ASN Report 2017

347 ASN report on the state of nuclear safety and radiation protection in France in 2017 Chapter 12  - EDF Nuclear Power Plants Seismic risks Although seismic activity in France is moderate or slight, EDF’s inclusion of this risk in the safety case for its nuclear power reactors is the subject of constant attention on the part of ASN, given the potential consequences for the safety of the facilities. Seismic protection measures are taken into account in the design of the facilities. They are periodically reviewed in line with changing knowledge and changes to the regulations, on the occasion of the periodic safety reviews. Basic Safety Rule (RFS) 2001-01 of 31st May 2001 defines the methodology for determining the seismic risk for surface BNIs (except for radioactive waste long-term repositories). This RFS is supplemented by ASN guide 2/01 of May 2006, which defines acceptable calculation methods for study of the seismic behaviour of nuclear buildings and particular structures such as embankments, tunnels and underground pipes, supports or tanks. Buildings and equipment important for the safety of NPPs are designed to withstand earthquakes of an intensity greater than the most severe earthquakes that have ever occurred in the region. EDF’s NPPs are thus designed for seismic levels with incorporation of the local geological features specific to each one. As part of the periodic safety reviews, the seismic reassessment consists in verifying the adequacy of the seismic design of the facility, taking account of advances in knowledge about seismic activity in the region of the site or in the methods for assessing the seismic behaviour of elements of the facility. The lessons learned from international experience feedback are also analysed and integrated into this framework. Changing knowledge leads EDF to reassess the seismic hazard during the periodic safety reviews, more specifically: ཛྷ ཛྷ third periodic safety reviews of the 1,300 MWe reactors; ཛྷ ཛྷ fourth periodic safety reviews of the 900 MWe reactors; ཛྷ ཛྷ second periodic safety reviews of the 1,450 MWe reactors. Following the Fukushima Daiichi accident, ASN asked EDF to define and install a “hardened safety core” of material and organisational measures to control the fundamental safety functions in extreme situations which, in the French context, are comparable to those which occurred in Japan on 11th March 2011. This “hardened safety core” shall notably be designed to withstand an earthquake of an exceptional level, exceeding the levels adopted in the design or periodic safety review of the installations. In order to define this exceptional level earthquake, ASN asked EDF to supplement the deterministic approach to defining the seismic hazard with a probabilistic approach, which would be more closely in line with international best practices. ASN considers that the assessments of the seismic hazards determined by EDF are acceptable, with the exception of those concerning the sites of Saint-Alban/Saint-Maurice, Fessenheim, Chinon and Chooz which are insufficient in the light of current knowledge. ASN therefore asked EDF: ཛྷ ཛྷ to reassess the seismic spectra for the SaintAlban/Saint- Maurice, Fessenheim, Chinon and Chooz sites in order to take account of the uncertainties; ཛྷ ཛྷ to define a working programme to verify the strength of the equipment and civil engineering structures and make any seismic reinforcements for the periodic safety reviews. FOCUS Seismic resistance fault in the Donzère-Mondragon canal embankment which protects the Tricastin NPP On 18th August 2017, EDF sent ASN notification of a significant safety event concerning a risk of failure of a part of the Donzère-Mondragon canal embankment for the largest earthquakes studied in the nuclear safety case. This significant safety event was rated level 2 on the INES scale. The resulting flooding would be such as to cause a nuclear fuel melt accident in the four reactors of the Tricastin NPP and would have made deployment of the on-site and off-site emergency management resources particularly difficult. After hearing EDF, ASN considered that the information presented by the licensee could not rule out the risk in the short term. It therefore required that EDF temporarily shut down the four reactors on the Tricastin NPP in a resolution dated 27th September 2017. EDF strengthened the portion of the embankment concerned, after carrying out additional geotechnical surveys. ASN carried out inspections during the works. The assessment carried out by IRSN on the embankment thus strengthened at the request of ASN confirms that there would be no breach in the event of a safe shutdown earthquake. ASN considered that, further to the investigations and the repairs carried out by EDF, the condition of the embankment allowed the restart of the EDF reactors. It thus gave its consent accordingly on 4th December 2017. ASN has begun to draft requirements, which will be the subject of a public consultation, in order to oversee the actions to be taken by EDF, in particular reinforced surveillance of the embankment, maintaining pre- positioned equipment in place and final reinforcement of the embankment, as rapidly as possible, so that it can withstand the extreme earthquake considered subsequent to the Fukushima Daiichi accident.