An $800,000 grant from the U.S. Department of Energy will allow Penn State researchers to investigate a new approach for removing rare-earth fission products from the molten salt baths where used nuclear fuel is electro-refined to recycle uranium and minimize nuclear waste.
Hojong Kim, assistant professor of materials science and engineering at Penn State, will use the grant toward efficient recovery of rare-earth elements using liquid metals.
"The electrorefining process is designed to separate the usable fraction of uranium metal, about 95 percent of the material, from the used nuclear fuel, using a salt bath," Kim said. "However, in this process, rare-earth fission products are dissolved into the salt, accumulate over time, and must be removed to reuse the salt bath and minimize the generation of additional nuclear waste."
Current methods for removing rare-earth elements are not efficient because of the multivalent states and high chemical reactivity of rare-earth metals.
For example, the recovery efficiency of neodymium, the most common rare-earth fission product, is less than 50 percent, yet Kim has demonstrated that the recovery efficiency can be higher than 90 percent using liquid bismuth metal, based on preliminary results in his laboratory.
Kim will target three common rare-earth elements found in used nuclear fuel -- neodymium, gadolinium and samarium.
Kim has identified an approach that leverages the strong chemical interactions between these elements and liquid metals so that they can be more easily removed. In prior research, Kim and his group applied this method and successfully recovered alkaline-earth fission products deemed impractical to recover from a molten salt bath.
"There is a fundamental problem as to why the recovery efficiency for rare-earth elements is so poor, so we proposed a new hypothesis and approach based on liquid metals to enhance the efficiency," Kim said. "In our preliminary work, we observed great gains in recovery efficiency using our approach."
The three-year project will focus on improving recovery efficiency and control of chemical selectivity to reduce the volume of nuclear waste. On a fundamental level, this research will contribute new thermodynamic and electrochemical properties of rare-earth alloys.
Kim's research group will investigate and validate thermodynamic properties of rare-earth alloys experimentally, while Zi-Kui Liu, distinguished professor of materials science and engineering at Penn State, will focus on high-throughput computational modeling of complex, multi-component alloy systems to accelerate the development of efficient rare-earth recovery processes. James Willit, of Argonne National Laboratory, will assess the feasibility of this approach in a simulated process environment with support from Kim's research group.
The research fits into Kim's goal of realizing materials sustainability.
"I envision that the materials cycle needs to be closed so that the bulk of the used materials are recycled and only a small fraction reaches landfills for minimal impact on our environment," Kim said. "That's what drives my research."