Which uranium isotope is commonly used as reactor fuel due to fissile nature?

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

Answer: (B)

Fissile materials can sustain a nuclear chain reaction with neutrons. Among uranium isotopes, only uranium-235 is fissile enough to be used directly as reactor fuel in typical reactors because it fissions readily with thermal (slow) neutrons and can maintain a chain reaction. Natural uranium contains only about 0.7% of this isotope, so enrichment is used to increase its share for practical operation. Uranium-238 does not fission with thermal neutrons; it’s fertile and can become plutonium-239, which can act as fuel but isn’t the direct fissile fuel in most designs. Uranium-236 is formed during fission and isn’t used as fuel. Uranium-234 is a decay product and also not used as reactor fuel. Thus, uranium-235 is the isotope commonly used as reactor fuel due to its fissile nature.