French PWR's ultimate heat sinks threatened by their environment

In 2009, several environmental phenomena affected some NPP's water intakes. The most significant event was the total loss of heat sink (trains A and B) on unit 4 of the Cruas power plant in December, by clogging the pumping station pre-filtration trash racks due to a massive arrival of vegetable matter. This incident that happened for the first time in France was classified as level 2 on the INES scale. Another event occurred in January 2009 at the Chooz B site. Very low temperatures led to rapid formation of frazil ice on the protective grid of the site located at the water intake. Partial blockage of this grid by ice lowered the water level at the entrance of the pumping station below the lowest safe level to keep the essential service water safety pumps in good working order. The full paper gives more events and details. As shown above, the total loss of ultimate heat sink could be a real risk which was not fully identified 40 years ago at the initial design of French plants. During the 80's, the Probabilistic Safety Assessments (PSA) developed at the IRSN brought to light those potential scenarios. Further to these studies, confirmed by the utility EDF, some modifications were implemented in all built reactors, in terms of design of systems as well as operating procedures. Moreover, the partial flooding of the 'Le Blayais' NPP in December 1999 led to a wide study of the risks of a total loss of heat sink on all units induced by external hazards or combinations of them. Modifications have been decided to guarantee that each NPP could cope with such situations affecting all its units, in particular in terms of sufficient steam generator water supply capacities until the heat sink recovery. (authors)