The Super-Phenix was the only full-scale, liquid-metal, fast-breeder reactor. That might sound esoteric, but for many years, a big chunk of the United States’ nuclear R&D program was devoted to just this kind of power plant.
Breeder reactors yield more fissile material than they use. In other words, you get more plutonium out than you put uranium and plutonium in. During an era when plutonium for weapons was considered essential and uranium supplies were thought to be scarce, breeders were a hot item.