How does uranium-235 differ from uranium-238?

Uranium-235 and uranium-238 are both isotopes of the element uranium, but they differ significantly in their natural abundance and nuclear properties.

Key Differences

• Natural Abundance: Uranium-238 is the most plentiful isotope, making up approximately 99.27 percent of naturally occurring uranium. In contrast, uranium-235 is relatively rare, constituting only about 0.72 percent of natural uranium.12
• Fissile Properties: Uranium-235 is the only naturally occurring fissile material, meaning its nucleus can undergo nuclear fission when it collides with a slow neutron. Because it fissions more readily and releases more neutrons per fission than other isotopes, it is essential for sustaining a chain reaction in nuclear reactors and atomic bombs.12
• Transmutation: While uranium-238 is not fissile with slow neutrons, it plays a critical role in nuclear technology. After absorbing neutrons and undergoing beta decay, it is transmuted into plutonium, which is a fissile material used in breeder reactors.2
• Radioactive Decay: Both isotopes are radioactive, but they have different half-lives. Uranium-238 has a half-life of approximately 4.5 billion years, while uranium-235 has a half-life of about 700 million years.12