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Thorium has six naturally occurring isotopes (atoms with the same number of protons and different neutrons are called isotopes). Of these, thorium-232 is the most commonly found thorium isotope in the earth's crust. Its half-life is 14 billion years, almost equal to the estimated age of the universe.
Thorium-232 is not disintegrated as a result of chain fission (core split) reactions. Therefore, it cannot be used directly in nuclear reactors. However, thorium-232 can turn into uranium-233, which can be used as a fuel in reactors as a result of various nuclear processes. Thorium-232 nucleus captures a neutron, forming the thorium-233 isotope. Thorium-233, whose half-life is very short (about 22 minutes), turns into protactinium-233 with beta decay. Protactinium-233, which has a half-life of about 27 days, also forms uranium-233 as a result of beta decay.
The idea of using thorium as a nuclear fuel dates back to the beginning of developments in nuclear energy. In the meeting held on different nuclear reactor designs in the USA in 1944, the issue of focusing on the development of reactor designs using thorium was also discussed. With the discovery of new natural uranium deposits, it was understood that uranium resources were not as limited as predicted, and research on thorium lost its importance over time. However, as of 2002, uranium extracted from underground every year can only meet half the annual need of nuclear reactors in the world. Thorium is about three times more abundant in nature than uranium.