How does deuterium oxide help control reactions in a nuclear reactor?

Deuterium oxide, commonly known as heavy water, serves as a critical moderator in nuclear reactors by slowing down fast-moving neutrons produced during fission.123

In a thermal reactor, neutrons emitted during fission are highly energetic and must be slowed to thermal energies to effectively induce further fission in fissile material. Deuterium oxide is highly efficient at this process because deuterium atoms have a low mass, allowing them to absorb energy from neutrons through scattering collisions.234

Unlike some other materials, deuterium oxide has the distinct advantage of being a liquid that slows neutrons down without significantly capturing them. By moderating the speed of these neutrons, it helps initiate and sustain a controlled chain reaction. Additionally, in many reactor designs, heavy water serves a dual purpose by also acting as a coolant to remove the thermal energy generated by the fission process.5234