- 261 - 3. Flooding This chapter presents the main findings drawn from the CSA reports relative to flooding submitted by the licensees, and ASN's conclusions after examining these reports. The following points were addressed in succession for each facility or group of facilities: Design of the facility; Measures to protect the facilities against the flood risk; Conformity of the facilities with the current frame of reference; Evaluation of the safety margins; Measures envisaged to reinforce the robustness of the facility to the flood risk. After examining the reports, ASN considered that improvement measures should be implemented on certain facilities. It will therefore request this. Some of these requests will be formal decisions taken by the ASN Commission, issued as legally enforceable requirements. 3.1 Design of the facilities ASN's rule No.I.2.e of 12 April 1984 for taking into account the risk of flooding applies to nuclear power plants (NPPs) operating a pressurised water reactor (PWR). It is therefore not strictly applicable to other facilities. Nevertheless, lacking another frame of reference, this rule can be considered for these other facilities. The draft guide relative to the protection of the facilities against external floods, currently under preparation and which was the subject of consultation in 2010, will apply to all basic nuclear installations (BNI), whatever their type. 3.2 Experimental reactors 3.2.1 Reactors operated by the CEA (Osiris, JHR, Phénix) Osiris Design of the facility With regard to the risk of flooding resulting from rainwater inflows, the CEA indicated in its CSA report that the site's storm water drainage system was designed for 10-year storm events. It points out moreover that the facility's drainage system is not designed for 100-year storm events at the bounds of the 70% confidence interval, but that the overflows that could occur under these circumstances would run towards the Corbeville stream below the site, and would therefore have no impact on the facility. With regard to the risk of flooding resulting from ground water infiltrations into the buildings, the CEA excluded the risk of flooding by ground water rise, as the water table is situated at a depth of more than 30 metres. Whatever the case, according to the CEA the flood risk is not such as to create a cliff-edge affect. Measures to protect the facilities against the flood risk Several constructional measures have been implemented to protect against a large infiltration of water into the Osiris reactor, such as: the situating of the general foundations inside a circular impermeable lining in reinforced concrete; the application of a multilayer bituminous coating under the basemat and rising 2 metres up the external wall of the containment; the presence of drains under the multilayer coating, leading to sumps and pumps installed in the annular space between the enclosure and the reinforced concrete lining; 10-cm high walls around the pipe passageways giving access to level -4 m from level 0.
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