In a fast-breeder reactor, water cannot be used as a coolant because it would moderate the neutrons. Liquid sodium metal may be used here as a coolant and heat-transfer fluid. Research on breeder reactors has essentially stopped in the United States because of concerns over nuclear proliferation since the plutonium bred in the reactor might be used for making weapons. Due to such concerns and the complexities of construction and operation, it is unlikely that breeder reactors will ever come into general operation within the next several decades, if ever. Some 435 nuclear power plants operating around the world generate about 345,000 MW e of electricity in 32 countries, about one-sixth the world's electricity supply.
Some countries depend vitally on the electricity generated by nuclear energy.
However, the United States, which led the world in early nuclear electric power development, was also first to be affected by its decline. Over the past two decades the US nuclear electric power industry has received no new domestic orders due to concerns over reactor safety, waste disposal, regulatory uncertainty, increased costs, and decreased electrical demand growth. These same pressures are now affecting nuclear power worldwide, although countries such as France and South Korea still have vigorous programs. A rebirth may be imminent because of impending solutions to some of these problems and because of the problem of global warming due to the release of greenhouse gases in the combustion of fossil fuels. The public has become suspicious of nuclear energy partly because of the elaborate methods used to address safety concerns.