The Nuclear Regulatory Commission’s regulations governing dry storage are found in 10 CFR Part 72. These are referred to as independent spent fuel storage installations (ISFSI) and are licensed by the NRC for a period of 20 years, renewable for another 20 years. They are designed to the same earthquake standards as a nuclear power plant. Each cask must meet specific requirements to be certified by the NRC, such that spent nuclear fuel can be safely stored for at least 100 years.
An actual test was done with an F-4 fighter plane that was catapulted into a large concrete barrier on a rocket sled at 480 mph. The engine caused the most damage; it penetrated about 2 inches into the concrete, which would not have caused any radioactive releases.
The strength and safety of these casks are designed to be the highest achievable, and incorporate multiple layers of engineered safeguards. The footprint of these casks also makes a possible airplane strike or other terrorist attack unlikely, as their size compared to an object like the World Trade Center or Pentagon is exceptionally smaller.
The storage pool at Diablo Canyon is being filled up, but it is not being crowded or overfilled. Federal law requires all plants to keep a full one-and-one-third reactor-core loads of empty space in the storage pool at all times. The safety record of the transportation of radioactive materials is impeccable, and no other industry can come close to claiming such a safety record.
The nuclear industry has more of a responsibility to keep the public’s safety in mind than any other industry, and strives to engineer these systems as passively safe and reliable as possible.