Uranium-238 is a naturally occurring isotope of uranium, making up about 99.3% of all uranium found in nature. It has a long half-life of approximately 4.5 billion years, which means it decays very slowly. This isotope is not directly usable as fuel in most nuclear reactors, but it can be converted into plutonium-239, a fissile material used in nuclear energy and weapons. Uranium-238 is primarily mined for its potential in nuclear applications and is also used in the production of depleted uranium, which has military and industrial uses. Additionally, it plays a role in the natural decay series that produces radon, a radioactive gas that can pose health risks.