What specific emergency scenarios require rapid boron injection?

Rapid boron injection is required in specific emergency scenarios where there is a need to quickly reduce reactor reactivity and shut down the nuclear chain reaction. Boron is a strong neutron absorber and acts as a "neutron poison", rapidly decreasing the number of neutrons available to sustain fission. One such scenario is an uncontrolled reactivity insertion event, where there is a sudden and unexpected increase in reactivity. This could be caused by the inadvertent withdrawal of control rods or by a sudden decrease in coolant temperature. In these cases, rapid boron injection is needed to quickly shut down the reactor and prevent a power excursion. Another scenario is a loss-of-coolant accident (LOCA), where there is a break in the reactor's primary coolant system and a rapid loss of coolant. In this case, boron injection is used to ensure that the reactor remains shut down, even with the loss of coolant. Rapid boron injection is also required in the event of a steam generator tube rupture (SGTR) in a pressurized water reactor (PWR). In this scenario, radioactive coolant from the primary system can leak into the secondary system. Boron injection is used to shut down the reactor and prevent further leakage. The emergency core cooling system (ECCS) includes accumulators which are tanks of borated water under pressure which are designed to inject borated water rapidly and automatically into the reactor core in the event of a LOCA.