The German casks are thick-wall metal (ductile cast iron) over 19″ thick. They don’t use or need concrete.
The Fukushima thick-wall casks for storing spent nuclear fuel assemblies are 10″ thick metal (carbon steel and lead). They survived the 9.0 earthquake and tsunami.
Either system is better than what we have. Switzerland uses both types of thick-wall casks but requires the highest technical standards of these types from the manufacturers. The Swiss are a great example of the best available technology standards for storing high level radioactive waste. They also have an on-site hot cell facility for inspecting and retrieving spent nuclear fuel rods.
Thin-wall stainless steel canisters are vulnerable to failure through undetectable microscopic cracks. No thin-wall canister has been evaluated for earthquake risks assuming there may be partial cracks.
Thin-wall canisters do not stop gamma or neutrons. The thin-wall canisters are normally stored in carbon steel lined concrete casks. The concrete casks have huge air vents for convection cooling. Thick-wall metal casks don’t have or need air vents.
In the U.S. there are some thick wall metal casks (both ductile cast iron and the steel/iron casks). However, rather than spending money to store these thick-wall casks in hardened buildings, as is done in most of the world, they leave them out in the environment where seals and bolts can prematurely rust.
Thin-wall canisters systems cannot be stored in buildings, since radionuclides are continuously being released through the air vents.
Some of the oldest canisters at San Onofre have inlet air vents that measure over 2000 counts per minute. The NRC refuses to tell us why these readings are so high. The NRC also refuses to tell us what the outlet air vent readings are on these older canisters. Instead, the NRC has eliminated the requirement to even measure radiation levels at the outlet air vents.
The oldest canisters at San Onofre have stainless steel clad rods so are less likely to explode when air enters canisters through cracks.
Most fuel rods now have Zirconium cladding instead of stainless steel. Due to the longer burnup time of the fuel in reactors (both medium and high burnup), the structure of both the cladding and uranium fuel pellets changes from ductile (flexible) to brittle, making them more likely to fail during transport. Also, some of the zirconium and uranium metal transforms into zirconium hydrides and uranium hydrides. This increases risks for hydrogen explosions when this material comes into contact with air. For example, zirconium hydrides in gas or small particle form will explode when exposed to a relatively small amount of air.
Cask venders will sell anything the U.S. wants. However, as long as the NRC has the authority to make exemptions to safety requirements, companies will never use the safest available technology.
ASME N3 standards are American Standards for Mechanical Engineers specifically designed for pressure vessels storing and/or transporting highly radioactive fuel waste and other highly radioactive waste.
Only thick-wall metal casks can meet these American standards. The NRC should not be allowed to give exemptions to these standards.
Canisters and casks are pressurized with helium instead of air to minimize metal corrosion and to prevent hydrogen explosions.
See oldest canisters/casks in this 2-page Inventory, sorted by state.