The reactor block of the Superphenix nuclear reactor in Creys-Malville. The Superphenix is a fast breeder reactor developed in France in the Isere department.

(Source: IAEA)

A reactor that, unlike thermal reactors, operates mainly with fast neutrons (neutrons in the energy range above 0.1 MeV) and does not need a moderator. Fast reactors are generally designed to use plutonium fuels and can produce, through transmutation of ²³⁸U, more plutonium than they consume, i.e., they can be operated as breeder reactors with a conversion ratio greater than unity.