What is the role of recirculation flow control in BWRs?

• A. It helps regulate reactivity by altering steam voids in the core.
• B. It changes turbine output directly.
• C. It controls feedwater temperature to the steam generators.
• D. It maintains containment pressure.

Answer: (A)

In a boiling water reactor, power is controlled by changing the amount of steam bubbles, or voids, in the core. Recirculation flow control does exactly that: by increasing the flow through the core, more water boils and more steam voids form. Because the reactor has a positive void coefficient, those additional voids push the reactivity higher, so the reactor power rises. Conversely, reducing recirculation flow lowers the void fraction, decreases reactivity, and power drops. This is why recirculation flow is the primary means of regulating reactor power in a BWR.

It doesn’t directly set turbine output, which depends on steam flow and turbine controls; it doesn’t control feedwater temperature to steam generators (BWRs don’t have separate steam generators like that), and it isn’t used to maintain containment pressure.