you turn it off.

“critical” is the normal operating state of reactor when it’s working. what you want to avoid is supercriticality, which means that power is rising. if it’s delayed supercritical but prompt subcritical, power rises and may or may not stop on its own at some point. when it’s prompt supercritical, you don’t even have time to ask https://www.nuclear-power.com/nuclear-power/reactor-physics/nuclear-fission-chain-reaction/reactor-criticality/