- 301 - Design measures taken to prevent loss of the ultimate cooling system or heat sink The chilled water production for the storage area ventilation loop heat exchangers is connected to the redundant electrical power supply. In the event of electrical supply by the backup network or loss of the process secondary chilled water circuit of building 500, the ventilation of the containments enters safeguard mode: heat removal is ensured by the safeguard air intake and by the THD (very high flow) extraction. When post-earthquake safeguard mode is entered, containment ventilation stops and cooling is ensured by natural convection through the walls of the glove boxes. Cooling of the STE rod storage area is ensured by the HD (high flow) ventilation, by the safeguard fresh air intake and by a specific ventilation system via two independent loops each ensuring 100% of the thermal load to remove, designed to earthquake design standards and safeguarded; these loops each comprise two air/water heat exchangers and a refrigerating unit integrating redundant equipment. The prevention measures against chilled water loss (process and ventilation) in building 500 (primary loop) are: protection against freezing ensured by an electrically backed-up low output pump; redundancy of the production equipment; maintaining of cooling of the backed-up and emergency equipment of the ventilation loop in the event of electrical power supply loss by a combined generator set/pump unit in the process primary chilled water loop. To guard against loss of the process cold water supply of building 500 (secondary loop), the primary loop/secondary loop heat exchangers are doubled and the circulation pumps are trebled. Conclusion on the measures planned to protect the facilities against the risk of loss of the ultimate cooling system / heat sink, possibly combined with aggravating factors If the cooling function is lost, the integrity of the STE storage geometry is jeopardised when the air temperature reaches 160°C, that is to say 11 hours after cooling stops. Restoring the function within less than 11 hours would seem difficult in a generalised post-earthquake emergency management situation. Moreover, the degradation of the neutron decoupling screens and a change in the geometry of the stacks of fuel rod trays could occur simultaneously and jeopardise control of the sub-criticality of the storage area before reaching 160° C. If the cooling function in the storage areas outside the STE is lost, the temperature that jeopardises the integrity of the geometry is reached after 5 days, if there are no added aggravating factors. The licensee considers that an external intervention to perform mechanical repairs on the ventilation system or the diesel generator sets, or to restore the electrical power supply using an external mobile generator set, can be carried out within this period of time . In the event of loss of cooling that could induce a criticality accident in the STE storage area, the licensee envisages adding boron to avoid a criticality accident. The boron would be mixed with water in a buffer tank, pumped to the storage area and sprayed over the fuel rod trays. ASN considers that the licensee must prove its capacity to restore then maintain the cooling function in the Mélox facility within a time compatible with the rise in temperature , that is to say: less than 11 hours for the STE storage station; less than 5 days for the other fissile material storage areas; and taking aggravating factors into account. ASN also considers that the licensee must substantiate the feasibility and effectiveness of spraying the fuel rod trays taking the intervention conditions into account (mist, temperature, radiation protection). It will issue a requirement on this subject. Loss of the main cooling system combined with total loss of the electrical power supplies Simultaneous loss of the cooling system and all the electrical power supplies leads to the same feared situation as in the previous section. It is the loss of cooling that requires the fastest response due to the rise in temperature in the fissile material storage areas.
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