Phénix (French for phoenix) was a small-scale (gross 264/net 233 MWe) prototype fast breeder reactor, located at the Marcoule nuclear site, near Orange, France.

It generated 590 MW of thermal power, and had a breeding ratio of 1.16 (16% more plutonium produced than consumed), but normally had to be stopped for refueling operations every two months.

[1] Plans for a French fast reactor date as far back as 1958's Rapsodie, and followed up in 1964 for a larger design with a power output of 1 GWe.

Only a small number of breeders, estimated to be around 20, would be required to fuel the fleet of about 200 light water reactors.

The fuel load is capable of running for about 90 days maximum, but in practice it normally ran for two month periods.

Phénix ran without problems through the 1970s and '80s, but in the early 1990s it began to demonstrate a number of unexplained behaviours, including large power transients.