Grab an ice cube (0°C) and throw it onto a car engine that has been running for over an hour (or a red hot frying pan). Notice how quickly the ice cube converts into steam? Liquid nitrogen is (as I'm sure we all know) a LOT colder than 0°C and quite clearly if water is being converted to steam this quickly the reactor is a lot hotter than 100°C.
Now go outside on a day where ice has accumulated on your windshield, now throw boiling water on it. Notice how you now have a $500 repair bill to fix the cracked windshield/windscreen?
In this case the reactor was designed to be cooled solely by liquid water (from my understanding) and you can't just pipe another coolant into the same systems and expect it to either work flawlessly or work for more than a few minutes. Pipes that are just fine at 10-100°C become incredibly brittle at -150°C or colder. Same goes for everything in the plant I'd imagine to some extent. If you were to bypass it all and start filling the rooms near the reactors with the stuff you'd see an INSANE amount of expansion as it converts from a liquid to a gas and might suffocate anyone around if there isn't something to deal with that gas. I can't even begin to start thinking of what would happen to the reactor vessel itself nor the fuel rods within but I'd imagine they'd react badly to temperatures that low.
However you might be able to temporarily retrofit the place to have liquid nitrogen cool down the water that is being fed into the reactor. Having the water buffer the effects would be a very good idea. Honestly I'm not an "expert" on any of this and I'm sure the technicians out there have considered it if it was possible. I wouldn't be surprised in the least if they don't have LN2 reserves on site to help just in this sort of an emergency but I don't think we will have the details on exactly how the coolant systems have failed for some time.
Here is hoping our Sendai guys are fine, just have dead communication links. :crying: