- 323 - controlling the level of water in the pool, above the pile block, with the pool acting as the ultimate cooling source. The loss of heavy water inventory in the pile block can be detected from the control room, the PCS, or the reactor building. The safeguard actions consist firstly in isolating the pile block (in an earthquake situation, the isolation function would not be available) and secondly in resupplying the pile block with water from the emergency water make-up system (CES), which can only be controlled from the PCS. The time required to resupply the pile block with water is estimated at 60 minutes if external means (motor-driven pump) are deployed. With regard to core cooling management after damage has occurred, it must be noted that the core could be situated in the reactor pool further to failure of the pile block. In this extreme hypothesis, control of the facility would still consist in maintaining the pool water level by using the CES. The resupply procedure is similar to that used for the pile block. The pool water level can be monitored from the control room and the PCS. After an earthquake, the information would only be available in the PCS, and it would be lost if the PCS became flooded. The licensee carried out a study which it qualifies as highly conservative and which concluded that the molten core cannot melt through the pool bottom lining (particularly in view of the difference in melting temperature between the aluminium matrix of the fuel and the steel of the pool). The licensee moreover considers that the risk of recriticality does exist but would not significantly aggravate the consequences of core meltdown, given the depth of the pool and the thickness of the concrete walls. With regard to the control of spent fuel element cooling in channel 2, the ILL considers that it relies on: controlling the water inventory in channel 2; controlling the "emergency rod drop" function for the fuel element present in the transfer cask. The information on the channel 2 water level is available in the control room and the PCS, and water makeup is carried out using the emergency water make-up system (CES) from the PCS. The licensee also indicates that water loss from the cask can be detected from the control room, the PCS or in the reactor building. The cask water temperature can also be monitored. If there is a confirmed drop in the cask water level, the shift supervisor can perform make-ups with heavy water or light water. If the water level cannot be restored, emergency dropdown is carried out; this can be done from the control room or the PCS. These dropdown systems are tested regularly. Emergency dropdown using the system takes 10 minutes. In the event of loss of the backed-up electrical power supplies, dropdown can be performed manually in 30 minutes in the reactor building. Lowering of the water level in channel 2 or the cask is detected by an alarm associated with the measurement of dose rate above channel 2. The diagnosis can be confirmed by two other sensors. These measurements are available in the PCS but would be lost in the event of an earthquake or flooding. With regard to containment management, the HFR has a double-walled containment: an inner containment in reinforced concrete and an outer containment consisting of a metal wall. The annular space between them is kept at a positive pressure of 135 mbar to prevent any direct leakage from the inner containment to the exterior. Management of environmental releases in these cases consists: maintaining positive pressure in the annular space between the inner concrete containment and the outer metal containment, associated with the static containment. The overpressure value is defined according to the risk of combustion of the cold and hot neutron sources: 15 mbar if combustion has already occurred, 75 mbar if it can be excluded; or limit the pressure rise in the inner concrete containment by making discharges through the EG (gaseous effluents) circuit equipped with two HEPA and PAI (Iodine trapping) filtration lines. The pressure rise in the inner containment can be due to a possible BORAX-type explosive reactivity accident, to the combustion of cold and hot neutron sources or the heating of the air in the hall by the residual power of the spent fuel elements, to heat transfers through the double-walled containment, or to leaks from the pressurised annular space and evaporation of water from the pool. Detection of an abnormal radiological environment in the reactor hall or the detection of an earthquake causes automatic isolation of the containment.
RkJQdWJsaXNoZXIy NjQ0NzU=