When nuclear fuel at a power plant is used up, the assemblies that hold it are placed in pools of circulating water and left to cool for years.
But the fuel remains extremely hot – several hundred degrees Fahrenheit – extremely radioactive and extremely dangerous.
After the fuel assemblies, each made up of multiple fuel rods filled with tiny pellets of uranium, cool to a required level, they are placed in a canister or cask while still underwater. The water is pumped out and helium, an inert gas, is injected.
Then the canisters are taken to a temporary storage area, either above or slightly below ground and usually near the power plant, since there’s still no permanent location to store spent nuclear fuel.
Unfortunately, once the casks are sealed, it’s nearly impossible to see what’s going on inside.
“It’s not something that you’re just going to put your head in and check,” said Sam Durbin, a mechanical engineer at Sandia National Laboratories. “You really have to have predictive capability to know what’s happening inside.”
So those in the industry rely on complex models – “computational fluid dynamics modeling” – to indicate conditions inside the casks, including temperature, to ensure conditions remain safe.
Durbin’s research at Sandia National Laboratories, funded by the U.S. Department of Energy and U.S. Nuclear Regulatory Commission, aims to help mathematicians make current models even more accurate.
Scientists there have built a simulated dry cask – complete with a 14-foot-long mock nuclear fuel assembly hooked up to 700 thermocouples – devices that measure temperature.
While real-life casks are capable of holding 68 to 89 fuel assemblies, the Sandia simulation is just using one.
Modelers can then extrapolate the data.
No radioactive materials are being used in the research.
New data collection is partly necessary because newer casks are designed to hold more fuel assemblies than the older models, and they use increased pressure inside.
Durbin and his team have collected three years of data from the simulated cask through the project but will continue to use the setup for additional research.
“The real value was providing this quality data set for modeling,” Durbin said. “You have this snapshot of the truth which you can then model.”
In the first round of research, Sandia researchers simulated conditions inside of an aboveground cask, of which there are thousands of real-life examples throughout the country.
Current work is simulating a dry cask system being stored underground, akin to the system used by Holtec International.
Holtec is proposing an interim spent nuclear fuel storage facility in southeast New Mexico that would collect and house thousands of the casks just under the desert’s surface until a permanent storage site is developed.
“I think this is very important work,” said Edwin Lyman, a senior scientist with the Union of Concerned Scientists’ Global Security Program.
Lyman said storing spent fuel in dry casks is generally safer than storage in fuel pools.
“But there are still questions about the integrity of the fuel when it’s been in these casks for a long time,” Lyman said.
After a permanent site is developed for the nation’s spent nuclear fuel stockpile, which is around 80,000 metric tons, the fuel will likely need to be repackaged, he said.
“A lot of it (spent fuel in dry casks) has already been there for decades and probably many decades to come,” Lyman said. “So you need to make sure if you’re storing fuel in the dry casks that it maintains its integrity so that when you have to move it eventually to take it to a repository it remains intact and doesn’t cause problems when you try to repackage and transport it.”