[smittywerben]

This matches my understanding. It's been a few years since I was into this stuff so bear with me.

The NSC is so we can dismantle the Sarcophagus (the old containment structure). The fear is a explosion caused by the collapsing Sarcophagus (or perhaps there is another structural concern) might let the Elephants Foot reach the waterbed underneath the empty man-made cooling reservoir.

Here is a cross-section[0] of reactor 4 that shows the Elephants Foot (corium) at the bottom. I wish it'd show how far deep until water. I can't find information as to whether water even exists, or if this was just speculation (but Pripyat is swampy/marshy).

[0] https://i.imgur.com/qozfDP1.png