New Technology Solves Unique Spent Nuclear Fuel Storage Issues
Since the 1970s, the commercial nuclear industry has been attempting to address the issue of excess spent nuclear fuel (SNF) storage and disposal. With industry focused on deployment of advanced small modular reactors (SMR) over the course of the next ten years, a viable solution is needed to handle SNF coming out of these reactors. Many of these reactors will utilize tri-structural isotropic (TRISO) fuel particles encased in graphite moderator. TRISO fuel particles consist of a carbon, oxygen and uranium fuel kernel that is encapsulated by three layers of materials that contain radioactive fission products under any conditions. This type of fuel is more structurally sound and resilient than traditional reactor fuel. A major drawback of the TRISO-based reactors is they typically discharge the largest volume of SNF in the industry, approximately 10-16 times more than more traditional light water reactors per unit of energy produced. This situation creates both short and long-term storage challenges.
Historically, various solutions were evaluated to process SNF, such as burning carbon with air, but were immature, inefficient, or ineffective for TRISO fuel. In 2013, Savannah River National Laboratory (SRNL) worked with the German national research institution at Jülich, to develop an alternative technology utilizing a vapor-based process to address these concerns. Between 2013 – 2019, SRNL worked off and on with Jülich on this technology thanks to development funding provided by the German government. SRNL currently possesses two U.S patents and one European patent on the vapor-based digestion process.
SRNL’s patented process can reduce the volume of the TRISO SNF by a factor of 20 by removing the graphite binder in which the small particles of fuel are dispersed, without damaging the particles of fuel. After removal, the fuel kernels can go into short-term interim storage in a repository. For long term storage and disposal, the fuel will need to be further immobilized. This can result in a significantly smaller interim storage footprint and reduce the number of containers and shipments needed to move the final waste to a repository. Additionally, because the SNF elements are not crushed and the graphite is digested using a vapor-based process, the potential for damaging the TRISO particles is greatly reduced, allowing the fuel to potentially be recovered or recycled from the kernels for future use or recovery of valuable elements and isotopes.
Top image depicts fuel dispersed in graphite. Bottom image depicts fuel after graphite has been removed. Photos by Robert Pierce.
SRNL Scientist Bob Pierce said, “Apart from the impact of the SNF volume on geological disposal, the SNF presents a sizable interim storage liability, a liability which diminishes the appeal of the high-temperature gas-cooled reactor technology. The SRNL vapor digestion process is able to overcome that barrier, and it is the technology best positioned to solve this issue in time for the generation of these wastes in the early- to mid-2030 time frame.”
Recovered fuel particles. Photo by Robert Pierce.
Since 2019, when the German collaboration ended, SRNL has continued to seek a means to further develop this technology. Recently, a DOE Office of Technology Transfer (OTT) project in conjunction with the University of South Carolina Columbia (USC) has been funded which will support this advancement. Shortly after the DOE-OTT project was awarded, SRNL was also awarded an Advanced Research Projects Agency-Energy project with Westinghouse and USC as its partners. Westinghouse is a leader in the development of microreactors that will utilize TRISO fuel. SRNL currently has appropriate equipment and facilities staged that will be reequipped through the OTT project and produce pilot scale data. Due to the volume and scale of the reactors, pilot scale will suffice for processing, though SRNL is capable of scaling up if a large facility is needed.
The two projects will span a little more than two years, with project completions in early 2027. The goal is to have a complete and mature, full process demonstration at an engineering scale. The vapor-based digestion technology can also be later coupled with other separation technologies outside of this scope to impact the commercial nuclear industry, the nuclear research community and other facilities within the DOE complex.
Engineering-Scale Test System. Photo by Robert Pierce.