- 189 - ASN considers that the improvements proposed, which comply with the CSA specifications, must be implemented. ASN will be issuing technical requirements concerning the implementation of these equipment modifications on all the NPPs in service, as the EPR design already comprises effective measures to deal with these risks. The Bugey and Fessenheim plants entail a particular risk of spent fuel pool damage in the event of a falling fuel transport container: in these plants, unlike the others, between the handling zones and the fuel building spent fuel pool, there is no seal separating the part of the BK supporting the pool from the heavy loads handling zone, which would prevent any transmission of loads in the event of a falling container. ASN considers that EDF should present a study of the possible additional measures to prevent or limit the consequences of a falling container accident in the fuel building, incorporating the extreme situations studied in the CSAs. ASN will issue a requirement on this subject. ASN also considers that the current provisions concerning the transfer tube and safe positioning of an assembly during the course of handling should be the subject of detailed studies by EDF. With regard to the transfer tube on the NPPs in operation, analysis of the CSA reports showed that for the CP0, CPY and 1300 MWe plant series, the transfer tube rupture margins for seismic stresses going beyond the designbasis earthquake, could be limited. Moreover, the transfer tube is hard to inspect. It is therefore difficult to demonstrate that the risk of tube break is virtually to be ruled out. ASN therefore considers that EDF must study changes to hardware or to operating conditions to prevent uncovering of an assembly during handling in the event of a transfer tube break. EDF must also study the possibility of modifications such as to limit a fall in the water inventory of the pools in the reactor and fuel buildings. ASN will issue a requirement on this subject. In the case of the EPR, the design of the reactor and fuel buildings, which rest on a common basemat, thus limiting differential displacements, would make it possible to envisage a second containment barrier around the transfer tube such as to prevent the risk of uncovering of an assembly during handling. As part of the analysis of the CSA reports, EDF stated that for technical reasons which it considered to be prohibitive, it did not envisage installing a system for automatic safe positioning of a fuel assembly when the ambient conditions ruled out access to the premises. EDF prefers having the fuel assembly secured by operators present in the reactor building or the fuel building, making provision for the material or organisational measures enabling them to do so, while the ambient conditions are still acceptable. The goal is to ensure the that fuel assembly can be made secure within a period of less than two hours. ASN considers that EDF must continue to carry out studies and look for solutions to counter the difficulties mentioned earlier, look for technical measures to prevent the risk of uncovering of a fuel assembly and ensure that an assembly being handled is safely positioned as rapidly as possible when the ambient conditions still allow access to the premises. ASN will issue a requirement on this subject. Hydrogen management Following the Fukushima accident, ASN asked EDF to examine the risks linked to the build-up of hydrogen in the buildings other than the containment, especially the fuel building. ASN in particular asked EDF to identify: The phenomena capable of generating hydrogen (radiolysis, zirconium/steam reactions); The possible build-up of hydrogen; The means implemented to prevent hydrogen explosion or detonation. As part of the CSAs, EDF states that the presence of fuel assemblies in the BK pool can lead to the production of hydrogen in normal operation by radiolysis of the water and that an additional analysis is being initiated to assess the possible risk in the absence of ventilation.
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