My thoughts..1. It was already leaking from the cooling tank. Why do they have to collapse the floor? If you let it be like that.. it will fill in no time.. u just have to seal the exit.2. You can you instant sealer to seal the cracks... It's like a paste of cement you just have to apply over it.. it is effective for leakage like that.3. If the core meltdown starts.. you can release the excess gasses into an absorbent or channeling it through an exhaust to be released deep under the sea. This way you can prevent explosion without or with less radiation leakage4. To prevent the radiation exposure through air... You have to sheild the whole reactor... Though this is difficult to achieve.. to minimise the extent of damage this has to be done. This will contain the radio active particles to some extent..
How did you get the video link