I do not understand, why are they fighting near the reactor? It only makes sense to keep troops of both sides away from something as dangerous.
Though I think we're seeing the Ukrainians are fierce when cornered, so probably isn't the morale breaker they think it is.
I'm guessing the Ukrainian troops fell back to the nuclear power plant hoping they wouldn't get attacked there.
This whole situation reminds me of the Mosul dam in Iraq, which was controlled by ISIS for awhile. The dam is kept stable by continuous grouting operations, which I believe were on hold for awhile.
If you want an actual explanation for the extension of NATO into the former communist countries, look no further. East Europeans have a collective memory about how the Russian military operates.