[en] 'COSI', a software developed by the Nuclear Energy Direction at CEA, the French Atomic Energy Commission, is a code simulating a pool of nuclear electricity generating plants with its associated fuel cycle facilities. This code has been designed to study short, medium and long term options for the introduction of various types of nuclear reactors and for the usage of associated nuclear materials. It permits to study transition scenarios and gives due consideration to isotopic composition essentially of uranium, plutonium, minor actinides and some fission products. (authors)