High-assay low-enriched uranium, enriched to between 5 and 20 percent U-235. It is still low-enriched uranium, not weapons-grade material.

Why is TRISO fuel called robust?

Each particle has its own ceramic and carbon coatings, led by a silicon carbide layer, that retain fission products at temperatures far above those in normal operation.

What is a microreactor?

A very small, factory-built reactor, typically 1 to 20 MWt according to DOE, small enough to transport by truck and intended for remote sites, bases or industrial users.

Why don't sodium-cooled reactors need high pressure?

Sodium boils at about 883 °C at atmospheric pressure, far above typical operating temperatures, so it stays liquid without pressurization.

Advanced reactor designs and fuels: HALEU, TRISO, sodium, helium and molten salt

Beyond small light-water reactors, many advanced designs change the fuel, the coolant or both. This page explains the main ideas at a general level, without ranking vendors.

Study text drafted from public-domain US government sources. Practice questions are reviewed before publication.

Fuel: HALEU and TRISO

High-assay low-enriched uranium (HALEU) is enriched to between 5 and 20 percent U-235, above today's commercial fuel but below the 20 percent threshold for low-enriched uranium. DOE notes that it allows smaller designs, longer operating cycles and better efficiency, which is why many microreactor and advanced designs depend on it. Securing a domestic HALEU supply is one of the practical challenges for these projects.

TRISO (tri-structural isotropic) fuel consists of tiny uranium kernels coated with layers of carbon and silicon carbide. The silicon carbide acts as a miniature pressure vessel that retains fission products at very high temperatures. Thousands of particles are packed into graphite pebbles, as in pebble-bed reactors, or into compacts in prismatic blocks.

Coolants: gas, liquid metal and salt

Helium is chemically inert, stays a gas and barely becomes radioactive, so it suits high-temperature gas-cooled reactors. Liquid sodium has a boiling point of about 883 °C, so sodium-cooled fast reactors run near atmospheric pressure, but sodium reacts with water and air, so designs use an intermediate loop and inert cover gas. Molten salts also allow high temperatures at low pressure, with their own chemistry and corrosion challenges. Heat-pipe microreactors move heat with sealed pipes that evaporate and condense a working fluid, with no pumps.

Fast-spectrum designs can breed fissile fuel from U-238 and fission long-lived actinides. Internationally, the Generation IV International Forum studies six systems: gas-cooled, lead-cooled and sodium-cooled fast reactors, molten salt reactors, supercritical-water-cooled reactors and very-high-temperature reactors. In the US, DOE's DOME test bed at Idaho National Laboratory hosts microreactor experiments.

Sample quiz

Practice questions for this section are being reviewed.

Practice questions for this section are still in review and will appear here once they have been checked.

Frequently asked questions

What is HALEU?: High-assay low-enriched uranium, enriched to between 5 and 20 percent U-235. It is still low-enriched uranium, not weapons-grade material.
Why is TRISO fuel called robust?: Each particle has its own ceramic and carbon coatings, led by a silicon carbide layer, that retain fission products at temperatures far above those in normal operation.
What is a microreactor?: A very small, factory-built reactor, typically 1 to 20 MWt according to DOE, small enough to transport by truck and intended for remote sites, bases or industrial users.
Why don't sodium-cooled reactors need high pressure?: Sodium boils at about 883 °C at atmospheric pressure, far above typical operating temperatures, so it stays liquid without pressurization.

Sources

U.S. DOE Office of Nuclear Energy, What is High-Assay Low-Enriched Uranium (HALEU)?
U.S. DOE Office of Nuclear Energy, TRISO Particles: The Most Robust Nuclear Fuel on Earth
U.S. DOE Office of Nuclear Energy, What is a Nuclear Microreactor? www.energy.gov
U.S. DOE Office of Nuclear Energy, Advanced Small Modular Reactors (SMRs) www.energy.gov
U.S. NRC Glossary (NRC Basic References) www.nrc.gov

Explanations on this page are original. Cited US government works are in the public domain; they do not endorse this site.