What is the effect of withdrawing control rods in a reactor?

• A. Increases neutron multiplication and moves toward supercritical
• B. Decreases neutron multiplication and moves toward subcritical
• C. Has no effect
• D. Increases boron concentration

Answer: (A)

Control rods are neutron absorbers, so moving them changes how many neutrons survive to cause fission. Withdrawing the rods reduces neutron absorption, allowing more neutrons to sustain the chain reaction. That increases the neutron multiplication factor and pushes the reactor toward, and into, a supercritical state if the withdrawal continues. In other words, reactivity rises. Boron concentration isn’t changed simply by pulling rods, so increasing boron concentration would actually decrease reactivity. The idea that there is no effect isn’t correct because the neutron population is directly affected by how many absorbers are left in the core.