Once the spent fuel from reactors is sufficiently cool, it is sometimes put into dry storage in the United States, in steel and concrete containers at ground level. Here these containers, or casks, are placed vertically. There are storage sites like this close to nuclear plants in 39 states. The techniques used guarantee protection against radiation. However, these systems are designed to be temporary. They are often close to cities or industrial areas, and therefore could not be used to isolate waste for thousands of years.