- 149 - EDF has provided solutions for the various risks of heat sink loss: 1. Extreme cold: to prevent the water intake from freezing up As soon as winter temperatures prevail, pumping station monitoring is tightened under the "Extreme Cold" procedures, and the systems involved in the heat sink are placed in "winter" configuration: infrastructures identified as sensitive are subject to tightened monitoring. 2. Extreme heat: to avoid loss of the heat sink due to low water levels During springtime, river-based NPPs step up monitoring in order to detect abnormal heat sink temperatures or levels. In case of alert, "extreme heat " or "low water" procedures enable monitoring to be adjusted and measures to be taken to protect the heat sink by adapting production if necessary. The coastal NPP sites are naturally protected against this risk. In practice, the NPPs concerned by heat sink low water situations are generally shut down well before the lowest safe water level is reached, to limit their environmental impact. 3. Oil spill: to avoid the water intake getting clogged up by an influx of hydrocarbons. Some NPP sites are protected from this risk by their geographical location. In such cases, a daily inspection patrol of the heat-sink related facilities suffices to check the quality of the cooling water. The other NPP sites (on the coastline, on an estuary , beside a navigable canal, etc.) however, are exposed to the risk. In 2003 EDF carried out a probabilistic assessment of the drift of an oil slick offshore of the sites situated on the English Channel and the North Sea. This study assessed the probability of arrival of an oil slick resulting from an accident as 2.10-3/year for the NPPS on the Normandy coast. Protection of the heat sink is based on design features and an operating doctrine that allows alerting of the NPP, surveillance of the oil slick in collaboration with the public authorities, and preventive shutdown of the plant units if there is a confirmed risk of an oil slick entering the intake channel. In the event of large-scale oil pollution offshore of the NPP, the alert is raised by the public authorities, and the situation generally leads to triggering of the "POLMAR" (maritime pollution) plan. Agreements between EDF, the French maritime authorities and Météo France (the French met office) enable the movement of an oil slick to be monitored and its position with respect to the water intakes of the nuclear sites to be communicated to EDF. Entry of an oil slick into the NPP surveillance zone results in the application of graded prevention actions to ensure the availability of the protection means, prepare plant until shutdown and, if necessary, implement the provisions of the on-site emergency plan: raw water consumption is limited as a precautionary measure to preserve the backup heat sink. The site plant units are gradually shut down to reduce the flow drawn in at the pumping station to the level required for reactor cooling; a floating pontoon equipped with vertically descending sheets situated in front of the intake waterway limits the ingress of a surface oil slick into the pumping station, subject to the preventive shutdown of the circulating water system (CRF) pumps, which reduces the inflow of water to just the essential service water (ESWS) flow required to cool the safety-related auxiliaries; the filters and their washing systems also help limit the hydrocarbon influx. These instructions can also be triggered by an observation made as a result of pumping station monitoring, by the appearance of a rotating drum filter clogging or circulation pump tripping alarm. EDF estimates that the ESWS system instrumentation, and the flow measurements in particular, remain operational up to a hydrocarbon level of 10%. 4. Clogging agents: to avoid obstruction of the water intake All the pumping stations have designed-in protection against massive influxes of clogging agents through lines of defence which vary from one site to another according to the particularities of the environment, but which typically are: At the water intake entry point, the first element met is a set of movable grids with widely spaced bars; At the pumping station entry point, the first element met is the "upstream" grid which has more closely spaced bars. A few metres downstream, one or two coarse filtration grids prevent the ingress of large floating objects. These coarse filtration grids are usually equipped with trash racks (one per grid) which raise any debris and direct it via a discharge channel to a waste collection bin.
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