The reactor roof not the building that houses the reactor...that was blown off yesterday.
Did u hear the reports of using helicopters to drop water on reactor 4?
If it was not exposed, do u think that would be logical?
There is a crack at the top of the containment structure at Reactor 4.
This hole/crack is allowing steam to escape from the containment area where the spent rods are being stored and cooled.
These rods are not as hot as the those that were active at the time of the earthquake and are much more slowly burning away the water exposing the zirconium cladding at times. When this metal oxidizes it produces Hydrogen.
This gas then escapes out of the breach and reacts with the air and explodes and or catches fire.
That's my understanding, may not be right but that is what i gather.
They cannot get water back up to this area at the moment due to the radiation. If they don't figure it out(helicopter drops aside) they will have a much bigger problem.