The CANDU reactor is a Canadian-invented, pressurized heavy water reactor developed initially in the late 1950s and 1960s. The acronym "CANDU", a registered trademark of Atomic Energy of Canada Limited, stands for "CANada Deuterium Uranium". This is a reference to its deuterium-oxide (heavy water) moderator and its use of uranium fuel (originally, natural uranium). All current power reactors in Canada are of the CANDU type. Canada markets this power reactor abroad.
The reactors are used in nuclear power plants to produce nuclear power from nuclear fuel.
The CANDU reactor is conceptually similar to most light water reactors, although it differs in the details.
The central functionality behind the CANDU design is heavy water moderation and on-line refuelling, which permits a range of fuel types to be used (including natural uranium, enriched uranium, thorium, and used fuel from Light Water Reactors). Significant fuel cost savings can be realized if the uranium does not have to be enriched, but simply formed into ceramic natural uranium-dioxide fuel. This saves not only on the construction of an enrichment plant, but also on the costs of processing the fuel.
In terms of safeguards against nuclear proliferation, CANDU reactors meet a similar level of international certification as other reactor designs.
The Advanced CANDU Reactor (ACR) is a Generation III+ design and is a further development of existing CANDU reactors designed by Atomic Energy of Canada Limited. It is a light-water-cooled reactor that incorporates features of both Pressurised heavy water reactors (PHWR) and Advanced Pressurized Water Reactors (APWR) technologies. It uses a similar design concept to the Steam Generating Heavy Water Reactor (SGHWR).
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)
For more information (background, pictures, experiments and references): CANDU Reactor