This was asked during our Physics class, and nobody knew the answer, so our professor gave it as our homework. Thanks to those who will answer my question =]
Because the reactor housing and storage silo's need to be strong incase some nutter tries to break into one with an aircraft or other such thing.
Due to the processes and the materials inside silos, explosions and fires are fairly common. The dome-shaped top can, believe or not, pop off, so all of the fire and stuff can go out the top instead of pushing out the walls. Everything is controlled, preventing deaths and injuries from the shrapnel that would go everywhere if the whole silo were to explode.
