NRC Blog Explains Value of Scale Model Cask Testing

Bernard White, senior project manager in the Division of Spent Fuel Storage and Transportation, says practicality, not cost, is the main reason to use it.

Bernard White, a senior project manager in the Nuclear Regulatory Commission's Division of Spent Fuel Storage and Transportation, writes in a recent post to the agency's blog that practicality, not cost, is the main reason to use scale models when testing the safety of casks used to transport the most radioactive cargo, including spent nuclear fuel. White writes that casks are evaluated, using scale models or components of the casks, for their ability to withstand vibration, water spray, free fall, stacking, penetration, and fire.

"The bottom line is scale-model testing provides the necessary information for the NRC staff to know that a cask loaded with spent fuel can be transported safely, even in the event of an accident," he explains. "Cost savings is a factor, but not the most important one. The biggest reason for using scale models is practicality. Transport casks for spent nuclear fuel are typically in the 25-ton to 125-ton range. There are very few testing facilities in the world that can put a 125-ton cask through the required tests. For example, during 30-foot drop test, the test cask must strike the surface in the position that would cause the most severe damage. Cask designers often perform several drops to ensure they identify the correct position. After the 30-foot drop, the cask is dropped 40 inches onto a cylindrical puncture bar, then placed in a fully-engulfing fire for 30 minutes. Casks are also immersed in water to ensure they don't leak. Measurements from these tests are plugged into computer programs that analyze the cask structure in great detail."

His article points out that NRC regulations specify that in the 30-foot drop test, the cask must hit an "unyielding" surface. "In a real-world accident, a 125-ton cask would damage any surface significantly. It requires considerably more engineering work to achieve an unyielding surface for a full-sized cask than for a scale model, with no measurable advantage. The rule-of-thumb for testing is the impact target should be 10 times the mass of the object that will strike it. So a 125-ton cask would need to hit a 1,250 ton surface. A 30-ton cask would only need a 300-ton target," White writes. "Scale models are easier to handle and can be used efficiently for many drop orientations to meet the multiple test requirements. If a test needs to be run again, it can be done much more easily with a scale model. Design changes are also more easily tested on models. Together with extensive analyses of a cask's ability to meet our regulatory requirements, the information from these tests allows the NRC to decide whether a cask can safely transport the radioactive contents."

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