- 299 - Loss of the function for diluting the hydrogen from radiolysis by stopping ventilation of the buildings housing the pools or by stopping the superchargers producing the air sweeping the fission products and fines storage tanks. According to the operator, the times to reach the Lower Flammability Limit (LFL) of hydrogen (4%) in the equipment identified in the feared situations can be estimated as follows: Unit Equipment Time R1 Fines transfer tanks 8 hours T2 Fines storage tanks 13 hours T2 Fines transfer tanks 20 hours R1 Fines storage tanks 23 hours T1 Fines recovery tanks 23 hours R1 Fines recovery tanks 25 hours T7 Alkali rinsing tank (not acidified) 25 hours R7 Alkali rinsing tank (not acidified) 28 hours R7 Fines suspension tank 48 hours Times to reach the LFL of hydrogen in the event of loss of radiolysis hydrogen dilution Furthermore, the cooling of the plutonium oxide storage pits in the BSI units for BNI 116, BST1 and its extension for BNI 117, is ensured by forced ventilation of the concrete pits containing these storage pits. The possibility of supplying power from backup generator sets designed to earthquake design standards is planned for in the design. In the event of loss of the forced ventilation of the pits, natural draft operation is provided for in the pits of the BST1 extension, which is of more recent design. With regard to the PuO2 storage pits in the BSI and BST1 units, which do not have natural draft cooling, the time for the concrete to reach a temperature of 160°C is estimated at 20 hours in the event of loss of cooling. The storage areas for vitrified fission product containers (T7 and R7 unit buffer storage areas, EEV/SE storage) have cooling systems that function by forced convection or, in the event of loss of the electrical power supplies, by natural draft. Measures taken to prevent these feared situations The measures to prevent these situations have been detailed in the preceding paragraphs. They are based on the multiplicity of electrical power sources and supply paths, the redundancy of the equipment ensuring the cooling of the pools and FP tanks on the one hand, and the dilution of hydrogen from radiolysis on the other, and the robustness of these equipment items in the event of an earthquake. Moreover, a study postulating the prolonged and joint outage of the electrical power supply and the cooling function revealed margins with respect to the times to attain the phenomena that could lead to radiological releases (heating of solid particles in the DPC, degradation of the cladding of spent fuel elements stored in pools, boiling of FP solution tanks, hydrogen explosion). Measures envisaged to reinforce the robustness of the facilities to the loss of the main cooling system combined with total loss of the off-site and emergency electrical power supplies The licensee has undertaken to submit a complementary study integrating the robustness to the loss of the natural convection of the PuO2 and glass storage pits, within one year. ASN considers that the equipment ensuring cooling of the pools and FP tanks must be rendered robust to external hazards to ensure their operability in accident conditions. It will issue a requirement on this subject.
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