US Sandia National Laboratories announced on 18 July that researchers had built a scaled test assembly that mimics a dry cask storage container for used nuclear fuel. This will be used to study how nuclear fuel temperatures change during storage and how the peak fuel temperatures affect the integrity of the metal cladding surrounding the fuel.

Sandia designed and constructed a dry-cask simulator to study how hot used nuclear fuel will become during storage, and how the fuel’s peak temperatures will affect the integrity of the cask over time. 

New cask designs store more used fuel, from 17 up to 37 assemblies, explained Samuel Durbin, a mechanical engineer at Sandia.  The additional storage capacity of modern dry casks provides cost savings to operators and utility customers, but new data and computer models were needed to help verify existing computer modelling predictions of fuel temperatures, under any conditions, and its effect on the integrity of the fuel cladding, Durbin said.

As part of a three-year project for the US Department of Energy (DOE) and the Nuclear Regulatory Commission (NRC), Durbin and Greg Koenig, an advanced nuclear fuel cycle technologist, led a Sandia team that designed and constructed a dry-cask simulator for boiling water reactor assemblies. They ran tests to measure the highest temperatures the cladding inside the canister would be exposed to under a variety of set conditions during decades of storage. 

“The simulator is fitted with more than 750 data-gathering instruments, and about 700 of those are thermocouples, or devices that measure temperature,” noted Koenig. “We have special programming software that takes user parameters and determines the hottest temperature within the simulation. We control the power and pressure and can have many different decay heats.”

Everything inside the cask is built to simulate the way it would be for a utility. The only difference is that Sandia’s cask simulator uses electrical heater rods the same length and diameter as fuel rods instead of used fuel assemblies. This means that no radioactive materials were used during the testing. 

“By modelling peak temperatures, we are providing knowledge about the fuel, which is important because a lot of phenomena are tied to the temperature the cladding sees while it’s sitting inside the dry cask," Durbin said.  "The better you know what temperatures the fuel will reach during a set of conditions, the better you can predict the integrity of the fuel and the cask.”

The team used the simulator to measure temperatures for both above- and below-ground storage systems. For below-ground storage, they positioned a wind machine over the cask to replicate crosswinds from 0-12mph. “Our simulator is very heavily instrumented so we can get a lot of quality information about the fuel,” Durbin said.

Utilities typically evaluate the performance of a dry cask through detailed analytical modelling of the cask’s thermal performance. The new data from Sandia could help regulators verify the accuracy of the analytical modelling.

Photo: Sandia National Laboratories designed and constructed a dry-cask simulator for boiling water reactor assemblies to study how hot spent nuclear fuel will get during storage. (Photo by Randy Montoy)