In the wake of the Teil earthquake of 11 November 2019, ASN takes stock of the earthquake resistance of the French nuclear power plants

When designing a nuclear power plant, how do you determine the earthquake intensity it must be able to withstand?

Earthquakes form part of the natural risks that nuclear facilities must be able to withstand. Seismic protection measures are taken at the installation design stage and reviewed every ten years during the periodic safety reviews to take account of developments in knowledge and changes in regulations.

In France, the characterisation of the seismic risk that each basic nuclear installation (BNI) must be able to withstand is based on a deterministic approach, detailed in basic safety rule RFS 2001-01. This rule is supplemented by ASN Guide 2/01 which defines the design provisions for the seismic protection of civil engineering structures.

The method consists in:

• firstly, determining the "maximum historically probable earthquake" (MHPE) which corresponds to a return period of about 1,000 years. -This level of earthquake can be considered as the most intense level "in human memory" identified in the region concerned;
• then defining the "safe shutdown earthquake" (SSE) which corresponds to an increase in the magnitude of the MHPE of 0.5 on the Richter scale. Furthermore, the SSE is positioned inclusively and conservatively as close as possible to the nuclear site within the seismotectonic zone to which it belongs

The SSE therefore integrates margins with respect to the historical earthquake recorded in the region in question: it is more intense and is positioned a close as possible to the nuclear site.

On some sites, the consideration of paleoseismicity data can lead to supplementing of the movements associated with the SSEs.

Have the lessons from the Fukushima accident been taken into account?

After the Fukushima nuclear power plant (NPP) accident, ASN asked EDF to check the robustness of its NPPs to an even higher earthquake level, the "hardened safety core earthquake" (HSCE), at which the main safety functions must continue to be ensured. The movements of the ground (accelerations) corresponding to the HSCE must be greater than those of the SSE increased by 50%, and greater than those of earthquakes with a return period of 20,000 years. To meet this requirement, EDF has defined a "hardened safety core" of equipment (such as the ultimate backup diesel generator sets) that can withstand the HSCE and is currently being deployed on the reactors.

Are the earthquake levels reassessed during the lifetime of a nuclear installation?

During their 10-yearly periodic safety reviews of the installations, the licensees reassess the earthquake intensity (SSE) to take into account in the safety case. This reassessment is made in the light of developments in historical knowledge and any earthquakes that have occurred since the last review.

Furthermore, ASN can ask that any event which could call into question the hypotheses considered for the design of a facility be taken into account.

The reassessments of earthquake levels regularly lead the licensees to reinforce parts of their installations. If the consequences of a predicted earthquake are unacceptable and the necessary reinforcements cannot be made, ASN can ask that the installation be closed. This was the case with the plutonium treatment facility (ATPu) at Cadarache.

ASN checks that all the installations are designed to withstand an earthquake. It should be noted that some long-term installations or parts of installations (low-level waste storage areas, etc.) or installations undergoing decommissioning (waste recovery or packaging equipment, etc.) can be subject to requirements as appropriate for the risks they represent.

 What are the consequences of the Teil earthquake on the Cruas nuclear power plant?

The Cruas NPP, like all the French NPPs, is equipped with a seismic monitoring system. During the earthquake of 11 November 2019, one of the five sensors exceeded the threshold

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beyond which the reactors must be shut down in order to carry out in-depth verifications. The ground movements recorded were nevertheless about five times lower than the level taken into consideration in the reactor design.

EDF thus carried out an in-depth diagnosis of its installations and has submitted it to ASN. EDF has in particular inspected the civil engineering structures and the condition of the equipment important to safety. At the request of ASN, EDF has carried out tests to check the functioning of the reactor safeguard systems.

The Cruas NPP nuclear island is built on seismic isolators which attenuate the seismic movements. EDF has inspected the condition of these isolators further to the earthquake of 11 November 2019.

During two inspections held on 20 and 22 November, ASN verified some of the EDF inspections by random sampling. The results of all these inspections were examined by ASN before the reactors were restarted.

ASN authorised reactors 2 and 4 to be put back into service on 6 December 2019, and reactor 3 on 11 December 2019.

 What are the consequences of the Teil earthquake on the Tricastin nuclear site?

With regard to the Tricastin NPP, none of the seismic monitoring sensors reached the threshold that makes in-depth verifications necessary.  Furthermore, no damage calling into

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question the safety of the nuclear installations of the Orano Cycle platform on the Tricastin site was observed.

With regard to the embankment protecting the Tricastin NPP, in 2017 EDF detected a risk of failure of one section of the embankment in the event of an earthquake reaching the SSE level. The flooding resulting from such a failure could potentially lead to a nuclear fuel melt accident in the four reactors of the Tricastin NPP and would make deployment of the on-site and off-site emergency management resources particularly difficult.

On 27 September 2017, ASN instructed EDF to reinforce the embankment section concerned so that it would withstand an earthquake of the SSE level, and imposed temporary shutdown of the four reactors. EDF carried out this work.

Following the Fukushima NPP accident, ASN instructed EDF to take measures to cope with "extreme" earthquakes (HSCE). ASN therefore asked EDF to further reinforce the embankment protecting the Tricastin NPP before the end of 2022 so that it can withstand an earthquake of HSCE level. Pending this reinforcement work, ASN has prescribed:

• tightened monitoring of the embankment;
• a plan of action should the water table level in the embankment of the Donzère-Mondragon canal rise;
• on-site human and material resources to repair damage to the embankment further to an earthquake.

 Will the Teil earthquake have any consequences on the resistance criteria of the Tricastin and Cruas NPPs?

Fine characterisation of the Teil earthquake will require a few more months. IRSN has provided some information further to the first investigations:

The seismic risk defined for the third periodic safety review of the Cruas and Tricastin NPPs is based on the earthquake of 8 August 1873 to calculate an MHPE with an Ms magnitude (surface wave magnitude) of 4.7 at a depth of 4 km. This ultimately leads to an SSE of Ms magnitude 5.2 at a depth of 4 km. EDF also uses the paleoseismic earthquake of Courthézon as a reference earthquake (magnitude 6.5 and an epicentral distance of 50 km for Cruas and 27 km for Tricastin).

On 14 November 2019, IRSN estimated that the earthquake of 11 November 2019 corresponds to an Ms magnitude of about 4.5 at a depth of about 2 km and displays characteristics similar to those of the MHPE.

ASN has asked EDF to determine, once it has been characterised and before March 2020 in any case, whether the MHPE (and therefore the SSE) of the Cruas and Tricastin NPPs should be reassessed in the wake of the Teil earthquake. If this is the case, EDF must determine whether these new levels will oblige it to reinforce its installations. ASN will review the entire process and adopt a position on the issue.

As regards the Orano platform at Tricastin, the licensee must propose a reassessment of the reference earthquake in 2022, as part of the periodic safety review of the Georges Besse 2 plant. This reassessment shall take into account the Teil earthquake and, if necessary, revise the characteristics of the reference earthquake for this platform.