What substance acts as the reactor coolant, flowing in and around the reactor to transfer heat?

• A. Air
• B. Water
• C. Helium
• D. Nitrogen

Answer: (B)

Water is the reactor coolant because it efficiently absorbs heat from the reactor and carries it away to generate steam. Its high heat capacity means it can take up a lot of heat with only a small rise in temperature, making it an effective heat transfer medium. In many reactors, water is kept under high pressure to stay liquid at high temperatures, allowing continuous circulation through the core to remove heat. Water also often acts as a neutron moderator in light-water reactors, aiding control of the reaction. Other gases like air, nitrogen, or helium don’t transfer heat as effectively (air and nitrogen have low heat capacities, and helium is used only in specific high-temperature designs), so they’re not the standard coolant in most reactors.