What is Mass Separation?

Published: July 2026

Uranium enrichment depends on a deceptively simple idea: isotopes of the same element behave almost identically in chemical reactions, but they have slightly different masses. Natural uranium is mostly uranium-238, with about 0.7% uranium-235, the isotope most useful for sustaining a nuclear chain reaction in common reactor designs. Because U-235 and U-238 are chemically similar, enrichment relies on physical separation, not ordinary chemistry, to increase the share of U-235 in the material.

Sources:

• Energy.gov: HALEU Frequently Asked Questions
• Nuclear Regulatory Commission: Uranium Enrichment

In uranium enrichment, “mass separation” refers to the process of separating uranium isotopes based on their slight weight difference. Before enrichment, uranium oxide is converted into uranium hexafluoride, or UF₆, because UF6 can exist as a gas at useful operating temperatures and fluorine has only one naturally occurring isotope, which avoids adding unwanted isotope-mass complexity to the separation process. Once in gaseous form, uranium isotopes can be separated by processes such as gas centrifugation, gaseous diffusion or laser separation, although centrifuge technology is the dominant commercial enrichment process today (source: NRC).

In a gas centrifuge, UF₆ gas spins at very high speed inside a rotor. The heavier U-238-bearing molecules tend to move slightly outward, while the lighter U-235-bearing molecules remain slightly closer to the center A single centrifuge only performs a small amount of separation, so enrichment facilities connect many centrifuges in cascades, gradually raising the concentration of U-235 while producing a separate depleted stream with less U-235 (source: NRC).

See also:

• World Nuclear News: Upgrading of third Russian uranium enrichment plant starts

For today’s commercial nuclear power plants, uranium is typically enriched from its natural level of about 0.7% U-235 to roughly 3% to 5% U-235 (source: NRC). High-assay low-enriched uranium, or HALEU, requires a higher assay: more than 5% and less than 20% U-235, according to the U.S. Department of Energy. That makes HALEU more concentrated than conventional low-enriched uranium, but still below the 20% threshold generally associated with highly enriched uranium (source: NRC).

Mass separation is therefore central to HALEU production because HALEU is not a different element or a chemically altered form of uranium. It is uranium with a higher proportion of U-235. Producing HALEU through enrichment requires additional separative work beyond conventional LEU, along with licensed enrichment, transport, deconversion and fuel fabrication infrastructure capable of handling uranium at higher assays (source: DOE).

HALEU matters because many advanced reactor developers expect higher-assay fuel to support smaller reactor cores, longer operating cycles, increased fuel efficiency and better fuel utilization. The Department of Energy has said the United States has limited commercial HALEU enrichment services available, which is why federal programs are supporting domestic enrichment capacity and related infrastructure. DOE has estimated domestic HALEU demand could reach 50 metric tons per year by 2035 as advanced reactors move toward deployment.

In short, mass separation is the enabling step that turns natural uranium into usable enriched uranium fuel. For HALEU, the same core principle applies, but the target enrichment level is higher, the supply chain requirements are more specialized and the strategic importance is growing as advanced nuclear technologies move closer to commercial use.

See also:

• Nuclear Regulatory Commission: High-Assay Low-Enriched Uranium (HALEU)