In this series we’ve been talking about storing spent nuclear fuel in dry casks. One major function of these casks is to cool the fuel. Keeping the spent fuel from getting too hot is one way to ensure casks will be safe. As the fuel cools, heat is transferred from inside the cask to the outside.
Our experts look at how the cask will perform this function. We require the cask and fuel to remain within a certain temperature range. Our review looks at four main areas:
Spent fuel releases heat as a result of its radioactive decay. This is called decay heat. A key function of dry storage casks is to move the decay heat from the cask to the outside environment to ensure the fuel and cask components do not get too hot. Our experts look at how that heat will move through the cask and into the environment.
The method used to remove heat has to be reliable and provable. Heat must also be removed in a way that is passive—meaning no electrical power or mechanical device is needed. Casks use conduction, convection and radiation to transfer the heat to the outside.
The graphic shows the three heat transfer methods. As you can see, conduction transfers heat from the burner through the pot to the handle. The process of heat rising (and cold falling) is known as convection. And the heat you feel coming off a radiator, or a hot stove, is known as radiant heat.
These methods work the same way in a storage cask. Where the canister or metal structure containing the fuel touches the fuel assemblies, heat is conducted toward the outside of the cask. Most casks have vents that allow outside air to flow naturally into the cask (but not into the canister) and cool the canister containing the fuel (convection). And most casks would be warm from radiant heat if you stood next to them. (The heat generated by a loaded spent fuel cask is typically less than is given off by a home-heating system.)
We limit how hot the cask components and fuel materials can get because we want to protect the cladding, or the metal tube that holds the fuel pellets. Limiting the heat is one important way we can ensure the cladding doesn’t degrade. The cask must keep spent fuel cladding below 752 degrees Fahrenheit during normal storage conditions—a limit that, based on the material properties of the cladding, will prevent it from degrading. The fuel must also remain below 1058 degrees in off-normal or accident conditions (such as if a cask were dropped while it is being positioned on the storage pad, or if a flood or snow were to block the vents).
We also confirm the pressure inside is below the design limit to make sure the pressure won’t impact the structure or operations. Our experts review applications for new cask designs carefully to verify the fuel cladding and cask component temperatures and the internal pressure will remain below specified limits.
Each storage cask is designed to withstand the effects from a certain amount of heat. This amount is called the heat load. We look at whether the designer correctly considered how the heat load will affect cask component and fuel temperatures. We review how this heat load was calculated.
We also verify that the cask designer looked at all the environmental conditions that can be expected because these will also affect the cask component and fuel temperatures. These may include wind speed and direction, temperature extremes, and a site’s elevation (which can affect internal pressure). To make sure the right values are considered, we verify they match the historical records for a site or region.
We review all of the methods used to prove that the storage system can handle the specified heat loads. We also verify any computer codes used in the analysis and the values that were plugged in. For example, we look at the material properties for cask components used in the code. We look at calculations for temperatures and pressure. We make sure the computer codes are the latest versions.
And we only allow designers to use codes that have been endorsed by experts. We might run our own analysis using a different computer code to see if our results match the application.
The analysis and review allow us to see whether and how the dry cask will meet the temperature limits. Our review ensures the temperature is maintained and the cladding is protected. Finally, our review confirms the cask designer used acceptable methods to analyze or test the system and evaluate the thermal design. If we have any questions or concerns, we ask the designer for more information.
Only when we are satisfied that our requirements are met will we approve the thermal analysis in a cask application.