Also, the experiment's requirement lead into the operators disabling almost every single safety mechanism. The automated systems disallowed the actions they were trying to do, so they disconnected most important security measures.
Operating like that you can make even hydroelectric power station extremely deadly, and kill a lot of people.
I can recommend Atomic Accidents: A History of Nuclear Meltdowns and Disasters: From the Ozark Mountains to Fukushima by James Mahaffey - it has a chapter on Chernobyl and goes into a fair amount of details:
Operating like that you can make even hydroelectric power station extremely deadly, and kill a lot of people.