Now the caveat is they can design all this stuff but they never put this sort of emergency into practice.. so this is real life case of whether human engineering did the right stuff or they didn't design it for some fallacy somewhere (ie something they never thought about how it would behave). I see more problems in real world engineering from the fact people can't think up everything...
I didn't study and I am just using my engineering intuition, but who will be able to go in there to repair anything once inside the concrete dome is contaminated with the radioactive material? Is the reactor pretty much to be sealed and considered dead? Or you can wait some 500 years and it will be operable again?!? Or perhaps they have enough Chinese labor they can bring in there to go in and clean it? Where does the contaminated water go? I suppose the pressured water released also contains radioactive particles at this point or somehow magically they can cool the reactor, but not have the water contact them? A lot of questions, but when you find the pros who offer their knowledge, you gotta ask!
I don't know the full situation, but I gather they vented the primary containment into the atmosphere (ie outside the secondary containment) just for the purpose that they can get people in there to work on the reactor and salvage it. At worst case, they would vent into the secondary containment then would have to vent the secondary containment into something else and get people in suits or robots in there etc. It may make that reactor unusable.
Because it's a boiling water reactor, the water comes in direct contact with nuclear material and when they vent pressure, it is this radioactive water that's put into the atmosphere. They're only venting part of the pressure, not the entire vessel, so water will remain where they can cool the reactor and replace it with new water that's colder I assume.
The radioactive release is hazardous, but news makes it sound like people will drop dead on contact.. no, it will cause cancer and people will die slowly over decades.. remember, Japan had Nagasaki and Hiroshima bombed and they built cities back on it. Ofcourse that was maybe 100 lbs of uranium and 30 lbs of plutonium vs maybe thousands of pounds of the stuff.. but it's only water that came into contact with it..
In Chernobyl, the actual core caught fire and released all that stuff.. that was pretty nasty.
I am surprised they didn't release pressure and let it build up to 2x design capacity.. that is negligent. Much better to risk some folks developing cancer over decades with release into atmosphere rather than risk catastrophic primary containment collapse by preventing the pressure valve to work as intended..
Btw, part of 3 mile island problem was related to the pressure valve not working.. intentionally NOT using it is very stupid.