Posted on May 30, 2024 by Natalie Houghtalen
Congratulations! If you have read our Nuclear Fuel 101, you are already well on your way to understanding the importance of the nuclear fuel supply chain. This second installation goes a bit further, discussing what happens after fuel is used, innovations in nuclear fuel and why new types of fuel are important for new reactor designs.
Most new advanced reactor designs incorporate passive and inherent safety features (like heat transfer) that remove the need for active, engineered safety systems (like a pump or valve). This allows designs to shut down naturally and safely in case of an event, like a total loss of power.
Advanced reactors are designed to be smaller, more affordable, safer, and more fuel efficient. Like other types of new reactors – fast reactors – can harness the energy from used fuel. This could reduce proliferation risks and decrease the volume of high-level nuclear waste by around 85%. Currently, the U.S. safely stores used nuclear fuel at more than 80 sites across 35 states.
After nuclear fuel is used in nuclear reactors, there are a few options for managing the fuel.
Storage. When removed from any type of reactor, nuclear fuel continues to emit both radiation and heat. Once the fuel has reached the end of its useful life, the assembly is loaded into a nearby storage pool to allow the heat and radiation levels to decrease. After several years, the fuel may be transferred to naturally-ventilated storage, typically in dry casks like the ones shown below at the Comanche Peak Nuclear Power Plant.
The Comanche Peak Nuclear Plant has provided jobs for 1,300 workers since 1990 and supplies Texans with 2,425 MWe, enough to power 500,000 homes.
Recycling. One factor that makes nuclear energy so unique is the ability to reprocess and recycle used fuel. In a reactor, uranium splits into two new atoms (or elements). Over time, these new atoms build up like dirt on a windshield until the fuel is no longer usable. Used fuel reprocessing removes that “dirt,” giving the remaining uranium a second life. Even after several years in a reactor, the fuel retains more than 90% of its potential energy. Globally, in 2019, recycled fuel replaced the need for more than 2,200 tons of new natural uranium, despite reprocessing capacity being limited to France, the U.K., Russia, Japan and India.
The Nuclear Fuel Cycle
Source: World Nuclear Association
There are a number of innovations in nuclear fuel that are currently in early stage use or research, development and demonstration (RD&D) phases.
Mixed oxide fuel (MOX) is the second, third, fourth and even fifth life of the low-enriched uranium (LEU) from light water reactors (LWRs). Programs that leverage MOX fuel exist in Europe, Russia, and Japan. MOX fuel has been used commercially since the 1980s, and currently fuels about 10% of France’s nuclear plants.
Making MOX fuel requires mixing depleted uranium with plutonium. Depleted uranium is the less-valuable coproduct from uranium enrichment. Plutonium is produced naturally in the reactor’s fuel during operation and can be extracted from used fuel during reprocessing.
MOX fuel can be used in qualified existing reactors alongside fresh fuel. They decrease the overall amount of waste produced per megawatt, reduce the consumption of natural uranium by about 20% with each recycle and can be recycled up to five times.
Tri-structural isotropic fuel (TRISO) is an innovation in fuel form that makes it impossible for the fuel to meltdown. The idea behind TRISO fuel is to give each piece of nuclear fuel, no bigger than the tip of a pen, its own containment and pressure vessel, two vital safety features in a full-sized plant. This feature makes the fuel durable even at very high temperatures. TRISO particles remain securely intact up to 3250°F, a temperature that exceeds even worst-case conditions in a reactor. For context, high-temperature reactors operate between 1350 and 1750°F, well below the melting threshold for TRISO. The U.S. Department of Defense and NASA both use TRISO fuel for their upcoming reactor programs because of its exceptional safety features.
Thorium as an alternative source of nuclear fuel has been a promising innovation over the past decade. Thorium occurs naturally as thorium-232. It is slightly radioactive, is about three times more abundant than uranium, and has a half-life that is three times the age of the Earth. In a reactor, it is possible to create more thorium than is used, although economic and technical hurdles remain.
India has established the long-term goal of a three-stage, thorium-based, closed-loop fuel cycle. Stage one involves pressurized water reactors fueled by natural uranium. In stage two, the used fuel from stage one will be reprocessed to recover plutonium, which will be used to fuel India’s fast reactors. Finally, stage three would involve fueling advanced heavy water reactors with thorium-plutonium fuel. Using thorium-based fuel could help diversify the fuel cycle, however thorium is expensive to extract, and additional RD&D is needed to capture its full potential.
Another potential composition for nuclear fuel is natural uranium (i.e., unenriched uranium) fuel pellets. Natural uranium does not require enrichment, and enrichment infrastructure is required for making nuclear weapons; therefore, if a country wants to reduce proliferation risk, it can opt to use natural uranium as a fuel source.
Two schools of reactors can take advantage of natural uranium: heavy water reactors and fast reactors. The most famous heavy water reactor design is the Canadian CANDU reactor which first went online in 1977. Canada has exported CANDU reactors to Argentina, China, India, Pakistan, Romania and South Korea. There are 30 CANDU reactors in operation globally. British-designed Magnox reactors also use natural uranium fuel, and have been in operation since 1956. Because natural uranium is unenriched, the storage and reprocessing is more simple and less expensive than for enriched uranium fuel.
Fast reactors are designed to operate with high-energy, fast neutrons (imagine neutrons moving 9000 miles per second). Fast reactors are less common today than CANDU reactors but dominate the field of advanced reactors because they are up to 60 times more fuel efficient than today’s LWRs.
Fast reactors were some of the first nuclear reactors built in the U.S. because they can extract more energy from fuel than traditional LWRs, and can use energy from material that would be considered “waste” in traditional LWRs. The Experimental Breeder Reactor (EBR-I and EBR-II) test facilities were operational from 1951 to 1964 and 1965 to 1994, respectively. EBR-I was the world’s first plant to generate electricity from atomic energy, and the combined test facilities successfully demonstrated a complete breeder reactor cycle with on-site fuel reprocessing.
France also took an early interest in recycling fuel in fast reactors. France’s Phénix prototype reactor, which operated from 1973 to 2009, was designed to maximize fuel utilization and recycle all of the plutonium it produced. The recycling facility, the Marcoule Pilot Plant has reprocessed a total 25 tons of fuel from the Phénix reactor.
Because some of the fuel was reprocessed multiple times, France was able to illustrate a closed-loop fuel cycle and demonstrate the value of the fast breeder reactor system.
What’s in Fuel
Source: Japan Nuclear Fuel Limited
Fuel form, composition and enrichment level create many combinations of fuel types that could service remote communities, military installations and massive metropolitan centers. Nuclear batteries power our space missions and nuclear reactors power our submarines, but the individual technologies taking advantage of that energy look drastically different.
Completely carbon-free, nuclear energy powers nearly 20% of U.S. electricity consumption using variations of the same fuel developed in the 1940s. More than 80 years later, new advanced reactor designs are gaining momentum and we could see them in the marketplace this decade. The fuels in development for these new designs enable the recycling of used fuel and prevent the reactor from melting down. Although the U.S. does not currently recycle any used nuclear fuel, in 2022 the Department of Energy awarded $38 million for twelve projects aimed at developing domestic recycling capacity.
Asking everyone to have a complete understanding of nuclear fuel is unnecessary, but it is important that people understand the magnitude of its potential to provide reliable, clean and affordable energy to our power grid.
