Also the reactor design was flawed - the control rods that should have stopped the reactor actually increased the reactivity while moving down into the reactor.

Also the reactor design was flawed - the control rods that should have stopped the reactor actually increased the reactivity while moving down into the reactor. Energy spikes were observed in different plants when shutting down the reactors, but they were classified to not show the reactor design (and by proxy the soviet union) in a bad light, so the operators had no idea of this critical fault.