Construction of new nuclear power plants is expected to restart shortly in the United States, and the number of nuclear plants worldwide is expected to jump beyond the 435 current plants producing 15% of the world's electricity. Most nuclear plants use nuclear fission of fissionable uranium-235 to produce large amounts of energy due to mass conversion into energy. That heat generation is removed by coolants which go on to produce electricity. The fissions generate fission products (FP) and neutrons absorbed into fertile uranium-238 to produce plutonium (Pu) and minor actinides (MA), consisting of americium (Am), curium (Cm), and neptunium (Np). There are two principal types of reactors: thermal and fast. Thermal reactors use a moderator to reduce their energy level, while fast reactors with no moderator operate at a much higher energy level and can better fission plutonium and minor actinides. Also, they produce more neutrons and can breed fissile material.