238U

238U is a naturally occurring radioactive isotope of uranium with mass number 238. It constitutes about 99.27% of natural uranium and has a half-life of 4.468 × 10^9 years. It decays by alpha emission to 234Th, initiating the long 238U decay series that ultimately yields stable 206Pb. The series includes a succession of alpha and beta decays through nuclides such as 234Th, 234Pa, 234U, 230Th, 226Ra, 222Rn, and 210Po, among others.

Natural occurrence and dating: 238U is the dominant uranium isotope in most rocks and soils and is present in seawater in trace amounts. It provides the basis for uranium–lead dating, particularly U-Pb dating, which determines ages of ancient igneous and metamorphic rocks by measuring the 238U to 206Pb decay.

Nuclear applications: 238U is fertile rather than fissile; when it captures a neutron it becomes 239U, which beta decays to 239Pu, a fissile material used in nuclear reactors and some weapons. 238U itself is not readily fissioned by thermal neutrons, but can participate in fast neutron fission and serves as a key starting material for breeding plutonium in reactors.

Health and safety: As an alpha emitter, 238U poses health risks primarily if ingested or inhaled; external exposure is limited. Its decay chain also produces radon gas, a health hazard in enclosed spaces and building materials containing natural uranium.