use as much of their fuel up as possible—not make more. That’s especially true when the reactor produces plutonium, which is easy to turn into nuclear weapons. Breeder reactors fell out of favor quite early in the history of nuclear power plants; today, countries like the U.S., the U.K., and Germany have long since abandoned their breeder development programs. The core of the Russian Fast Breeder Reactor on June 27, 2017 in Zarechny, Svedlovsk Oblast, Russia. Pallava BaglaGetty Images That’s not the case in China. The CFR-600 is a sodium-cooled fast-neutron reactor, which means instead of using water, like most of the world’s working nuclear power plants, it’s cooled by liquid sodium, with a much broader temperature range and less interactivity than water. And fast neutrons have more natural energy than the alternative, thermal neutrons, which must be held in a temperature-controlled medium in order to act. Fast reactors don’t require the medium.