- 98 - Main operating provisions Operating principle in the event of an earthquake: In order to be able to rapidly take adequate steps to bring the plant units to the shutdown state felt to be safest for each one, and maintain it in this state, or to continue with operations, RFS I.3.b recommends the installation of seismic instrumentation for pressurised water reactors. The procedure to be followed then depends on the level of the earthquake in relation to the Half Design Response Spectrum (½ DRS: spectrum corresponding to an earthquake which should not modify the behaviour of the facility with regard to an SSE occurring subsequently and the spectrum of which is half the DBE). if the ½ DRS threshold is not exceeded, each unit can continue to operate provided that a visual inspection is carried out on structures and equipment. if the ½ DRS threshold is exceeded, the units must go to the shutdown state considered for each unit to be the safest. The resumption of operation may only be initiated with the approval of ASN. The operation of this seismic instrumentation was the subject of a series of targeted inspections by ASN in 2011 (see § 1 of this chapter). During these targeted inspections, ASN on certain sites identified nonconformity of the seismic instrumentation with RFS I.3.b, problems with operator interpretation of the measurements taken by this instrumentation, and a lack of clarity in the reactor shutdown procedures. These deviations can delay reactor shutdown as specified in RFS I.3.b, or could even lead to this decision not being taken. Moreover, the required inspection following the occurrence of the ½ DRS, defined by RFS I.3.b and constituting a prerequisite for restart of the reactors on the site, is not clearly defined. ASN will require that EDF perform a conformity check on its facilities with respect to RFS 1.3.b. Furthermore, even though a degree of training has been dispensed, the exercises triggered by ASN during the inspections showed that on most sites, the operators had problems in analysing the data produced by the seismic instrumentation, which could delay shutdown of the reactors or even lead to this decision not being taken. ASN will require that EDF define and monitor an operations personnel training programme to enhance their preparation for a possible earthquake. ASN will also require that EDF study the advantages and drawbacks of implementing automatic shutdown of its reactors in the event of seismic loading, enabling the reactor to be shutdown to a safe state appropriate to each site, if the seismic level corresponding to a spectrum with half the amplitude of the design response spectrum is exceeded. Protection against the indirect effects of the earthquake SSC failure, "event earthquake" approach In addition to the design-basis earthquake resistance of the IPS equipment necessary in the event of an earthquake, the safety approach was supplemented by an approach called the "event earthquake", the aim of which it to prevent damage to an equipment item necessary in the event of an earthquake by an item or structure not seismic-classified. This approach is being implemented on the occasion of the ten-yearly outage inspections. This only considers direct mechanical damage or direct spraying of mechanical or electromechanical equipment. The hypotheses adopted by EDF in the approach are as follows: equipment that is not designed to withstand an earthquake can fail and thus constitute a potential hazard. seismic-classified equipment must not have its function or integrity compromised by failure of an item that is not seismic-classified.
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