(238U). The abundant isotope of uranium of which it comprises 99%. It is not fissionable but will form plutonium-239 as a result of bombardment by neutrons in a reactor. Its half-life is 4.51 × 109 years. It will be used in breeder reactors, together with plutonium, where its energy potential will be exploited by transmuting it to fissionable plutonium.See Breeder.