New Fuel Cycles for New Reactors
The next generation of nuclear technology features improvements in fuel design and fuel cycles.
Beyond improving the economics of an individual reactor, new fuels can boost global economic ties. Nuclear fuel is the most traded nuclear commodity globally; however, nuclear fuel types are closely related to reactor designs, so long-term nuclear trade relationships can become well-established once a given technology is chosen. These energy trade relationships can have important energy security and geopolitical implications.
The Advanced Fission Fuel Cycle
The traditional fission fuel cycle to support conventional light-water reactors involves:
- Mining and milling uranium ore
- Conversion to a chemically useful form
- Enrichment to commercial grade (~4% U-235)
- Fabrication of fuel forms
- Fission in a reactor
- Long term storage and disposal of long-term waste
Although recycling or reprocessing is possible with this fuel cycle, it has been limited in practice. This fuel cycle produces fission products, activation products, and spent fuel that require long-term storage. It also produces low-level radioactive waste, which is relatively straightforward to handle but still requires short-term storage.
By comparison, advanced nuclear fuel cycles have many fuel options and in some cases enable reuse, recycling, and reduction of used nuclear fuel. Advanced fuel cycles offer more diverse feedstock options, including unenriched uranium, low enriched uranium (~4% U-235), high-assay low enriched uranium (less than 20% U-235), previously used fuel, thorium, or even plutonium or other materials from recycled warheads.
By enabling more resilient, flexible, and affordable nuclear energy, new nuclear fuel types for fission enable advanced nuclear energy to play a key role in mitigating climate change. Fuel choices in many designs feature inherent safety features that offer a more cost-efficient and more reliable system than conventional active systems. Greater fuel efficiency can reduce nuclear operating costs, waste output, and mining requirements. Some designs can reduce refueling requirements and timelines, increasing plant availability and thus lowering average costs.
Long-Term Waste Storage and Disposal
Even with innovations, long-term storage and disposal is needed for civilian and military nuclear. From a technical perspective, safe short- and long-term waste storage has been demonstrated and is routinely practiced in some countries. The amount of space spent fuel requires is not large. The total spent fuel produced by all US nuclear plants to date roughly totals the same volume of waste produced by coal every hour and all commercial waste since the 1950’s would only cover a football field to a height of ten yards.
However, permanent geologic repositories have proven politically controversial. To manage the nuclear waste challenge, the Blue Ribbon Commission on America’s Nuclear Future made several recommendations, including:
- A consent-based approach to siting future nuclear waste management facilities
- Prompt efforts to develop one or more consolidated storage facilities.
- Support for continued U.S. innovation in nuclear energy technology and for workforce development
In recent years, consent-based siting has had success in Sweden and Finland. New technologies, such as borehole disposal, or alternative waste arrangements, like private services agreements, could offer new opportunities to safely, responsible manage long-term waste. A recent report by the Breakthrough Institute explores how new policies for decentralized storage and other methods can address waste issues.
The Emerging Fusion Fuel Cycle
There are many types of fusion reactions, both in stars and engineered. For commercial fusion energy, most designs plan to use the reaction between two heavy isotopes of hydrogen, deuterium and tritium. Deuterium is abundant in water and can be separated relatively easily, although a lack of market demand means there is currently no large-scale supply.
Tritium, as a relatively short-lived radioactive isotope, does not occur naturally in any significant quantity. Tritium can be produced and separated as a byproduct of heavy-water “CANDU” -type fission reactors, subsequently to be used in fusion reactors. However, the quantities produced are not sufficient to sustain any commercial fusion program. Instead, so-called breeding blankets in fusion reactors containing lithium, specifically the isotope lithium-6, can be used to produce tritium. The interaction of the neutron from the DT reaction with lithium-6 produces a tritium atom which can then be separated and reintroduced as fuel. The primary inputs for the DT fusion reaction are therefore actually lithium and deuterium. It is expected that small quantities of tritium will be required for initial operation of the first commercial fusion reactors, but these can then breed sufficient quantities for the start-up of other reactors without the need for CANDU tritium.
From a waste perspective, fusion does not face the same concerns as nuclear fission. If a fusion reactor can be designed to avoid the use of materials that result in the production of long-lived radioactive products and transuranic elements, as current fusion companies are working towards, fusion waste storage requirements will include:
- Low-level waste. As with fission waste, moderate amounts of irradiated materials must be stored responsibly for a short time period.
- Short-term storage of tritiated materials. Due to a 12.3 year half-life, over 99% of tritium decays over 82 years.
- Long-term storage of intermediate-level neutron activated materials. The two primary activation products, niobium and molybdenum, are from steels in the reactor structure; future innovations may reduce the activation of fusion materials and lower or eliminate the need for long-term waste storage.