Along with Scott Manley’s excellent video that @Whack-a-Mole posted above, I would also recommend this video: What /Actually/ Happened at Chernobyl - YouTube The whole “graphite tips” thing is one of the main inaccuracies of the HBO series. If after the two videos you’re still wondering about the last push over the edge of the neutron flux and the reaction, we dove into it a bit more here: Chernobyl Control Rod Graphite "Tips"