Superphénix (French pronunciation: [sypɛʁfeniks]; English: Superphoenix, SPX) was a nuclear power station prototype on the Rhône river at Creys-Malville in France, close to the border with Switzerland. Superphénix was a 1,242 MWe fast breeder reactor with the twin goals of reprocessing nuclear fuel from France's line of conventional nuclear reactors, while also being an economical generator of power on its own. As of 2024^[update], Superphénix remains the largest breeder reactor ever built.

Construction began in 1976, the reactor went critical in 1985 and was connected to the grid in 1986. The project suffered cost overruns, delays and enormous public protests. Overall, the reactor totalized a very low operation factor of 14.4%. Despite many technical issues related to being a first-of-a-kind project most of its downtime was caused by administrative procedure: the plant was technically capable of resuming operations but was forbidden to do so. Technical problems were solved over time and, by 1996, the plant had reached an availability of 95%.

The plant was powered down in December 1996 for maintenance, and while it was closed it was subject to court challenges that prevented its restart. In June 1997, the newly appointed Prime Minister, Lionel Jospin, announced that Superphénix would be closed permanently; this was made official by ministerial decree in December 1998.

France had considered the problem of plutonium production just after the end of World War II. At the time, the conventional solution to this problem was to use a graphite moderated air or water cooled reactor fueled with natural uranium, such as the UNGG. Such designs have little economic value in terms of power production, but are simple solutions to the problem of "breeding" plutonium fuel, which can then be separated from the original uranium fuel with chemical processing.

It had long been known that another solution to the breeder reactor design was to replace the graphite with liquid sodium metal. The graphite is used as a moderator, slowing the neutrons released in the nuclear reactions to a speed that makes other uranium atoms receptive to them. If the natural uranium fuel is replaced with fuel sensitive to fast neutrons, typically highly enriched uranium or plutonium, the reaction can run without the use of a moderator.

While this design eliminates the need for a moderator, the core still needs to be cooled. Ideally the coolant would be both highly efficient, allowing the core size to be reduced, as well as being largely transparent to neutrons. The most studied example of such a material is liquid sodium, although salts and other metals have also been used.

This not only greatly reduces the size of the reactor, but the fast neutrons from a single reaction are capable of causing several breeding reactions. By surrounding the core with additional fertile material such as natural uranium, or even nuclear waste from other reactors, the breeding reaction will take place in a larger volume and in otherwise useless materials. This section is known as the blanket. Such a design also has the quality that it generates more fuel than it consumes, as long as the breeding ratio is greater than 1.

Such a design has three major advantages over conventional military designs.