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ORNL receives spent fuel canister to support long-term storage studies

Seeking solutions for nuclear waste disposal: Canister allows ORNL researchers to demonstrate findings Credit: Kaushik Banerjee/ORNL, U.S. Dept. of Energy

The receipt of a nuclear fuel canister is boosting the research of an Oak Ridge National Laboratory team investigating methods to help the nation effectively dispose of nuclear waste for the long term.

“The high-level radioactive waste generated from a nuclear power reactor to supply a person’s electricity needs over a 100-year lifetime would be about the size of 100 sheets of paper,” said Rose Montgomery, leader of ORNL’s Used Fuel and Nuclear Material Disposition group. “My team’s goal is to help the U.S. Department of Energy in its mission to reduce that volume even further and to figure out long-term solutions for storing, handling and disposing of the nation’s nuclear fuel currently being held in interim storage.”

The canister, originally slated for use by the San Onofre Nuclear Generating Station in California but never used to store spent fuel, stands over 16 feet tall and weighs about 22 tons empty – roughly the equivalent of five small pickup trucks. ORNL is one of three national labs selected to receive canisters from the U.S. Department of Energy for ongoing nuclear storage research projects. 

The canisters play an essential role in the nuclear power industry, which is a major component of the nation’s carbon-free electricity production.

“The nuclear energy industry is unique among power generation options in that its used fuel is inertly stored in sealed canisters that are robustly designed as required by the stringent requirements of the Nuclear Regulatory Commission, the Department of Energy, and the Environmental Protection Agency,” Montgomery said. “The used fuel is tracked and traceable, and it can be retrieved at any time for reprocessing and reuse.”

At a nuclear power plant, fuel rods containing uranium are used to generate electricity. At the end of their power production cycles, the fuel rods – which are hot in regard both to temperature and radioactivity – are stored under water in specially designed pools to cool for about five years. Then, while still under water, the rods are transferred into large storage canisters. When a canister is moved to dry storage, water in the canister is removed by drawing it out through a long, narrow pipe inside of the canister.

“These dual-purpose canisters, which are proven to be safe and reliable for both transportation and interim storage, weren’t originally designed to be a permanent disposal solution,” Montgomery said. “Our team is part of a multi-lab effort to explore enhancements that could extend capabilities of the canisters for the extreme long term – not just for a decade or a century but for a million years.”

The team spans multiple disciplines from across the laboratory, drawing on nuclear energy researchers such as Montgomery and Kaushik Banerjee, who is now at Pacific Northwest National Laboratory, modeling and simulation experts Adrian Sabau and Emilian Popov, remote robotics engineer Venugopal Varma, mechanical engineer Eliott Fountain, and others.

“One focus of this research project is to find the best way to fill the void spaces surrounding the rods inside the canisters to mitigate any post-disposal risk of the canisters going critical,” Montgomery said.

If the team can find a way to make the fuel canisters appropriate for direct, permanent, long-term storage, the benefits include avoiding the cost and complexity of repackaging spent fuel into smaller purpose-built disposal containers, eliminating the need to dispose of the existing canisters as low-level radioactive waste, and potentially decreasing the risk to workers who otherwise would be involved in the repackaging.

ORNL research to date includes modeling and simulation to determine what substances would work best as fillers in the canisters and experiments performed on a small-scale canister model fabricated at the lab.

“Having a full-scale, industrial-use canister on site will allow us to conduct a demonstration project with experiments that test the findings of our simulation model,” Sabau said. “With modeling and simulation, we’re investigating what heated material would work best – melted wax, molten metal, low-melt glass. With a demonstration using the full-scale canister, we’ll be able to validate our findings.”

One potential method for injecting the molten filler is to use the drain pipe that is already inside the canister. This would allow workers to inject filler without opening the sealed canister. A challenge, however, is keeping the filler material hot enough to flow through the 16-foot length of the narrow pipe, as is ensuring the filler can penetrate the complexity of the fuel assemblies.

“It is especially difficult to ensure that the filler can move through the fuel assembly’s spacer grids,” Montgomery said, holding up a sample of the silver mesh-like component. “Inside the power plant, the spacer grids performed important heat transfer functions within the reactor, but inside the storage canister they are obstacles to the flow of molten fillers. We are performing tests to analyze whether the inserted material can fill enough of the void spaces in the canister to prevent any risk of criticality before it cools and solidifies in the canister.”

 “Safety for the long term is our ultimate goal,” said Montgomery.

The team’s research is supported by DOE’s Office of Nuclear Energy.

UT-Battelle manages ORNL for the DOE’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science. — Amy Reed