Construction of France’s first experimental sodium-cooled reactor, Rapsodie, started in 1962 and it went critical on 28 January 1967 with a nominal capacity of 20 megawatts thermal (MWt). At the end of 1967, its power was increased to 24 MWt, and in 1970, after core redesign, to 40 MWt. Its operating power was reduced to 22 MWt in June 1980 to minimize the thermal stresses thought to be the source of cracks in the reactor vessel. The reactor operated until April 1983, when it was shut down permanently.

Rapsodie was a loop-type reactor, with the heat exchanger between the primary and secondary sodium loops outside the reactor vessel. It was as close as possible to the basic design imagined for commercial applications (molten-sodium coolant, reactor material, power density, etc). The core contained 31.5 kilograms (kg) of plutonium-239 and 79.5 kg of uranium-235. The mean duration of reactor runs was 80 days and the fuel reached burn-ups of 102,000 MWd/t.2