The reason they can do that is because they are continuously venting.
I also believe they are doing more than venting. Remember the report yesterday about how they increased the amount of sea water by 9 times in reactor core 1(I believe) in order to cool it down.
Now it does not take much understanding to figure out that the coolant outlet is therefore wide open with the water running onto the land and into the sea. I believe that there no longer is a coolant loop. They are simply blasting water in and letting it run out into the environment. A continuing last effort attempt to prevent total meltdown. This prevents steam pressure buildup.
Ever hit stainless steel with water when it’s hot? Welcome to the steel pretzel factory.
I don't think they've vented the containment shells on 1 through 3 for the last couple of days. I think that the reactors are hot, but stable. It's the spent fuel pools that are of greater concern now.
That sounds like a reasonable assumption based on reports but I'm wondering....
That is releasing a heck of a lot of radiation into the environment. Why not just let it go into meltdown, wait fives years to cool and then clean it up. The plants are toast anyway, it's not like there is anything to save at this point beyond reducing exterior contamination.
I have a similar take. That the MOX is headed towards a potential (albeit theoretical) alpha-mode failure and/or RPV breach; and that any mitigating action is permissible, steam explosion perhaps almost preferable to an alpha ejection or (perhaps inevitable) corium breach of the RPV.