Uranium-238 () is the most common isotope of uranium found in nature, accounting for approximately 99.27% of natural uranium. It is a primordial nuclide, meaning it has existed in its current form since the formation of the Earth. With a half-life of about 4.47 billion years, roughly the age of the solar system, it is exceptionally stable for a radioactive element.
Uranium-238 is the head of the Uranium Decay Series, also known as the radium series. It undergoes a long sequence of alpha and beta decays before reaching stability as Lead-206. Key intermediate products in this chain include Radium-226 and Radon-222. In undisturbed geological formations, this chain typically exists in secular equilibrium.
is a fertile material rather than a fissile one. While it cannot sustain a nuclear chain reaction on its own in a thermal reactor, it can absorb a neutron to become , which then undergoes two beta decays to become Plutonium-239 (), which is highly fissile. This transmutation process is a core principle of breeder reactor technology.
Due to its extremely long half-life, is used extensively in geochronology. The Uranium-Lead dating method relies on the ratio of to to determine the age of ancient rocks and minerals like zircon. In the industrial sector, is the primary component of Uranium Mining. The tailings from these operations often contain high concentrations of the decay progeny, presenting a long-term NORM management challenge.
Uranium-238 itself is an alpha emitter. Because alpha particles have low penetration, it primarily poses an internal hazard if inhaled as dust or ingested. Chemically, uranium is also a heavy metal toxin, primarily affecting the kidneys. However, from a radiological perspective, the greatest risk often comes from its daughter product, Radon gas, which can accumulate in enclosed spaces.