Dry Cask 101: Making Sure They’ll Hold Up

Steven Everard
Structural Engineer

CASK_101finalEvaluating the structure of a spent fuel storage cask is a key part of our licensing process. In its application, the cask designer must provide an evaluation that shows the system will be strong and stable enough to resist loads that may be placed on it. NRC structural and materials engineers scrutinize this evaluation to make sure the design meets our regulatory requirements.

In an application, casks designers must provide evidence the cask system will:

  • Maintain confinement of the spent nuclear fuel
  • Maintain the fuel in a subcritical condition
  • Provide radiation shielding
  • Maintain the ability to retrieve or recover the fuel if necessary

In our structural review, we make sure the system can perform those functions even after experiencing a load, such as if the cask were dropped. We look at the structural design and analysis of the system under all credible loads for normal conditions—that is, planned operations and environmental conditions that can be expected to occur often during storage.

We also look at off-normal conditions, accidents and natural phenomenon events. “Off-normal” describes the maximum conditions that can be expected to from time-to-time, but not regularly. An example is the highest pushing or pulling force on a horizontal canister when it is being placed inside the storage overpack. Accident conditions and natural phenomenon include a dropped cask, earthquakes, tornadoes, flooding and any other credible accident or environmental condition that could affect the structural integrity of the system. These requirements are outlined in 10 CFR Part 72.

The structural review looks at whether the cask designer evaluated the proper loading conditions. It will also ensure the designer evaluated the system’s response to those loads accurately and completely. The reviewers must verify whether the resulting stresses in the material meet the acceptance criteria in the appropriate code.

As we explained in an earlier post, codes and standards are guidelines typically developed over many years of experience and through industry-wide and government agreement. Some of the more common codes an applicant may use come from the American Society of Mechanical Engineers, the American Society of Civil Engineers, the American Concrete Institute, the American Institute of Steel Construction and the American Welding Society.

Not all loads are likely to occur at one time, but some might occur together. So we look at several different combinations of loads that can be expected at the same time. These include dead loads (which come just from the weight of the material), live loads, (which come from the movement of the system or people and things near it), and environmental loads (including snow, ice, wind, temperature and seismic). For example, the cask could experience a dead load, live load, snow load and wind load together. But it is not reasonable to expect the cask to be in a snow storm, a tornado and an earthquake at the same time.

These cases are analyzed to determine the stresses placed on the material used to construct the cask system. This analysis may be completed by either hand calculations or by a computer model. Typically, we only look at the maximum stresses in the different materials—since lesser stresses would not be as challenging to the system.

The maximum stresses from the analysis are compared to the allowable stresses from the appropriate code to determine a margin of safety. These design margins are typically large. This is because designs must be robust enough to withstand the accident scenarios. To be conservative, we and the designers overestimate loads and underestimate material strength. Doing this adds conservatisms and enhances our assurance that the design is adequate.

Dry Casks 101: Managing Heat

CASK_101finalCaylee Johanson
Mechanical Engineer

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.

Heat Radiation Transparent 2The 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.