The French Commissariat à l'Energie Atomique (C.E.A.) has had the considerable advantage of operating for several years, two different fast reactors, RAPSODIE and PHENIX. It is intended to show the most important consequences of such a possibility with special emphasis on the aspects which are relevant to the behavior of the metallic materials.

We shall emphasize the statistical experience it allowed us to gain with respect to the basic phenomena which are responsible for the deformation and failure of the cladding and the ducts and to the overall behavior of the subassemblies. We shall also mention the fruitful experience we accumulated on the breaches and their consequences to the reactor operation.