ASN Report 2018

• 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 31 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: ∙ ∙ third periodic safety reviews of the 1,300 MWe reactors; ∙ ∙ fourth periodic safety reviews of the 900 MWe reactors; ∙ ∙ second periodic safety reviews of the 1,450 MWe reactors. Following the Fukushima Daiichi NPP accident, ASN instructed 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 that which occurred in Japan on 11 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 NPPs of Saint-Alban/Saint-Maurice, Fessenheim, Chinon and Chooz which are insufficient in the light of current knowledge. ASN therefore asked EDF in 2015 and 2016: ∙ ∙ to reassess the seismic spectra for the SaintAlban/Saint- Maurice, Fessenheim, Chinon and Chooz Nuclear power plants in order to take account of the uncertainties; ∙ ∙ 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. • Heat wave and drought risks During the heat waves in recent decades, some of the rivers used to cool NPPs experienced a reduction in their flow rate and significant warming. Significant temperature rises were also observed in certain NPP facilities housing heat-sensitive equipment. EDF took account of this experience feedback and initiated reassessments of the operation of its facilities in air and water temperature conditions more extreme than those initially included in the design. In parallel with development of these “extreme heat” baseline safety requirements, EDF initiated the deployment of priority modifications (such as an increase in the capacity of certain heat exchangers) and adopted operating practices optimising the cooling capacity of the equipment and improving the resistance of equipment susceptible to high temperatures. For the periodic safety review of the 1,300 MWe reactors, EDF has initiated a modifications programme on its facilities designed to provide protection against the effects of a heat wave situation. The capacity of certain cooling systems for equipment required for the nuclear safety case will in particular be improved. EDF has also initiated a climatic monitoring programme in order to anticipate climate changes, which could compromise the temperature hypotheses adopted in its baseline safety standards. With regard to the 900 MWe reactors, ASN gave its consent in 2012 to the implementation of the baseline safety requirements and integration of the resulting modifications. ASN also asked EDF to take account of the comments it made during this examination process with a view to drafting and implementing baseline requirements applicable to other types of reactors. In 2016, ASN asked EDF to take account of the lessons learned from the heat wave events of 2015 and 2016, along with their effects on the facilities, in the studies planned for the fourth periodic safety reviews of the 900 MWe reactors. The conclusions of these studies could, as applicable, be taken into account in the revision of the studies concerning other types of reactors. • Lightning risks The measures taken to address the lightning risk for the NPPs is currently primarily based on the steps specified by the regulations applicable to installations classified for protection of the environment. These regulations require the performance of a lightning risk assessment to identify whether or not it is necessary to protect the various buildings, along with a technical study to determine the nature of the protection systems to be installed (mainly lightning arresters and lightning conductors), where they are installed and how they are checked and maintained. For the purposes of the fourth periodic safety review of the 900 MWe reactors, ASN informed EDF that the application of this order would not enable a sufficient level of safety to be attained on the NPPs and asked EDF to transmit a new methodology more particularly taking account of plausible combinations with other hazards and the secondary effects of lightning (fires, loss of off-site electrical power supplies). The new methodology proposed by EDF is currently being reviewed by ASN. ASN report on the state of nuclear safety and radiation protection in France in 2018  289 10 – EDF NUCLEAR POWER PLANTS 10