- 262 - Constructional measures have also been implemented in the hot workshop building to limit the risk of water infiltration. The CEA therefore considers that the design measures taken, as much for the arrangement of the electrical equipment qualified as important for safety as for the routing of the water pipes, preclude the risk of a flood being the cause of simultaneous loss of the Osiris reactor's protection channels (the CEA specified that the most penalising consequences for the facility in case of flooding were the total loss of the electrical power supplies, with the exception of the lines supplied directly by the ultimate backup generator set (GUS) (extraction fans), and the loss of nuclear ventilation). Furthermore, the licensee indicates that operating provisions for firstly alerting to the imminence of a flood, and secondly mitigating its consequences, have been defined, and comprise: monitoring the alert given by Météo France (the French Met Office) or the Centre; performing inspection patrols; level sensors that can start a lift pump or send an alert to the control room; the presence in the facility of mobile pumping means, and immersible and motor-driven pumps on the site that can pump up the water and direct it to outlets. Conformity of the facilities with the current frame of reference As a general rule the CEA considers that the conformity of the essential equipment and the potential threats is confirmed by the periodic inspections and tests, the maintenance operations and the actions carried out further to the periodic safety reviews. For Osiris in particular, and with respect to the flooding risk, the licensee states that conformity is ensured by applying the following measures: performing daily inspection rounds to monitor the premises where there is a risk of water ingress; taking three-monthly measurements in the external drain manholes to monitor any water inflows in the immediate vicinity of the facility buildings. The CEA specifies that no leaks through the basemat have been detected; taking samples from the sumps collecting the water from the drains situated under the multilayer coating of the Osiris reactor in order to monitor the sealing against a water table rise and leaks from the water block. The CEA indicates that the last televisual inspection concluded that the general condition of the pipes and grids was good; every 10 years, performing leak tests of the multilayer coating covering the lower part of the containment and the basemat; inspection of the drain situated under the canal by video camera, cleaning and testing its operation. The licensee considers its overall condition satisfactory; 4-monthly testing of automatic starting of the lift pumps situated in the annular space sumps. It can be pointed out that the targeted conformity review carried out in 2009 as part of the Osiris facility's periodic safety review revealed no major nonconformities. Evaluation of the safety margins To estimate the safety margins with respect to a 100-year storm event, the CEA reviewed its 2005 study relative to the risks of saturation of the facility's storm water drainage systems further to a 100-year flood situation. It considered a 100-year storm event corresponding to the upper bound of the 95% confidence interval. The results show that with these rainfall levels it is always the drains situated to the south, southeast of the facility's buildings that would become saturated. The volume of water resulting from the overflow of these drains was estimated at 221 m3 for intense rainfall periods lasting 30 minutes. Given the flatness of the land near the facility, the CEA considers that such an overflow volume would lead to a water wavefront of less than 50 cm height and could lead to water ingresses in the retention areas of certain tanks, as well as in the diesel generator room, then to the -4 m level of the crown gallery before reaching the annular space. The CEA does not consider that this event could produce a cliff-edge effect. It nevertheless plans measures to reinforce the robustness of the facility, particularly to limit the impact of water ingress into the non-fixed tank retention areas and the gallery.
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