- 293 - Loss of the cooling systems / heat sink Design measures taken to prevent loss of the ultimate cooling system or the heat sink The cooling water used to remove the power transmitted by the reactor's heavy water and demineralised water system heat exchangers is river water drawn from the river Drac, into which it is also discharged after use. The temperature of the water drawn varies from 5°C to over 15°C depending on the season. The water necessary for the needs of the reactor is routed from the pumping station by three separate networks, each with its own pumps. In the event of loss of the heat sink, the reactor would be stopped by the emergency shutdown function associated with the core inlet water temperature (37°C). According to the operating rules, the shift personnel would activate shutdown of the main reactor coolant pumps and leave the pumps of the shutdown cooling system (CRA) in operation for one hour before switching to natural convection cooling mode. Under these conditions, the heavy water temperature at the fuel element outlet would reach about 61°C maximum before stabilising, because the power transmitted to the pool water (440 m3) by the pile block structures and the pipes is then of the same order as the residual power of the core. The ILL evaluates the drop in the pool water level at 40 cm/day. It has calculated that the pool water would not boil. Under these conditions, the ILL would have more than 15 days to restore the pool water inventory. With regard to channel 2 and the transfer cask, the drop in level would be less than 40 cm/day (for 400 kW of spent fuel element (SFE) power). The absence of any intervention for 4 or 5 days would have no impact on the fuel elements because the stored SFEs have more than 6 metres of water above them and the top of the cask heat exchanger (operating in natural convection) is covered by more than 3 metres of water. Nevertheless, the level of activity above channel 2 could significantly increase. By lowering the fuel elements, the ILL would have more than 15 days to restore the channel 2 water inventory. The ILL indicates that it has the emergency water make-up system (CES), designed to earthquake design standards (see "earthquake" section), to provide the necessary water make-up by pumping water from a 120 m3 tank or by pumping the water from room A11 with another pump if the water level in this room is sufficient. The CES can only be put into service from the PCS. Water make-up is also possible from the exterior via a seismic containment penetration situated beside the truck door, by means of a motor-driven pump and a flexible hose between the motor-driven pump and the containment penetration. Conclusion on the planned measures to protect the facilities against the risk of loss of the ultimate cooling system or the heat sink The facility is relatively insensitive to loss of the heat sink, therefore it can be considered acceptable that the HFR has just one heat sink. The ILL has nevertheless undertaken to implement the improvements mentioned below. Measures envisaged to reinforce the robustness of the facility to loss of the ultimate cooling system or the heat sink The ILL has undertaken to apply the following measures: install the ultimate reflooding system (CRU), designed to earthquake design standards (mentioned on the "earthquake" section). The safety injection will be ensured passively by gravity as long as the pool water level is sufficiently high. The reserve of water in the pool is sufficiently large to guarantee the water inventory in the short term, allowing the CES to be brought into service; in addition to the CRU, install two wells in the water table in the immediate vicinity of the reactor building to reinforce the robustness of the facility to an earthquake exceeding the DBE, which will also give greater robustness in the management of the total loss of heat sinks scenario. This is because it will enable external water to be injected into the pool and channel 2 at a much higher flow rate, i.e. 250 m3/h per well, and with total redundancy. This system will be designed to operate up to an earthquake intensity of DBE + σ and a water level of 216.20 m NGF; install an intermediate system that triggers rod drop automatically if secondary flow is lost for more than 10 seconds, in accordance with a previous study proposed by the ILL; during the next periodic safety review, consolidate demonstration of the capacity to remove the residual power immediately after reactor shutdown using the natural convection valves alone, taking into consideration the problems that could affect the reactor coolant pump flywheels and the primary and secondary cooling systems in an earthquake of intensity DBE + σ. In this context, the
RkJQdWJsaXNoZXIy NjQ0NzU=