A calandria can be defined as one of two distinct types of devices. A nuclear power station design in Canada, known as the CANadian Deuterium Uranium (CANDU) reactor, uses a calandria reactor core which is based on the use of heavy water, or deuterium, and natural uranium fuel. A calandria may also be defined as a thermosiphon boiler design, which is a sealed chamber that functions as a heat exchanger through calandria evaporation of chemical solutions without the need for an electrical pump to facilitate the process.
The Canadian design for the CANDU reactor was first developed in the late 1950s, and, as of 2011, 29 updated models now exist in seven countries around the world, including Canada itself, South Korea, and China, as well as India, Argentina, Romania, and Pakistan. Due to the versatility of design of a CANDU reactor, which allows it to use a variety of fissionable materials including natural uranium and reprocessed uranium as well as thorium and plutonium, it is a direct competitor for light-water reactor (LWR) models. Light-water reactors use ordinary water as a coolant and neutron moderator, and are in operation in 359 locations around the globe as of 2011, including many additional scaled-down models on submarines and other Navy vessels.
The calandria in a CANDU reactor is the actual reactor core. It contains immersed fuel bundles when in full operation, which are bathed in deuterium heavy water as a form of coolant and transferring medium for heat. The water is channeled from the calandria to a steam-generating system that powers turbines for electrical energy generation. Unlike many other modern nuclear reactor designs, the CANDU system does not have a high pressure containment vessel. The calandria's low pressure design allows it to be refueled without shutdown as well.
A calandria evaporator is a form of chemical reactor chamber used to concentrate compounds, such as liquors and fruit juices, by precipitating out lighter and heavier elements through the use of a heat exchanger. Both nuclear reactor chambers and chemical reaction chambers based on calandria principles utilize the idea of vertical tubes. In the CANDU system, these are filled with nuclear fuel, and, in a chemical reactor, the source material. Circulation occurs through the tubes and a liquid medium that surrounds them, with the fluid propelled by different levels of specific gravity in the compounds. A central pipe known as a downcomer facilitates the separation process through thermosiphon effects, where the convection of fluid through the tubes occurs as heat is exchanged naturally in the system.