Spent nuclear fuel is about 95 percent uranium. About 1 percent are other heavy elements such as curium, americium and plutonium-239, best known as fuel for nuclear weapons. Each has an extremely long half-life — some take hundreds of thousands of years to lose all of their radioactive potency. The rest, about 4 percent, is a cocktail of byproducts of fission that break down over much shorter time periods, such as cesium-137 and strontium-90, which break down completely in about 300 years.