Noun
uranium-234 (uncountable)
(physics) A radioactive isotope of uranium, 92U, having a half-life of about 250,000 years; it accounts for about 55 parts per million of natural uranium; it is a member of the uranium-238 decay chain (aka "uranium series").
Uranium-thorium dating method main A relatively short-range dating technique is based on the decay of uranium-234 into thorium-230, a substance with a half-life of about 80,000 years. Source: Internet
This is because uranium-238 decays directly to thorium-234, which with a half-life of 24 days decays to protactinium-234, which in turn decays in a matter of hours to the long-lived uranium-234. Source: Internet