A boiler is a closed vessel in which water or other fluid is heated under pressure. The fluid is then circulated out of the boiler for use in various processes or heating applications. Construction of boilers is mainly limited to copper, steel, stainless steel, and cast iron. The source of heat for a boiler is combustion of any of several fuels, such as wood, coal, oil, or natural gas. Electric boilers use resistance or immersion type heating elements. Nuclear fission is also used as a heat source for generating steam. Heat recovery steam generators use the heat rejected from other processes such as gas turbines.