Among Primordial nuclides, the four shortest-lived have been verified as Th, U, K, and U, Uranium-235, an isotope of uranium.
The four shortest-lived primordial nuclides (i.e. nuclides with shortest half-lives) to have been indisputably experimentally verified are  Th  (1.4 x 10 years),  U  (4.5 x 10 years),  K  (1.25 x 10 years), and  U  (7.0 x 10 years). Uranium-235 (U) is an isotope of uranium making up about 0.72% of natural uranium. 
