system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What actually happened at the V.I Lenin powerplant to cause the Chernobyl accident?

I've read a lot about this topic, but a lot of it is contradictory, circumstantial, or just plain confusing. Can anyone lend some concise enlightenment?
gpt
From memory, and in eli5 terms:  
They were running a test, which set up conditions that rapidly heated the reactor. The folks in the control room were unclear on what was happening, reacted the wrong way, and caused a huge release of heat.  
Because of the design of the reactor, there was room between the top of the reactor and the top of the heated water.  This meant that when the heat spiked, the water close to the fuel boiled instantly, threw the rest of the water upward like a giant, multi-ton hammer, smashed the reactor to bits, exposed the fuel rods to air and caught the graphite that was supposed to help protect things on fire instead.   
The roof of the entire building blew off and bits of it also caught fire, and everything, including a lot of the fuel, was able to burn into the atmosphere.  
The Soviet government refused to say anything was wrong until multiple days later, when radioactive particles were detected falling in Sweden.  
Many Russian workers and soldiers died (or received lethal radiation doses then died later) trying to put out fires and cool down the remaining reactor fuel. Eventually this included tactics like dropping bags of sand onto the hole in the building from helicopters.   
The whole city around Chernobyl had to be evacuated and remains so to this day.  
Edit: should say Soviet workers and soldiers, not just Russian chernobyl is in what is now Ukraine.