The fission process releases large amounts of energy, about 50 million times that of coal on a weight basis, which means that very little uranium fuel is required. No combustion is involved so that there are no emissions but fission generates radioac­tive materials, some of which have very long lives. There are also considerable prob­lems in the reprocessing and disposal of spent fuel, the possibility of leaks or accidents, and the decommissioning of the power stations at the end of their working life. The accidents at the nuclear-generating plants at Three Mile Island and Chernobyl have shown that, despite very stringent safety arrangements, accidents can occur. This has made the public wary of nuclear power and more likely to accept alternative sources of power. Nuclear power is also regarded as not sustainable as there is a limited supply of uranium and its production involves the production of greenhouse gases.