The most critical issue was probably the reactor design's positive void coefficient. The RBMK design was created to use less enriched fuel than other designs, so it had to have performance characteristics that would permit a stable reaction when that fuel was used. To get that "feature", the design had a positive void coefficient.
This meant that it to work in such a way that reactivity would increase if coolant boiled away or other voids (spaces without nuclear moderating materials) formed in the fuel assembly. To keep reactions under control, it was necessary to ensure that there was always some material present to moderate the reaction.
Almost every other reactor design will essentially shut itself off if coolant is lost (negative void coefficient). This is a safety factor in case situations unexpected by the designers occur. In the event something really strange happens, the reactor turns off.
With a positive void coefficient, if something unforeseen by the designers happens, the reactor ramps up to higher output. Combine that with human mistakes in a certain sequence and safety systems that don't operate 100% correctly when the reactor is overheating and running away and you can get an out of control excursion as happened at Chernobyl.
This meant that it to work in such a way that reactivity would increase if coolant boiled away or other voids (spaces without nuclear moderating materials) formed in the fuel assembly. To keep reactions under control, it was necessary to ensure that there was always some material present to moderate the reaction.
Almost every other reactor design will essentially shut itself off if coolant is lost (negative void coefficient). This is a safety factor in case situations unexpected by the designers occur. In the event something really strange happens, the reactor turns off.
With a positive void coefficient, if something unforeseen by the designers happens, the reactor ramps up to higher output. Combine that with human mistakes in a certain sequence and safety systems that don't operate 100% correctly when the reactor is overheating and running away and you can get an out of control excursion as happened at Chernobyl.