Backgrounder on High Burnup Spent Fuel
Nuclear fuel is removed from a reactor every few years when it can no longer economically sustain a chain reaction. This "spent" fuel remains radioactive and must be managed. Initially, it goes into a pool onsite for cooling and storage. Some utilities are moving their spent fuel after it has cooled for several years in the pool into NRC-certified dry storage casks. These casks are specially designed to contain the radioactivity and allow hot spent fuel to cool further.
What is burnup?
"Burnup" is a way to measure the uranium burned in the reactor. It is expressed in gigawatt-days per metric ton of uranium (GWd/MTU). Burnup depends on how long the fuel is in the core and the power level it reaches. The burnup level affects the fuel's temperature, radioactivity and physical makeup.
In a reactor, uranium fuel fissions—splits apart and releases energy—and the metal cladding that encases the fuel reacts with cooling water. This reaction forms oxide on the outside (similar to rust) and releases hydrogen. These processes begin slowly then start to accelerate as the fuel reaches burnup of 45 GWd/MTU. Anything above this level is considered high burnup.
Over time, burnup has increased, allowing utilities to get more power out of their fuel before replacing it. Average burnup, around 35 GWd/MTU two decades ago, is over 45 GWd/MTU today. How hot and radioactive spent fuel is depends on burnup, the fuel's initial makeup and conditions in the core. All these factors must be taken into account in designing dry storage and transport systems for spent fuel.
Is it safe to store and transport high burnup fuel?
To be certified by the NRC, dry cask designs must meet transportation requirements in 10 CFR Part 71 or storage requirements in 10 CFR Part 72. The NRC approves designs only after a full safety review. Based on these reviews, the NRC has certified numerous cask designs for spent fuel storage and transportation. Operating experience since dry storage began in 1986 and short-term tests show both low and high burnup spent fuel can be stored and transported safely.
More casks are available for low than for high burnup spent fuel. Because it has been used longer, there is a great deal more data on low burnup fuel. There is enough data on high burnup fuel that the NRC has been able to certify some high burnup spent fuel storage casks for an initial term of 20 or 40 years. Some systems have also been approved for transporting high burnup spent fuel.
Why does burnup matter?
Burnup is important to the NRC's review of spent fuel cask designs because each system has limits on temperature and radioactivity. When the cask is being dried, pressure increases inside and the fuel heats up. This may cause the cladding to become more brittle when it cools. These changes depend on burnup and the type of cladding, and need to be accounted for in high burnup spent fuel systems. A great deal of work has been done to understand the conditions that make different types of fuel cladding more brittle.
Testing continues on high burnup spent fuel and the set of available data is growing. Cask designers use the results to redesign their casks for higher burnups and additional fuel types. As more data becomes available, the NRC expects to be able to certify more casks. Cask designers also need this data to demonstrate the long-term safety of their systems so they can continue storing spent fuel beyond the initial license term.
How does the NRC make sure it remains safe?
The NRC assures safety by requiring many layers of protection. Casks provide several layers and the fuel cladding itself is an important layer. The regulations are designed to ensure the casks will hold up and the cladding will not break during storage or in a transport accident. The NRC carefully reviews each cask application to see if it meets the requirements. As part of this review, the NRC does its own analysis to confirm information in the application.
The NRC also does inspections before and during loading of dry casks to ensure the correct fuel goes into the right storage systems. Fuel with burnup higher than the NRC certificate allows cannot be loaded. It must remain in pool storage until a cask approved for higher burnup becomes available. The NRC also inspects loaded casks every few years.
What confirmatory research is being done?
Testing has provided a lot of information on how different types of cladding on spent fuel will behave, and this work continues. Planning has begun for an important new study, run jointly by the nuclear industry and the Department of Energy, with regulatory oversight by the NRC. In this study, high burnup spent fuel will be loaded into a cask fitted with instruments to provide temperature readings and allow gas sampling. Those readings, combined with tests on the fuel assemblies and inspection of the cask's interior after years of dry storage, will provide a much better understanding of what happens to high burnup spent fuel in a storage cask as it cools over time.
Work is underway to create better models to determine how much cladding actually may become brittle and the impact of vibration during transport. The NRC is also monitoring work going on internationally.
All this work will help cask designers, users and regulators better understand how to ensure high burnup spent fuel will remain safe in long-term dry storage and during transportation to a centralized storage or disposal facility.