Evaluating 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.