Reactor Coolant

a liquid or gaseous substance that is passed through the core of a nuclear reactor and removes from the core the heat released as a result of the fission reaction.

In power reactors, the coolant flows from the reactor to a steam generator, where steam is produced to drive the turbine. Alternatively, the coolant itself, whether steam or gas, may serve as the working fluid for driving the turbine. In research reactors, such as materials-testing reactors, and in special-purpose reactors, such as isotope-production reactors, coolants are used only to remove heat from the core.

Reactor coolants must meet the following requirements: small neutron-absorption cross section (in thermal reactors), low moderation of neutrons (in fast reactors), chemical stability under conditions of intense irradiation, low corrosiveness with respect to the structural materials with which the coolant is in contact, high heat-transfer coefficient, high specific heat, and low working pressure at high temperatures.

In thermal reactors, the coolant used may be light water, heavy water, water vapor, an organic liquid, or carbon dioxide. The coolants employed in fast reactors include liquid metals— primarily sodium—and gases, such as water vapor and helium. In many cases, the coolant also serves as a moderator.