- 230 - For each key SSC, the licensee then identified the equipment items whose failure could lead to a mechanical threat on the SSC (falling load) and the hazards that could affect them in view of their environment (fire, explosion, internal flooding, etc.). The potential sources of threat then undergo an assessment of their robustness to external hazards identical to that to which the key SSCs are subjected. ASN considers that the procedure adopted by the ILL for the JHR is satisfactory. Fuel cycle facilities operated by AREVA AREVA identified the key SSCs on the basis of the feared situations (transposition of the notion of severe accident used for reactors). The key SSCs are those that ensure, in these feared situations, continuing operability of the important safety functions that are necessary to place and maintain the facility in a safe condition. The feared situations considered for AREVA's facilities correspond to situations of "release of a hazard potential" that can challenge the technical bases of the PUIs and PPIs (on-site and off-site emergency plans respectively). Given the number of feared situations that can be envisaged, based in particular on the accidents considered in the on-site emergency plan (PUI), priorities have been defined according to the hazard potential and its release kinetics. Thus, for the La Hague site, the analysis is based on the potential source term in accident situations beyond the design basis, the kinetics leading to the feared situation (less than 48 hours save exceptions) and the robustness of the facilities (particularly the stability of the structures and the containment barriers) rendering their failure plausible, even for the extreme hazards studied. The Mélox plant analysis is also based on the potential source term in accident situations beyond the design bases and the kinetics leading to the feared situation. AREVA identifies the key SSCs as those which, in the feared situations, ensure the important safety functions necessary to achieve and maintain a safe condition after an event. The safety functions concerned are, for example, the stability of civil engineering structures, maintaining containment integrity; the cooling of radioactive substances, dilution of hydrogen from radiolysis. The key SSCs defined in this way are either "structural" (civil engineering elements) or "functional" (essentially active equipment fulfilling specific functions). Feared situations for the Tricastin site facilities The Georges Besse I plant The potential hazard sources for the environment and populations are uranium hexafluoride and hydrofluoric acid (HF) present in: the gaseous diffusion cascade; the auxiliary building U; the Programme Resource Department building (DRP) (REC1: UF6 container reception and reconditioning unit). Loss of electrical power supply and cascade cooling have no impact on the safety or the environment. During the operating phase, the feared situations are: for the diffusion cascade (which can contain 800 tonnes of UF6), opening of breaches leading to the release of hydrofluoric acid (HF); for the auxiliary building U and the DRP unit, rupture of numerous pipes and loss of sealing of the metal containments leading to rapid evaporation of the liquid UF6 (up to 114 tonnes in building U and 40 tonnes in the DRP unit). The feared situations during the rinsing phase are the same as during operation, but with a reduced source term for UF6. Eurodif concludes in its report that the radius of the zone of significant hazards for human life is compatible with the envelope radius of the PPI. ASN considers that the identification of feared situations for the Georges Besse I plant in the complementary safety assessments is satisfactory
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