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

Enrichment aims to raise the concentration of the fissile isotope uranium 235.

Separating uranium from its ores is milling and refining, a step done before enrichment.

Raising the share of U-238 would work against making usable reactor fuel.

Adding more moderator is a reactor design matter, not part of enrichment.

Natural uranium has too little U-235 to sustain a chain reaction in most reactors.

Gas centrifuges and gaseous diffusion are the common methods used for enrichment.