There are a lot of specifics still not known. However, I do know that reactors do not explode. The explosion had to have been caused by something else in the building not nuclear (like a transformer or something).
Exactly!
Based on my "analysis of the video"
and more recent reports on the news networks the explosion was caused by Hydrogen. That leads me to believe it was the generator that exploded because Hydrogen is used to cool the generator. The generator in the turbine building, not the reactor containment building.
On the other hand, these nuclear plants are boiling water design and the same water that circulates through the reactor also is heated to steam and passes through the turbine to spin that and the generator to make power. So that might explain the concern over coolant loss and core exposure.
FWIW - I worked 30+ years in power generation (as does slow modem) with most in nuclear - so much of this equipment is familiar. I would not want to be part of that response team.