With the TV show doing a great job at delivering its explanation in a manner that most people can easily understand, I felt I wanted to do a more detailed description. So I cover basic reactor physics, explain how the RBMK reactor works, how Xenon 135 works, Why the control rods included graphite tips, and why the reactor became unstable and ran away.

Many of the diagrams here are from https://www.nuclear-power.net and they have Lots more information on Nuclear Physics
https://www.nuclear-power.net

And of course I highly recommend the TV show:
https://www.hbo.com/chernobyl