What are Light Water Reactors?

• A. A nuclear reactor that uses H2O, or light water, as both moderator and coolant
• B. A reactor that uses heavy water as moderator and liquid metal as coolant
• C. A reactor that operates with steam directly produced in the core
• D. A reactor using gaseous coolants exclusively

Answer: (A)

Light water reactors are defined by using ordinary water (H2O) as both the neutron moderator and the coolant. This choice of moderator and coolant is what sets them apart from other reactor types that might use heavy water (D2O), gases, or liquid metals. In practice, the two common varieties of light water reactors are pressurized water reactors and boiling water reactors. In a pressurized design, the water is kept under high pressure to stay liquid as it passes through the reactor core, transferring heat to a secondary loop that produces steam. In a boiling design, the water boils in the core and generates steam directly to drive the turbine. The use of light water means the fuel can be slightly enriched to sustain fission, since ordinary water absorbs neutrons more than heavy water.

The other options don’t define light water reactors. Using heavy water as moderator or liquid metal as coolant describes a different class of reactors. Using gaseous coolants points to gas-cooled reactors. And while boiling in the core can occur in light water reactors, that detail describes a specific variant rather than the defining feature, which is that ordinary water serves both as moderator and coolant.