The isotope 235U, with an abundance of only 0.7% in natural uranium, is commonly used to produce electricity in nuclear fission reactors. This isotope has the distinctive and useful property of undergoing nuclear fission through interaction with thermal-energy neutrons (neutrons with average speeds of only a few km/s). The other main isotope of uranium, 238U, does not undergo nuclear fission with thermal neutrons, but it does capture neutrons to form the isotope 239Np which then decays to 239Pu. This isotope of plutonium undergoes nuclear fission with thermal neutrons with a higher probability than that of 235U. The energy released in the fission of 235U and 239Pu, mainly in the form of kinetic energy of the fission fragments, provides the heat to run the turbines that generate electricity at a nuclear fission power plant.