Don't you remember Fukushima? All it takes it for the cooling systems to be compromised. A simple power cut can do that if the backup systems fail. In the case of Fukushima, the tsunami flooded the backup generators.

Thankfully, this does appear to not be as serious as it could have been. Of course Russia is being condemned for this, but it's war, it might not have been Russia. If NATO are looking for excuses to get involved, a nuclear crisis would be an excellent way to do so. Not saying that's what I think happened, but I'm not ruling it out either. Attacking nuclear power plants is ridiculously stupid for any party to do, more so Ukraine or Russia themselves.

Whoever did this is fucking nuts. Russia is the most likely culprit, for obvious reasons.