The most egregious ones were absolutely known, but fixing them would not fulfil 5 years plans, so it was not done. We absolutely knew what is a scrubber and that burning brown coal will release sulfur which will fall down as sulfurous acid. But comrade, can the power plant work without a scrubber and filtration system? Yes it can. So build it without it.

Chernobyl had exactly same problem - no containment around reactor. Why would you build it when power plant can work without it?

The main problem of communism vs environmentalism is that to get environmentalism working you need to question and complain to authorities and demand solutions for obvious problems which authorities are causing - there were no private enterprises, everything was owned by the state. But if secret police will just threaten the complainer with punishment, then you have solved the problem. No complain = no problem. Welcome to everyday realities of communism comrade.