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Heavy- water reactor programs were started in Canada, France, Germany, Italy, Japan, Sweden, Switzerland, the United Kingdom, the United States of America and the former USSR. Each country built research and prototype power reactors, some operating successfully for a number of years, but only the heavy water moderated, heavy water cooled version developed in Canada (also known as CANDU) proceeded to the stage of commercial implementation to become one of the three internationally competitive reactor types available at the end of the 20th century and which has been exported to a number of countries.

Heavy-water reactor (HWR) is a nuclear reactor, commonly using unenriched natural uranium as its fuel, that uses heavy water (deuterium oxide D2O) as its coolant and neutron moderator. the heavy water coolant is kept under pressure, allowing it to be heated to higher temperatures without boiling. While heavy water is significantly more expensive than ordinary light water, it creates greatly enhanced neutron economy, allowing the reactor to operate without fuel-enrichment facilities and enhancing the ability of the reactor to make use of alternate fuel cyles.

HWR technology offers fuel flexibility, low operating costs and a high level of safety, and therefore represents an important option for countries considering nuclear power programmes.

-         Heavy water reactors: status and projected development: IAEA, 2002.

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