For reactor fuel, uranium enrichment is primarily intended to increase the…

Concept

Isotope enrichment changes the relative proportions of isotopes in a material without changing the chemical identity of the element.

For uranium fuel, the relevant distinction is between the fissile isotope that can sustain a thermal-neutron chain reaction and the more abundant fertile isotope.

Application

Natural uranium contains mostly U-238 and only a small fraction of fissile U-235. Enrichment raises the proportion of U-235 so the material can meet the fissile-content requirement of many reactor fuels.

Contrast among the values

• U-234 is a naturally occurring trace isotope and is not the principal fuel isotope targeted by standard uranium enrichment.

• U-235 is fissile with thermal neutrons and is the isotope whose fraction is raised in enriched uranium.

• U-238 is the dominant fertile isotope in natural uranium; its relative fraction falls as U-235 is enriched.

• U-239 is a short-lived isotope produced after neutron capture by U-238 and is not a significant constituent of natural uranium.

Cross-check

The leftover depleted-uranium stream contains a lower proportion of U-235 and is correspondingly richer in U-238, confirming which fraction the enrichment stream increases.

Result

The concentration increased by uranium enrichment is that of U-235.