In case of nuclear energy, the objective of the enrichment process is

Concept

Enrichment is an isotope-separation stage that raises the relative abundance of a selected isotope above its natural level.

Natural uranium is about 0.7% uranium-235 and about 99.3% uranium-238; uranium-235 is fissile and can sustain a chain reaction.

Application

For reactor fuel, enrichment changes the isotopic composition so that the proportion of fissile uranium-235 is higher than in natural uranium.

Contrast

• Separating uranium from its ore is part of mining and milling, before isotope enrichment.

• Increasing uranium-238 would not create the higher fissile fraction required for ordinary reactor fuel.

• Changing the amount of moderator is a reactor-operation choice, not an isotope-separation objective.

Cross-check

Natural uranium already contains mostly uranium-238, whereas common reactor fuel requires a uranium-235 fraction above its natural abundance. Therefore the enrichment process increases the concentration of fissile uranium-235.

Result

The objective is increasing the concentration of fissile uranium-235.