Weird story but I taught a Russian guy on my mission (in Poland) who claimed to have developed a piece of software used in all of the USSR's reactors. He built a backdoor into it (which was common in those days) and so got early access to find out what happened before the rest of the world.
He said he was shocked at how honest the state media was when they finally reported everything since most people assumed they lied about everything. The only real lie he found is they didn't go into specifics of why the cooling system failed but that the initial problems were caused by a large crane mounted on railroad tracks that was overloaded and fell over and smashed some cooling equipment.

According to this guy, the official accounts still don't (or at least didn't) mention that fact.
No idea if it's true but it was interesting nonetheless.