U-238, or Uranium-238, is a naturally occurring isotope of uranium, making up about 99.3% of natural uranium. It has a long half-life of approximately 4.5 billion years, which means it decays very slowly. U-238 is not directly usable as fuel in most nuclear reactors, but it can be converted into Plutonium-239, a fissile material used in nuclear weapons and some types of reactors. U-238 is important in the field of nuclear energy and geology. It is used in radiometric dating to determine the age of rocks and minerals. Additionally, U-238 can absorb neutrons and undergo a process called neutron capture, which can lead to the production of other isotopes, contributing to the understanding of nuclear reactions and the behavior of materials under radiation.