​CANDU Reactors

What is CANDU

CANDU ® stands for "CANada Deuterium Uranium".
It's a Canadian-designed power reactor of PHWR type (Pressurized Heavy Water Reactor) that uses heavy water (deuterium oxide) for moderator and coolant, and natural uranium for fuel.

CANDU-specific features and advantages

Use of natural uranium as a fuel

  • CANDU is the most efficient of all reactors in using uranium: it uses about 15% less uranium than a pressurized water reactor for each megawatt of electricity produced
  • Use of natural uranium widens the source of supply and makes fuel fabrication easier. Most countries can manufacture the relatively inexpensive fuel
  • There is no need for uranium enrichment facility
  • Fuel reprocessing is not needed, so costs, facilities and waste disposal associated with reprocessing are avoided
  • CANDU reactors can be fuelled with a number of other low-fissile content fuels, including spent fuel from light water reactors. This reduces dependency on uranium in the event of future supply shortages and price increases

Use of heavy water as a moderator

  • Heavy water (deuterium oxide) is highly efficient because of its low neutron absorption and affords the highest neutron economy of all commercial reactor systems. As a result chain reaction in the reactor is possible with natural uranium fuel
  • Heavy water used in CANDU reactors is readily available. It can be produced locally, using proven technology. Heavy water lasts beyond the life of the plant and can be re-used

CANDU reactor core design

  • Reactor core comprising small diameter fuel channels rather that one large pressure vessel
  • Allows on-power refueling - extremely high capability factors are possible
  • The moveable fuel bundles in the pressure tubes allow maximum burn-up of all the fuel in the reactor core
  • Extends life expectancy of the reactor because major core components like fuel channels are accessible for repairs when needed

To learn more about CANDU reactors visit CANTEACH site , "The Canadian Nuclear FAQ" by Dr. Jeremy Whitlock.

 Highlights of CANDU History

1898: Ernest Rutherford was appointed professor of experimental physics at McGill University, Montreal, and set up a world centre for research into the structure of the atom and into radioactivity
1930: Gilbert Labine discovered Canada's first uranium deposit at Great Bear Lake

Hahn and Strassman gave the first report on nuclear fission and Joliot-Curie, Halban and Kowarski found proof of it


The first Canadian experimental work on nuclear fission began at National Research Council laboratories in Ottawa

1942: The first self-sustaining chain reaction was achieved at the University of Chicago

The United States, Britain and Canada signed an agreement in Quebec City for joint action on nuclear fission ... British "heavy water" team from Cambridge University began transferring to NRC's Montreal Laboratories at the University de Montreal; it included Halban and Kowarski...Canada's first heavy water was produced by Cominco in Trail, B. C.


Construction began at Chalk River, the site chosen for the Canadian "heavy water" project


ZEEP (Zero Energy Experimental Pile), Canada's first reactor, started operating at Chalk River on September 5

1946: Canada's Atomic Energy Control Act was passed into law
1947: NRX (National Research Experimental) reactor began operating at Chalk River Laboratories (CRL) on July 22 and went on to become one of the highest flux reactors in the world; it operated until 1992
1951: The world's first commercial cobalt-60 radiotherapy unit was installed at Victoria Hospital in London, Ontario. Canada went on to supply cobalt-60 machines to 70 countries where they were eventually used to administer half a million treatments annually
1952: The Canadian government formed the Crown corporation, Atomic Energy of Canada Limited on April 1

On December 12 the NRX reactor suffered a major accident, but was returned to service in 14 months

1953: Canada's first uranium mine opened at Beaverlodge, Ontario

A partnership between AECL, Ontario Hydro and Canadian General Electric was formed to build Canada's first nuclear power plant, NPD (Nuclear Power Demonstration)

1957: NRU (National Research Universal) reactor started up on November 3; it is still considered one of the world's finest for its versatility and high neutron flux
1958: The name CANDU (Canada Deuterium Uranium) was chosen for the Canadian nuclear power station design, and the Nuclear Plant Division was established in Toronto

Bertram Brockhouse's triple-axis spectrometer was installed at the NRU reactor

The world's first tandem Van de Graaff accelerator began operation at CRL


AECL announced the establishment of a second research facility to be built at Whiteshell, Manitoba


Work began on the 200-megawatt CANDU prototype at Douglas Point on Lake Huron

CIRUS, a research reactor supplied by Canada to India, started up


NPD at Rolphton, Ontario, started supplying Canada's first nuclear-generated electricity to the grid on June 4

1963: AECL negotiated a contract with Deuterium of Canada Ltd. for the supply of heavy water from Canada's first large-scale heavy water plant to be built at Glace Bay, Nova Scotia

Plans were announced for a 1,000-megawatt nuclear generating station to be built at Pickering, Ontario

1965: The first organically cooled research reactor in Canada, WR-1, went into service at Whiteshell on November 1
1967: The Douglas Point Station produced its first electricity
1968: Ontario Hydro announced plans to build the Bruce Generating Station with four AECL designed 750 MW CANDU reactors
1970: The prototype of SLOWPOKE, a small low-cost reactor for research, was started up at

CRL...The CANDUBLW prototype, Gentilly-1, was also started up

1971: Units 1 and 2 at Pickering Generating Station A and Kanupp, a reactor built in Pakistan by Canadian General Electric, began operating ... Canadian General Electric's heavy water plant at Port Hawkesbury, Nova Scotia, went into production and AECL took over responsibility for reconstruction of the Glace Bay Heavy Water Plant
1972: Two AECL-designed reactors began operating: RAPS 1 in India and Unit 3 at Pickering

With Unit 4 on line, the Pickering Station produced more electricity than any other nuclear power station in the world


Site preparation began in Rio Tercero, Argentina, and Point Lepreau, New Brunswick, for 600 MW CANDU reactors, and at Pickering for another four-unit station

1975: The Port Hawkesbury and Glace Bay Heavy Water Plants were acquired by AECL

The University of Toronto acquired the first commercial SLOWPOKE reactor; three other Canadian universities also placed orders

1977: Units 1 and 2 at Bruce A went into operation

Pickering Unit 3 had the highest capacity factor in the world.


