Increasing the ratio of fissile Uranium (U-235) in natural uranium is known as:

Correct answer: Enrichment

Explanation: Enrichment is the process of increasing the fraction (ratio) of the fissile isotope uranium-235 (U-235) in natural uranium, which is required to make the uranium suitable for most reactor fuels or other applications.

• Natural uranium contains about 0.7% U-235; enrichment raises this percentage.

• Typical light-water reactor fuel is enriched to about 3–5% U-235; weapons require much higher enrichment levels.

• Common enrichment methods include gas centrifuges and gaseous diffusion.

Why the other choices are not correct:

• Enhancement: A generic term, not the standard technical term for isotope separation or raising U-235 fraction.

• Concentration: Refers broadly to amount per unit volume or mass, not specifically the isotopic enrichment process.

• Accumulation: Means gathering or build-up; it does not describe the technical separation process used to increase the U-235 ratio.