They are dealing with several situations that confused the public.
1. The primary containment, the reinforced concrete building that houses the reactor was never breached. Pressure from that is vented into the sheet metal and structural steel building on top of the reactor. That is where the hyfrogen ignited after it was vented.
2. The reactor vents into a torus, larged water filled structure, which reduces pressure.
3. The spent fuel pools, stainless steel lined concrete, which are outside the primary containment and which are probably now exposed have been the target of the helicopter water drops and the water from the fire engine deck guns.
4. In this country both the reactor, RPV, and the steel lined reinforced containment buildings are ASME pressure vessels.
That is true - the reactor vessel is where they're injecting sea water. The steam is vented into the large torus which is filled with cooler water. This reduces pressure by at least partially condensing the steam back down to water. The Torus is inside the containment structure as well. They have had to "burp" the containment structure on 1, 2, and 3 a couple of times because the pressure inside the containment was too high as a result of the torus not being adequate to cool the steam coming out of the reactor.
3. The spent fuel pools, stainless steel lined concrete, which are outside the primary containment and which are probably now exposed have been the target of the helicopter water drops and the water from the fire engine deck guns.
They also have one of those concrete booms up now to spray water into unit 4's pond since access to the spent fuel pond apparently is limited to using a hole in the side of the building. BTW, units 5 and 6 are using their normal cooling process to cool the spent fuel pools as well as the main reactor cores. They may be salvageable.
What coolants do not expand under high temperature? (they would have to also be mighty plentiful)