- 324 - Provisions for improving the robustness of the facility with respect to containment management in the event of an earthquake have been adopted in the CSAs. They are presented in the "earthquake" section. In addition, the licensee has also assessed the hydrogen risk due primarily to the "cold neutron sources". The conclusion is that the average hydrogen concentration in the hall would not exceed 0.3%, that is to say one tenth of the lower explosive limit (LEL) of hydrogen in air. However, the means of detection and management of a deuterium leak would be rendered unavailable in the event of an earthquake or flood. Facility monitoring measures (required instrumentation) Control of the facility in an earthquake situation is ensured from the PCS, which constitutes the fall-back centre, situated 170 metres from the reactor. This control centre is supposed to amply withstand an earthquake of the SSE intensity and the explosion of a tanker truck on the A480 motorway. It has an air purification and pressurisation system to protect against any external pollution of the air. In the event of an earthquake, the PCS allows: monitoring of the water levels in the pile block, the reactor pool and channel 2; the CES to be put into service and monitoring of the level of the associated water sources; monitoring of the radiological environment in the reactor building and at the bottom of the chimney stack (dose rate measurements); energising of the emergency fan housed by the PCS, which enables positive pressure to be maintained in the annular space; passive depressurisation of the reactor building hall. Reminder: on completion of the nonconformity processing planned in the near future, an active depressurisation function will be available after an earthquake. Nevertheless, the information available at the PCS does not allow the "residual power removal" function to be properly diagnosed. This is because the licensee has no information on either the temperature of the heavy water in the core or the position of the natural convection valves which are supposed to fulfil the function when the reactor is shut down. The ILL will examine the possibility of having information concerning the most relevant physical parameters of the reactor relative to the objective, instead of information on the status of the systems responsible for fulfilling the function. Likewise, the information available in the PCS does not allow a true diagnosis of the state of the "control of environmental releases" function. This is because the licensee only has a measurement of the dose rate at the bottom of the chimney. The PCS power supply is delivered from one of the non-backed up auxiliary panels of the reactor. It is backed up by a 100 kVA earthquake-qualified diesel generator set that is started manually. This diesel generator set has an autonomy of 20 hours under full load. The ILL nevertheless estimates that in practice the autonomy would exceed 40 hours since the high-power equipment items will function intermittently or even not at all (gaseous effluent heaters, emergency water make-up system). Furthermore, the fuel can be replenished from the fuel available in the 2 main tanks of the facility's diesel generator sets. If necessary, an external mobile diesel generator set can be deployed to supply the PCS with power, but in this case the ILL would have to count on the mobile means of the CEA. Measures envisaged to reinforce the existing measures and proposed studies With regard to a severe accident situation and given the analysis carried out for the ILL's CSA, the licensee's major improvement commitment is to build, for the planned restarting in 2014, a new emergency operations control centre (PCS) called PCS3, meeting the following requirements: earthquake design basis: SSE with margins higher than 2 to cover the "ultimate plausible earthquake, SSE + ; flood design basis: 216.2 m NGF - the maximum water level physically possible on the site; explosion design basis: 80 mbar, corresponding to the explosion of a 20-tonne LPG tanker truck on the A480 motorway;
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