The first door must be closed before the second door will open. There is an electronic interlock that will prevent opening the second door until the first is closed. This keeps the containment sealed and prevents the release of any air from containment to the rest of the plant. Leaving containment requires going through the same procedure. Control Rod Drives Control rods are one way operators adjust the fission taking place in the nuclear reactor.
Control rods contain boron and hafnium, which absorb neutrons.
Neutrons are essential to starting and sustaining a chain reaction. When fully inserted, the control rods shut down the reaction completely. As the control rods are withdrawn, the neutrons are free to create fission and the chain reaction starts up again. This usually is done very slowly, but the reactor can be shut down from full power in just a few seconds if necessary. The reactor is always started up slowly.