Under a joint agreement between the federal and Ontario governments, AECL was made lead agency for studies to immobilize and dispose of high-level radioactive wastes from nuclear power plants

1979: Site clearing began for the first CANDU reactor in Romania
1981: For the first time, nuclear stations in Ontario produced more electricity than either coal fired or hydroelectric stations

AECL received the go-ahead to construct an underground research laboratory at Lac du Bonnet, Manitoba, to study methods of storing nuclear fuel wastes


CANDU held seven of the top 10 places for lifetime performance among the world's reactors Korea

Three CANDU-6 units in New Brunswick, Quebec, and South Korea went into commercial service

The first of 4 Pickering A units was taken out of service for re-tubing the reactor

1984: The fourth CANDU-6 unit went into commercial service in Argentina

The Douglas Point reactor was retired from service on May 5

1985: Because of over-supply of heavy water, the two plants in Nova Scotia were closed

The first beam was extracted from AECL's superconducting cyclotron at CRL


The NPD reactor was retired from service in May

CANDU received the honour of being included in the top 10 Canadian engineering achievements of the past century, in celebrations marking Canada's engineering centennial

1989: Darlington Unit 2 produced its first power
1990: Darlington Unit 2 went into commercial service

South Korea purchased a second CANDU reactor from Canada


The full Tandem Accelerator Superconducting Cyclotron (TASCC) facility was commissioned at CRL

The Dualspec neutron spectrometer was commissioned at NRU


Darlington Unit 1 went into commercial service

South Korea purchased a third and fourth CANDU 6 reactor from Canada


Darlington Units 3 and 4 went into commercial service

Point Lepreau was the leader in lifetime performance among more than 400 nuclear power reactors worldwide

The last of the 4 Pickering A reactors to be re-tubed was returned to service

The NRX reactor at CRL was permanently shut down on April 8


On April 10 Pickering 7 broke the world record for continuous operation at 713 days; on October 7 it was closed for maintenance after setting an 894 day record ...AECL submitted the Environmental Impact Statement for the concept for disposal of used nuclear fuel to the Canadian Environmental Assessment Agency ...Bertram Brockhouse was awarded the Nobel Prize for Physics for neutron scattering work done at NRU (see 1958)


September 5 marked the 50th anniversary of the start-up of the ZEEP facility at Chalk River

South Korea's HANARO research reactor, based on AECL's MAPLE technology, started up

The NRU reactor passed its 1,500th day of operation without a shutdown of more than 130 hours a record unparalleled worldwide

On October 8 Bruce 2 was shut down for an indefinite lay-up due to aging concerns and boiler tube integrity


In January, Bruce Unit 3 became the first CANDU reactor in the world to reach 100 million MWh of lifetime gross energy production, enough to supply a city the size of Thunder Bay for 90 years

On November 26, AECL signed a $4 billion contract for the sale of two CANDU 6 reactors to China

Cernavoda 1 went critical for the first time on April 16 and began commercial operation on December 2

1997: On July 1, Wolsong 2 achieved commercial operation

On October 16 Bruce 1 was shut down for an indefinite lay-up due to aging concerns

At the end of December all Pickering A units were shut down for an indefinite lay-up as part of Ontario Hydro’s Nuclear Asset Optimization Plan (NAOP)

1998: Wolsong 3, South Korea's third CANDU, went critical for the first time on February 20 and achieved commercial operation on July 1

On March 17 and April 9 Bruce Units 4 and 3, respectively, were shut down for an indefinite lay-up as part of Ontario Hydro’s Nuclear Asset Optimization Plan (NAOP)

RAPS-2 was returned to service after a 4-year planned outage for re-tubing of the reactor

1999: Wolsong 4, South Korea's fourth CANDU, went critical for the first time on April 10 and achieved commercial operation on October 1
2000: Four PHWRs in India, RAPS-3 and 4 and Kaiga-1 and 2, went into commercial operation on June 1, December 23, March 5 and November 16, respectively
2001: On May 14 Bruce Power and Ontario Power Generation announced the successful financial closing of the lease transaction for facilities at the Bruce nuclear site
2002: On January 9 MAPS-2 began a planned outage for re-tubing of the reactor

50-years anniversary of Atomic Energy of Canada Limited

40 years of nuclear power in Canada

Qinshan 4, the first of two Chinese CANDU units, went into service on December 31

Did you know that...

  • In 1987, the CANDu reactor, along with the CN Tower and the Alouette space satellite, was ranked as one of Canada's top ten engineering achievments during the previous 100 years.
  • There were 438 nuclear reactors in operation around the world in January 2002, 32 of them are of CANDU type.
  • Canada is the world's leading exporter of uranium, producing over one third of the world's uranium mine output
  • Heavy water (D2O) is 10% heavier than ordinary water and has a neutron moderating ratio 80 times higher than ordinary water