Thorium cycle. Computer artwork showing the nuclear fuel cycle that uses the naturally abundant 232 isotope of thorium (Th-232) as the fertile material to produce energy. From top right: Natural thorium (Th-232) absorbs a neutron (blue sphere) from uranium fission to become Th-233. Th-233 decays quickly to protactinium-233, which decays slowly over a month to uranium-233, an ideal fuel. Uranium-233 fissions, releasing energy (white flash) and neutrons (blue spheres) to continue the cycle.