During a LOCA, what is the role of accumulator injection in the ECCS?

During a Loss-of-Coolant Accident (LOCA), the role of accumulator injection in the Emergency Core Cooling System (ECCS) is to rapidly provide a large volume of coolant to the reactor core in the early stages of the accident. A LOCA occurs when there is a break in the reactor's primary coolant system, leading to a rapid loss of coolant and a decrease in reactor pressure. Accumulators are large tanks filled with borated water and pressurized with nitrogen gas. They are connected to the reactor vessel via check valves. When the reactor pressure drops below the accumulator pressure (typically around 600 psi), the check valves open automatically, and the pressurized nitrogen forces the borated water into the reactor core. This injection of coolant serves to quickly cool the fuel rods and prevent them from overheating and potentially melting, a condition known as core damage. The accumulator injection phase is crucial because it provides immediate cooling before other ECCS components, such as high-pressure or low-pressure injection pumps, can start up and deliver sufficient coolant. Accumulators are designed to be passive, meaning they do not rely on external power sources to operate. This makes them highly reliable in the event of a LOCA, as they are not susceptible to electrical failures. The borated water injected by the accumulators also helps to shut down the nuclear chain reaction by absorbing neutrons. The boron acts as a neutron poison, reducing the reactor's reactivity and preventing a power excursion. The volume and pressure of the accumulators are carefully designed to provide sufficient cooling capacity for the initial phase of a LOCA, buying time for the other ECCS components to take over and provide long-term cooling.