality is reached, the reactor can increase its power.

Testing in Four phases

“Nearly 200 tests on nuclear island and 230 tests on turbine island are performed at different power levels,” explained Mustonen. The first tests involve physics tests, during which core compliance is verified. Several physics metrics are recorded: neutron flux in different areas of the core (flux maps), reactivity, neutron feedback, control rod efficiency, etc. The measured values are compared with predictions of computational codes modeling reactor physics and are then used in safety tests. The parameters of the protection system are also adjusted.

In the first phase (D1), the power increased from 0% to 5%. This phase ended on January 3, 2022. “Zero Power Physics Tests (ZPPT) have been successful and we will build on it,” confirmed Lamminpää. Since January 4, 2022, OL3 has been in the second phase (D2) which involves increasing the power to 30%. During the third phase (D3), the reactor will reach 80% before moving to full power in the fourth and final phase (D4) over several months”.
Lamminpää added, “the next big step is the first synchronization of the turbine generator to the grid, which will happen at 25% power.”
Once in service, the EPR will provide 14% of the country’s electricity.