- 29 - ASN finds that non-compliance with these requirements is liable to compromise the qualification of the accelerometers and accelerographs. The free field6 sensor was also checked. Some inspections showed that the free field sensor was unable to perform its functions (traces of corrosion on the free field sensor accelerometer at Saint-Alban, faulty sensor owing to electromagnetic disturbance at Dampierre). A number of potential anomalies were also recorded during the field visits or document analyses: certain check-points that are not explicit enough to guarantee the actual free field operation of the sensor (Blayais); at Civaux, during the exercise which involved applying the operating procedure to the shock perception criterion, the inspectors noted that the USB flash-drive installed in the EAU rack and designed to replace the flash-drive in place when its recording capacity is no longer sufficient, was defective; at Bugey, during the field visit, the inspectors found that the free field sensor was simply placed on the ground, making it vulnerable to shocks, etc. Other anomalies were recorded on certain sites, with no traceability regarding their resolution (Paluel for example). Identification and positioning of sensors The inspectors queried the positioning of the sensors (especially the free field sensors). Some sites were unable to provide any justification (Tricastin, Penly, Flamanville). The inspections also highlighted non-compliance with RFS I.3.b on several sites. RFS I.3.b (§ 2.2.3.1) stipulates that a 3-axis accelerometer be located "on the basemat of another building housing systems important for safety and the foundations of which are different from those of the reactor building". Some sites failed to comply with this stipulation; this is the case at Cruas, Nogent, Penly and Bugey. For these last three sites, the accelerometer associated with this requirement is the sensor placed on the floor at level 0 of the nuclear auxiliaries building, about fifteen metres above the basemat. The RFS states (§ 5.2.2) that free field devices must be "sufficiently far from all sources of vibrations or significant shocks as could disrupt the seismic measurements". At Penly, the accelerometer is in fact situated on the site platform at the edge of a road where vehicle traffic can create vibrations. The Saint-Alban accelerometer is located in the basement of the safety building. No justification for this choice of location was presented. ASN finds that the correct positioning must be checked for all the sensors required by RFS I.3.b; if necessary, they will be relocated. Instrumentation maintenance and calibration The inspectors noted that the seismic instrumentation accelerometers are regularly maintained and periodically checked (Paluel, Golfech, Penly, Nogent, Cattenom). However, the exact content of the maintenance operations is not always described in the official operating documents (at Chooz for example). RFS I.3.b (§ 2.2.4) requires that the devices also be calibrated, in particular because the loading response may drift over time, given that the sensors comprise electrotechnical components. Some sites are unable to state whether or not the devices have been calibrated since they were installed (Paluel, Golfech, Penly, Nogent, Fessenheim) or whether their response to a real mechanical loading had been tested (Cattenom). Automatic calibration of the sensors is performed daily when ordered by the EAU seismic rack, however the technology of these sensors and the various parameters measured during these calibrations were not presented to the inspectors (Nogent, Chooz) and it was impossible to demonstrate that the mechanical characteristics can be checked by means of these calibrations. 6 Free field corresponds to locations where soil movements can be considered to be undisturbed by the proximity of heavy buildings; RFS I.3.b considers that a point is in free field if its distance from the heavy buildings (nuclear islands, turbine halls, cooling towers) is at least 100 metres.
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