Why Is Uranium-235 Isotope Used In Nuclear Fission Reactors?

There are three naturally occurring isotopes of the uranium element (there are a total of 19 isotopes in the lab). Those found in nature are uranium-234, uranium-235 and uranium-238. In fact, these three isotopes of uranium are radioactive. The half-life of uranium-238 is about 4.5 billion years, while uranium-235 is about 700 million years.

In a nuclear reactor, when a neutron hits a heavy nucleus, such as uranium-235, it is captured by the nucleus and allows chain fission reactions to begin. The neutron, which allows the start of chain reactions in uranium-235, is low energy (goes at a few kilometers per second). In uranium-238, neutron must be high-energy to start the chain reactions.

The nucleus consists of protons and neutrons. However, the mass of the nucleus is always lower than the total mass of protons and neutrons. This mass difference is a measure of the nuclear binding energy that holds the nucleus together. Therefore, in order for a nucleus to divide, energy is required as much as the nucleus binding energy. The binding energy increases up to a certain atomic number as the mass number, that is, the total number of protons and neutrons. In nuclei with an atomic number close to 60, the binding energy is highest in the iron atom, for example. Then, as the mass number increases, the binding energy decreases.

The ratio of the number of protons in the nucleus to the number of neutrons also affects the stability of the nucleus. In nuclei with a double number of protons and neutrons, particles in the nucleus (protons with protons, neutrons with neutrons) are found in pairs, which gives the nucleus stability. Many nuclei with a proton and neutron number are more stable and their nuclear energy is high. In uranium-234 and uranium-238, the neutron number and mass number are double, while in uranium-235 the neutron number and mass number are single. For this reason, uranium-235, the binding energy of which is smaller than uranium-234 and uranium-238, can be divided more easily.