Water-Moderated Water-Cooled Reactor

a nuclear reactor in which water serves as the neutron moderator and coolant. Structurally, such a reactor is a water-filled tank in which the fuel assemblies (fuel element assemblies) making up the core are submerged. Generated by circulating pumps, the stream of water passing through this core carries off the liberated heat. Natural circulation is often used in low-power reactors.

There are two types of water-moderated water-cooled power reactors—pressurized-water reactors and boiling-water reactors. In the former, the water is not heated to the boiling point; in steam generators the water transfers the heat obtained to the water of the secondary loop, which is converted into live steam (for instance, in the reactors at the Novovoronezh Nuclear Power Plant). In boiling-water reactors the water is partially converted into steam as it passes through the core. The steam-and-water mixture is separated after emerging from the reactor or in the reactor itself—the steam is directed to the turbine while the water is returned to the reactor core. In order to produce steam suitable for use in the turbines, a high pressure is maintained in power reactors: 7 meganewtons/m² (70 kilograms-force/cm²) in boiling-water reactors and 10-20 meganewtons/m² (100-200 kilograms-force/cm²) in pressurized-water reactors. Water-moderated water-cooled reactors, in which the water moves under a pressure substantially lower than in power reactors, are used as research reactors.

As a result of the high moderating characteristics of water and its excellent qualities as a coolant, water-moderated water-cooled reactors are highly compact and make it possible to develop a great power density (per unit volume of the core). Therefore they are relatively inexpensive to construct. The reactors are simple and reliable in operation; they have become widely used as power and research facilities.