|Fig. 1: This is the CANDU reactor in Qinshan. (Courtesy of Atomic Energy of Canada Limited. Source: Wikimedia Commons)|
CANDU, which stands for Canada Deuterium Uranium, is a pressurized heavy water reactor. It was developed by Canada during World War II. The design uses heavy water which contains primarily the element deuterium and the main fuel type is uranium.
There is a primary cooling loop containing pressurized heavy water that is heated by fission reactions in the core. The heat is transferred to a secondary cooling loop containing light-water through a heat exchanger. This then powers a steam turbine connected to an electrical generator. The exhaust steam from the turbines is condensed and returned to the steam generator using cooling water from a nearby body of water. 
As a heavy water moderator, CANDU sustains fewer losses of neutrons which means that it can sustain chain reaction better with less fuel. Heavy water can be produced locally, lasts beyond the life of the plant, and can be re-used. Additionally, CANDU can use natural uranium instead of enriched uranium, which is less expensive.  It also uses pressure tubes rather than one large pressure vessel which make it easy to refuel. CANDU reactors can also be refueled while it is in operation.  CANDU reactors can also burn Thorium which is found in much more abundance and is cheaper than uranium. 
Currently, CANDU reactors are used in Canada, India, China, Pakistan, South Korea, Argentina, and Romania.  Some CANDU plants have suffered cost overruns during construction, such as the Darlington Nuclear Generating Station near Toronto. The CANDU reactors built in Qinshan (Fig. 1), however, were constructed on time and within budget. It has also been shown that refurbished units have historically showed poorer performance. However, risks associated with CANDU reactors include potential experience of violent power excursions which can lead to a release of radioactive material into the environment and spent fuel discharged could be more easily used to extract plutonium for nuclear weapons. 
© Allen Yu. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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