Nuclear propulsion in space can be envisaged in two ways:

--electric nuclear propulsion in which the heat created by a nuclear reactor is converted "into electric energy used to produce either a beam of fast ions, or a plasma, which is ejected into space and plays the role of propulsion fluid";

--"thermal nuclear propulsion in which the fluid used for propulsion is carried to a very high temperature by passage through the core of a nuclear reactor and ejected at great speed into space" [CEA 95].

The CEA in collaboration with the Centre national d'études spatiales (National Center of Spatial Studies, CNES), has studied both ways. During the eighties, the CEA and the CNES, pursuing the first way, defined a spatial generator Erato, "a reference model" of 200 kW. The generator included " a fast neutron reactor contained in a metallic vessel using as fuel highly enriched uranium (93%) in the form of uranium nitrate (UN) and cooled by liquid lithium . . . ; a primary lithium-helium thermal exchanger . . . ; a secondary circuit composed of four independent loops each containing a gas turbine in which warm helium expands . . . ; a cold source . . . ; a protective shield [Clef iv.87].

In 1992, the CEA, pursuing, in collaboration with the CNES, the second way, carried out an advance study of a thermal propulsion reactor for lifting of loads of about three tons between a terrestrial orbit 600 km high and a lunar orbit. The system included a reservoir of about twelve tons of liquid hydrogen, a turbopump assuring the circulation of the hydrogen, the nuclear reactor operating at very high temperature, and a nozzle. A conventional rocket of the Ariane V type would carry the whole into orbit [CEA 95]. In the summer of 1995 the CEA undertook a study of the technical-economic feasibility of such a device, in association with Phillips Laboratory in the United States. The CEA cofinanced the study [CEARev l.viii.95].

The CEA is also interested in the fabrication of RTG (Radioisotopic Thermoelectric Generators) and RHU (Radioisotopic Heater Units).

The isotopic generators such as RTG, "use the energy generated in the form of radiation by a radioactive source. Their power is scarcely more than several kWe. Nevertheless, entirely static" and having a life of ten years or more, they are "the devices most likely to furnish the electric energy indispensable to spatial devices for long-lasting missions" [Clef (iv). 94].

The RHU, low-power radioisotopic sources, are used to keep certain components at a given temperature during a mission. Plutonium 238 in the form of dioxide is the radioactive source used. The CEA affirmed that it and Cogéma could fabricate the plutonium 238 and that "all the capabilities exist . . in the CEA and in French industry to undertake the development and fabrication of RHU . . . In the longer term, the development of RTG can also be envisaged."