Which uranium isotope is fissile and used as fuel in nuclear reactions?

• A. Uranium-238
• B. Uranium-235
• C. Uranium-236
• D. Uranium-234

Answer: (B)

Fissile materials can sustain a chain reaction when they absorb a neutron, releasing energy and more neutrons that keep the reaction going. Among uranium isotopes, uranium-235 is fissile and readily fissions upon absorbing a thermal (slow) neutron, making it the primary fuel used in most nuclear reactors and many weapons. Uranium-238, while abundant, is not fissile with thermal neutrons; it is fertile, meaning it can capture a neutron and eventually become plutonium-239, which can also fuel reactions but is not itself fissile in the initial capture. Uranium-236 is formed in reactors from neutron interactions and isn’t used as a stand-alone fuel. Uranium-234 is a decay product and not employed as fuel. So uranium-235 is the isotope that is fissile and used as reactor fuel.