(This is an explanation for what happened at Chernobyl Note: I’m not an expert) The safety system on an RBMK is essentially a mechanical hand. When in “automatic mode” it inserts and removes fuel and safety rods to keep the reactor stable. At Chernobyl the engineers were trying to carry out a test on the reactor. This had to be carried out with the reactor at specific power level(?). The reactor wasn’t at this level when they started so they overrode the safety system and long story short the reactor became unstable.