Which method is typically used for power and reactivity control in PWRs?

In pressurized water reactors (PWRs), the method that is typically used for power and reactivity control is to dissolve boron in the coolant. Boron acts as a neutron absorber; it efficiently captures neutrons and reduces the number of free neutrons available to sustain the fission chain reaction. By adjusting the concentration of boron in the coolant, operators can effectively control the reactivity of the reactor.

This technique allows for precise management of the reactor's power output. When more boron is dissolved in the coolant, it decreases the reactivity, thereby lowering the power output. Conversely, decreasing boron concentration allows more neutrons to participate in fission, increasing reactivity and power output as necessary. This method is essential for maintaining safe operational parameters and providing a means for both operational control and emergency shutdown (or scram) capabilities.

The other methods mentioned either do not directly address reactivity control or are typically not used in PWRs. For instance, using gas injectors and expanding the reactor core are not conventional practices for regulating power in PWRs, and increasing fuel temperature typically leads to increased reactivity rather than control. Therefore, dissolving boron in the coolant is a well-established and effective method employed specifically