Why must U235 be enriched?

Uranium-235 (U-235) must be enriched primarily because it is not naturally abundant in the uranium ore. In nature, uranium consists mostly of uranium-238 (U-238), with only about 0.7% being U-235, which is the isotope that can sustain a nuclear fission chain reaction. For U-235 to be effectively used in nuclear reactors or for nuclear weapons, it needs to be concentrated or enriched to a higher percentage to increase the likelihood of fission events.

Enrichment increases the proportion of U-235 relative to U-238, enabling a chain reaction to occur more readily. This is critical for the efficiency and effectiveness of nuclear fuel, as reactors rely on a minimum concentration of fissile material to operate safely and effectively. Without this enrichment, the low concentration of U-235 in natural uranium would not be sufficient to sustain a controlled nuclear reaction, making it vital for practical applications in energy production and other fields.