Chernobyl rundown

So, Chernobyl. What is it?

Chernobyl is a nuclear power plant in Ukraine. Formerly Soviet Russia.

Now, it lies empty and closed within an exclusion zone. 

What happened?

In April of 1986, the reactor would melt down. (prompt critical) destroying the several ton roof, tossing it in the air like a toy ball. It was made of heavy lead and graphite.

What went wrong?