- 147 - The water intake structure varies from one site to another. For riverside NPPs, it usually consists of: a deflector panel; a floating skimmer boom to limit the entry of floating debris; waterways that can supply several under-river tunnels. Each waterway is equipped with removable trash racks. The water intake supplies the under-river tunnels which open out in a settling pit at the entrance to the plant unit intake channel. This intake channel divides to serve the pumping stations of each pair of plant units. Starting upstream and working downstream, the equipment used for the transit and filtration of the raw water, comprises the advance grids (widely spaced bars, no trash rack), the preliminary filtration grids (more closely spaced bars, equipped with trash rack), a filtering system (chain filters or rotating drum screens), and lastly the suction pumps. The water transits chiefly through specially built channels, streams or concrete water pipes. Raw water suction, delivery and filtration are ensured between minimum and a maximum levels called the lowest and highest safe water level respectively. The calculation of these levels takes into account the specific environment of the site. Taking the various design criteria into account ultimately determines: the shape and height of the dykes, the depth of the pipes, the setting and dimensions of the filtration system, the setting of the filtration system cleaning and disposal systems, the setting of the safety pumps. The last 3 points determine the form and depth of the pumping station. The reactors in service are designed to have an autonomy of at least 100 hours after a heat sink loss. If the heat sink loss affects all a site's reactors simultaneously, the targeted autonomy announced by EDF is 24 hours for seashore NPPs and 60 hours for riverside NPPs in the case of an unpredictable hazard (e.g. sudden influx of clogging material), and 72 hours in case of a predictable hazard (e.g. a climatic event such as extreme cold + frazil ice) in which case the tanks can be filled to maximum level as a preventive measure. The heat sink is usually the natural environment to which the nuclear facilities are connected, but other heat sinks do exist in the NPP, which are used according to the status of the plant units and also serve to cool down the core or the spent fuel pool: Equipment or system used as "Heat Sinks": Equipment or systems used Heat Sink Normal operation Steam generators (SG) o Normal feedwater o Auxiliary feedwater to steam generators (EFWS) and turbine bypass system (GCT-a). Steam generators EFWS water, demineralised water, raw water, turbine bypass system (GCT-a) Residual heat removal system (RHRS) CCWS (component cooldown system) water cooled by the ESWS (essential service water system Safety injection system (SIS) PTR (Reactor cavity and spent fuel pool cooling and treatment system) tank water Accident operation Containment spray system (CSS) o CCWS water cooled by the ESWS o PTR tank water
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