Nuclear Fuel 101

Posted on January 17, 2024 by Natalie Houghtalen

Updated on Jan 17, 2024 by Natalie Houghtalen and Jack Ridilla. Originally posted on July 30, 2019 by Natalie Houghtalen

Nuclear fuel burns, but not in the way you might think, like wood, where carbon bonds are broken to create heat. Instead, when the fuel burns, nuclear reactions break the atoms themselves. Nuclear energy has zero emissions and only creates a small amount of well-accounted-for waste. In fact, the amount of waste for one year of an American person’s energy use would be the size of a gummy bear, and a lifetime of energy use would fit in a soda can.

There are two main types of reactors used to generate electricity worldwide: pressurized water reactors and boiling water reactors. Both run on low-enriched uranium fuel pellets that heat water and drive a steam turbine. They were first developed in the 1950s and fall under the “light-water reactor” umbrella. Today, around 18 percent of the United States’ total electricity demand is met with only 92 reactors.

All nuclear reactors function on the same principle: a neutron hits a uranium-235 atom causing it to break apart into two big pieces and three new neutrons. This process is called nuclear fission, and uranium-235 is the only naturally occurring isotope that can do this. The two big pieces shoot out, bump into other atoms, and slow down. This process, which is essentially nuclear rug burn, creates the heat used to boil water and ultimately generate power.

Nuclear Fission Process

Nuclear interactions are all around us. In fact, 50 percent of the heat in the Earth’s core comes from radioactivity. This is an incredible, natural process. The question is, how do we harness it to create electricity?

Nuclear Fuel Energy Density

Mining and Refining

Uranium is a mineral found worldwide and its ore is mined in over a dozen countries, including the U.S. After the ore is mined, the uranium is separated out and dried into a powder, called “yellowcake,” which is between 80-90% pure uranium oxide.

Conversion

In order to get ready for enrichment, the powder has to be converted into a gas. There are two processes that do this: wet and dry conversion. The wet conversion process is used in Canada, France, China and Russia, while the dry process is used primarily in the U.S. During the conversion process, purified uranium oxide is transformed into uranium hexafluoride. Conveniently, this gas is also a powder at room temperature, making it easy to transport in 14-ton cylinders to the next process stage: enrichment.

Enrichment

Natural uranium contains three different types of uranium atoms, known as “isotopes.” Each isotope has the same number of protons, but a different number of neutrons. Adding the protons and neutrons together gives the number after the atom’s name (i.e., uranium-235 and uranium-238). The higher the number, the heavier the atom. Uranium-235 is the isotope that produces heat in today’s nuclear power plants. For it to be useful as fuel, the concentration of uranium-235 has to be increased using a process called enrichment. The fuel used in traditional U.S. reactors, low-enriched uranium (LEU) fuel, requires an increase in the concentration from 0.72% to 3-5%. Many new reactors require fuel at enrichments of 15-20%, called high-assay, low-enriched uranium (HALEU) fuel. Both of these fuels are nowhere near the enrichment required for nuclear weapons which would be 85% or more.

Uranium Enrichment by Product

There are several enrichment technologies that can accomplish this, but the most widely used is the gas centrifuge. During this process, a cylinder filled with gas is spun until the heavier, uranium-238 gas moves toward the walls of the centrifuge and the lighter, uranium-235 gas remains in the center. The lighter gas, with the higher concentration of uranium-235, is moved into the next centrifuge to repeat the enrichment process until the desired concentration is reached. Nuclear fuel enrichment capacity is primarily located in Russia and China, which collectively own nearly 60% of global capacity. Currently, 8% of global enrichment capacity is located in the U.S., but the U.S. Department of Energy (DOE) is working with American companies to stand-up a domestic supply chain for LEU and HALEU fuel. A robust and reliable supply of advanced nuclear fuel will play an important role in securing a clean energy future.

Deconversion and Pellet Manufacturing

After enrichment, the enriched uranium gas is deconverted into a powder that can be shaped into a small ceramic pellet. Ceramic is a great material for stovetops, coffee cups and nuclear fuel; it does not expand, crumble or melt easily. It also contains fission products within the pellet.

Fuel Assembly Fabrication

Pellets are stacked into rods 13 feet long and bundled into a fuel assembly. There are about 150 assemblies in each reactor core. About one-third of the assemblies are replaced every 18 to 24 months, and one bundle, on average, has a useful life of five years.

Manufacture of a Fuel Assembly

Fuel Assembly

Nuclear energy provides reliable, efficient and emissions-free power to our grid. Keeping our nuclear facilities online is critical if the U.S. is to achieve a low-emissions future while maintaining a reliable supply of electricity. To unleash even more American nuclear energy potential and lead in innovation, the U.S. must secure the nuclear fuel supply chain.