Fast neutrons are produced by nuclear processes such as nuclear fission. These neutrons may be emitted during either spontaneous or induced nuclear fission. Neutrons are produced copiously in nuclear fission and fusion. Thermal neutrons are more likely than fast neutrons to cause fission. Radionuclides are produced as an unavoidable result of nuclear fission and thermonuclear explosions. Most nuclear fuels contain heavy fissile elements that are capable of nuclear fission. These neutrons are produced by the spontaneous fission of plutonium-238. The reactors produce heat through nuclear fission which heats water. The neutrons released by fusion will fission U-238. Technetium-99 has a fission product yield of 6.0507% for thermal neutron fission of uranium-235. Some of them produce neutrons, called delayed neutrons, which contribute to the fission chain reaction. The time between absorbing the neutron and undergoing fission is measured in nanoseconds. This includes physics research in support of nuclear fission and nuclear fusion energy. It contained of enriched uranium, although less than a kilogram underwent nuclear fission. Xe is of considerable significance in the operation of nuclear fission reactors.