Sandia simulates used fuel storage cask temperatures

Thursday, 19 July 2018

The inaccessibility of the interior of an actual storage cask and the high radioactivity of the used fuel make it difficult to monitor the temperature, Sandia said.

During a three-year project for the Department of Energy and the Nuclear Regulatory Commission, a team at Sandia designed and constructed a dry cask simulator for boiling water reactor assemblies. Everything inside the cask was built to closely simulate the way it would be for a utility storing used nuclear fuel. However, instead of actual used fuel, the simulator features electrical heaters shaped like fuel rods.

The simulator was used to determine the maximum temperatures the cladding inside the canister would be exposed to under a variety of set conditions during decades of storage. The team used the dry-cask 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.

"The simulator is fitted with more than 750 data-gathering instruments, and about 700 of those are thermocouples, or devices that measure temperature," said joint leader of the project Samuel Durbin. "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."

He added, "A lot of survivability and degradation issues are driven by the hottest part of the fuel. By modelling these 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. So, 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."

Durbin said new cask designs have the capacity to store more fuel assemblies, currently up to 37. "They are increasing the pressure in the casks to increase convection, which transfers the heat out to the environment while the cask shields the radiation," he said.

Utilities typically evaluate the performance of a dry cask through detailed analytical modelling of the cask's thermal performance. This analysis is used to demonstrate performance and regulatory compliance before commissioning a new cask, and the Nuclear Regulatory Commission independently verifies the results. Durbin said the new data from Sandia could help regulators verify the accuracy of the analytical modelling.