U is the only nuclide existing in nature ( in any appreciable amount ) that is fissile with thermal neutrons .
Natural uranium is 99.284 % U isotope , with U only constituting about 0.711 % of its mass .
U is the only U existing in nature ( in any appreciable amount ) that is fissile with thermal neutrons .
Natural uranium is 99.284 % U isotope , with U only constituting about 0.711 % of its mass .