Studying Spent-Fuel Canister to Support Long-Term Storage
“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.
