Nuclear fuels are obtained from inorganic minerals extracted by mining. Although they are at least partially consumed when used in nuclear reactors for the production of heat, they differ from fossil fuels in the way they release energy. Burning of fossil fuels, such as coal, oil and gas, is a chemical reaction. Nuclear fuels, such as uranium, are destroyed by a process of spontaneous disintegration, called fission, and prompted by natural radioactivity. If the process is left to occur naturally in uranium-bearing rock, the rate of change is imperceptibly small. In a man-made nuclear reactor the energy-releasing processes of disintegration, which in the natural state happen slowly over thousands of millions of years, are compressed into minutes. The release of energy is harnessed to generate steam which drives electricity generators.