Man, wtf happened at Unit 2? I keep repeating the NHK broadcast and I'm not sure whether the suppression pool itself suffered damage and not the reactor vessel, or both, or what.
They mentioned radiation going up right after the explosion, which could indicate a reactor vessel or other primary piping rupture, but could also just be from trapped radioactive contamination in the suppression pool being thrown out by the explosion.
Either way it's important to make sure they don't allow the concrete building shell to get damaged at Unit 2 if they want to avoid further airborne release.
It's almost as if they're speaking in code. They don't want to spread panic, but they also can't hide the reality too much from those who understand engineering.
My theory is that the vessel pressure vented into the suppression pool which caused the drop in pressure. That could have made a loud noise, perhaps one they had never heard in testing. Since the pool is a metal torus containing water held up by concrete and tightly coupled to the other parts it's a rigid, massful, structure. It may have been damaged by the quake.
If the radiation is explained by the spent fuel fire at unit 4, the question remains whether or not the containment on unit 2 is still effective.
They mentioned radiation going up right after the explosion, which could indicate a reactor vessel or other primary piping rupture, but could also just be from trapped radioactive contamination in the suppression pool being thrown out by the explosion.
Either way it's important to make sure they don't allow the concrete building shell to get damaged at Unit 2 if they want to avoid further airborne release.