Explain the difference between prompt and delayed criticality.

The difference between prompt and delayed criticality lies in the source of neutrons that sustain the chain reaction. Prompt criticality occurs when the chain reaction is sustained solely by prompt neutrons, which are released almost instantaneously during the fission process. Delayed criticality occurs when the chain reaction is sustained by both prompt and delayed neutrons, which are emitted from the decay of certain fission products seconds or even minutes after the fission event. In a reactor operating at delayed criticality, the reactor power can be controlled and adjusted by moving control rods or changing the boron concentration. The delayed neutrons provide a time buffer that allows operators and control systems to respond to changes in reactivity and maintain a stable power level. If a reactor were to become prompt critical, the reactor power would increase extremely rapidly, making it difficult or impossible for operators or automatic control systems to respond in time to prevent an overshoot or even a runaway reaction. This is because the chain reaction is sustained solely by prompt neutrons, which are released almost instantaneously. The power would increase exponentially with a very short time constant, potentially leading to fuel damage or other serious consequences. Therefore, nuclear reactors are designed to operate at delayed criticality, and safety systems are in place to prevent them from becoming prompt critical. The term "critical" refers to the state where the chain reaction is self-sustaining, meaning that the number of neutrons produced in each generation of fissions is equal to the number of neutrons lost. When a reactor is critical, its power level remains constant. When a reactor is supercritical, its power level increases. The distinction between prompt and delayed criticality is crucial for understanding reactor dynamics and ensuring safe operation.