In a Pressurized Water Reactor (PWR), which substance is dissolved in the moderator/coolant to control the neutron population?

• A. Boron
• B. Helium
• C. Gadolinium
• D. Hafnium

Answer: (A)

In a PWR, controlling the neutron population is achieved by using a soluble neutron absorber in the reactor coolant. The substance dissolved in the primary coolant is boron, typically in the form of boric acid. Boron-10 has a high probability of capturing neutrons, especially thermal neutrons, so its presence reduces the number of neutrons available to sustain fission. By adjusting how much boron is in the coolant, the reactor can finely tune reactivity as fuel burns and the core tends toward less neutronic excess over time. This soluble poison is complemented by mechanical control rods, but the boron concentration provides a flexible, continuous means of reactivity control.

Helium is inert and would not affect neutron population. Gadolinium and hafnium are strong neutron absorbers used as solid burnable poisons or in control rods, respectively, rather than dissolved in the coolant.