What is the difference between HALEU and LEU?

Published: July 2026

As advanced nuclear reactors move closer to commercial deployment, one fuel term is appearing more frequently in discussions about the future of nuclear energy: HALEU, or High-Assay Low-Enriched Uranium. While most operating nuclear power plants today use LEU (Low-Enriched Uranium), many advanced reactor designs require HALEU to achieve their intended performance, efficiency, and operating characteristics.

• Sources: [nrc.gov], [energy.gov]

This article explains the differences between HALEU and LEU, why enrichment levels matter, and how these fuels support different reactor technologies.

• Source: U.S. Nuclear Regulatory Commission (NRC) and U.S. Department of Energy

Low-Enriched Uranium (LEU) is uranium that has been enriched so that the concentration of uranium-235 (U-235) is greater than natural uranium but remains below 20%.

The NRC notes that traditional commercial reactor fuel is typically enriched to between 3% and 5% U-235, making it the standard fuel for today’s operating light-water reactors.

Most of the world’s commercial nuclear power plants use LEU because their reactor designs were specifically engineered around this enrichment range (Source: nrc.gov).

The DOE defines HALEU as uranium enriched between 5% and 19.75% U-235. Because HALEU remains below the 20% enrichment threshold, it is still considered low-enriched uranium rather than highly enriched uranium (HEU).

• Source: NRC HALEU FAQs

U-235 is the uranium isotope that readily undergoes fission in most reactor designs. Increasing its concentration allows reactor designers greater flexibility in fuel and core design.

According to the NRC, fuel enriched to between 5% and 10% U-235 could potentially be used in operating light-water reactors, while fuel enriched to between 5% and 20% U-235 is proposed for use in many advanced non-light-water reactor designs.

The DOE states that HALEU contains a higher amount of fissile material, which can make fuel more efficient for certain advanced reactor applications.

Many next-generation reactor developers have designed their systems around HALEU fuel.

According to the DOE, most advanced reactor designs require HALEU to achieve characteristics such as:

• Smaller reactor designs
• Longer operating cycles
• Increased efficiency relative to current technologies

The NRC notes that several advanced reactor developers propose using HALEU-containing fuel in advanced reactor concepts, including designs that utilize TRISO fuel and molten salt reactor technologies.

• Sources: NRC and DOE

The DOE has identified HALEU availability as a critical requirement for the deployment of advanced reactors in the United States. The agency established the HALEU Availability Program under the Energy Act of 2020 to help ensure access to HALEU for domestic research, demonstration, and commercial applications.

According to the DOE, most advanced reactor designs require HALEU and supply limitations could delay reactor deployment if adequate fuel production capacity is not developed.

In January 2026, the DOE announced major investments to expand domestic LEU and HALEU enrichment infrastructure and support the development of a U.S.-based nuclear fuel supply chain.

Read more:

• DOE: HALEU Enrichment Services
• NuclearNewswire: DOE awards $2.7B for HALEU and LEU enrichment

What does HALEU stand for?

HALEU stands for High-Assay Low-Enriched Uranium, uranium enriched to greater than 5% and less than 20% U-235.

What does LEU stand for?

LEU stands for Low-Enriched Uranium. Traditional commercial reactor fuel is generally enriched between 3% and 5% U-235.

Is HALEU the Same as Highly Enriched Uranium?

No.

The NRC explicitly states that uranium enriched to 20% or greater U-235 is classified as highly enriched uranium (HEU). HALEU remains below that threshold and therefore remains a form of low-enriched uranium.

This distinction is important because HALEU occupies a middle ground between the enrichment levels used in today’s commercial reactors and the higher enrichment levels

Why do advanced reactors want HALEU?

The DOE states that most advanced reactor designs require HALEU and use it to support reactor performance characteristics such as smaller designs, longer operating cycles, and increased efficiency.

Does the United States currently produce HALEU?

The DOE states that commercial HALEU supply remains limited and has established programs to expand domestic enrichment capabilities.