CANDU Reactors – buyer beware

December 31, 1969

Every nuclear reactor is a disaster waiting to happen — and CANDU reactors are no exception. Canadian utilities are no longer building CANDU reactors because of their high cost and poor performance. CANDUs have a number of serious technical, and safety problems, as well as the unique environmental problem of tritium emissions.

However, Atomic Energy of Canada Ltd. (AECL), a Canadian government agency, is still trying to sell CANDUs — let the buyer beware! CANDU reactors will inevitably worsen the economic and environmental situation of importing countries.

The CANDU has been a marketing failure. Despite 35 years of effort, only six commercial CANDU reactors have been sold outside of Canada. Canada has only 22 commercial power reactors, one of which is being shut down early due to technical problems. All of these reactors have been heavily subsidized by Canadian taxpayers. CANDUs account for only about 5% of power reactors in operation and under construction worldwide. With the failure of CANDU sales, it is not likely that the Canadian government will continue to support the nuclear industry as it has in the past. Thus, CANDU buyers may be left without adequate research and technical support in the future.

CANDU Reactors: Not Economic

The cost of new CANDU reactors are mainly determined by the initial capital cost, and the performance of the reactor over time. In addition there are operating and maintenance costs and the need for large ongoing capital expenditures.

Initial Capital Cost — Because of the need for heavy water to be used as moderator and coolant, the CANDU is even more expensive than other reactor systems. High capital cost has killed nuclear power expansion in Canada — in the last 25 years, the cost of CANDU reactors has more than doubled in real terms.

Cost overruns are one of the most serious risks for CANDU purchasers. Canadian utilities have never estimated capital costs accurately. Ontario Hydro’s Darlington Nuclear Station (four 881 MW reactors) was estimated in 1978 at $3.95 billion, but by 1993, the cost was over $14 billion — an increase of over 250%. The Point Lepreau Nuclear Station in the Canadian province of New Brunswick is a single 600 MW reactor, similar to the standard CANDU-6 offered by AECL for export. It was originally estimated at $500 million but cost $1.25 billion when it started in 1983.

All of AECL’s commercial exports to date have been single unit 600 MW reactors (CANDU-6). AECL does not reveal the selling price of its reactors publicly.

Performance — AECL likes to brag that performance of CANDU reactors has been superior to other reactor designs. In fact, CANDU performance follows a pattern similar to other reactors. CANDUs improve their performance in the first few years of operation in a “learning curve”. However, after ten years, performance begins to decline at an increasing rate. As performance declines, costs increase proportionally.

Capacity Factor is the reactor’s actual electricity production divided by “perfect” output — what the reactor would produce if it always operated at its design rating. By 1994, the average lifetime Capacity Factor at Ontario Hydro’s 20 reactors was 73.85% — significantly below the target performance of 80%. For the single year 1993, Ontario Hydro’s reactors had a Capacity Factor of 64.8%.

Capital Modifications — CANDU reactors also require very high annual capital expenditures which increase with time. By 1993, capital modifications at Ontario Hydro CANDU reactors had risen to about $40 per kW of capacity — about twice the amount (in constant dollars) that was being spent in the mid- to late 1980s. In 1993, Ontario Hydro spent $530 million (in capital alone) on its twenty CANDU reactors.

Operation & Maintenance — Performance of nuclear plants is closely related to spending on operation, maintenance & administration (OM&A). Given a flat level of OM&A, performance will decline as a nuclear plant grows older. By 1993, Ontario Hydro was spending over $1 billion ($109 CDN) per year on OM&A for its twenty reactors — an average of about $51 million per reactor. In the late 1980s, Ontario Hydro’s OM&A expenditures increased in real terms at annual rates of about 20%

CANDU Reactors: Unsafe

In the five-year period from 1989 to 1993, there were over 900 incidents at Ontario’s five nuclear stations that required reporting to the federal nuclear regulatory agency. These events included: failure of operating or safety systems, breaches of security, radiation releases in excess of allowable limits, and exposure of workers to excessive radiation.

The CANDU and its prototypes have experienced some of the world’s most serious accidents:

  • In 1952, the NRX (a 40 MW reactor that was used to supply plutonium to the US military) at AECL’s Chalk River site in Ontario, had the world’s first major nuclear accident. Fuel melting, and an explosion destroyed the reactor core, and there was a large radiation release.
  • In 1958, an irradiated fuel element at the NRU (another reactor at Chalk River) broke off and caught fire after being removed from the reactor. 600 men (mostly Canadian and American soldiers) were involved in the clean-up of the radioactive contamination.
  • On August 1, 1983, a pressure tube in Pickering Reactor #2 had a one-metre rupture due to embrittlement, dumping coolant into the reactor building.
  • In January 1990, a computer problem caused a Loss of Coolant Accident resulting in a 12-tonne leak of heavy water from a fuelling machine on Bruce Reactor #4.
  • In August 1992, a tube-break in the moderator heat exchanger on Pickering Reactor #1 dumped 3,000 litres of radiation-contaminated heavy water into Lake Ontario. It was the largest tritium release in CANDU history, causing the shutdown of a nearby water supply plant.
  • In December 1994, a valve failure at Pickering Reactor #2 led to 140 tonnes of heavy water being dumped out of the reactor. For the first time in CANDU history, the Emergency Coolant Injection System was used to avoid a melt-down.
  • In May 1995, a valve failure caused a 25-tonne leak of radioactive heavy water at Bruce Reactor #5.

There are also a number of “generic” concerns about safety at CANDU reactors.

Positive Void Effect — Drastic increases in the rate of the nuclear chain reaction can occur if coolant does not circulate properly in the core, leaving a ” positive void” or space. This can lead to a loss of reactor control.

Flux Tilts — The flow of neutrons can vary beyond the specified limits in various sections of the reactor core, leading to a loss of control and fuel melting. There have been numerous flux tilts at CANDU reactors.

Reactor Explosions — Steam explosions are possible if melted fuel contacts the moderator. Hydrogen explosions are also possible in CANDU reactors.

CANDU Reactors:

Environmental Impacts

Even if a severe accident is avoided, routine radioactive pollution from CANDU reactors can lead to environmental and public health problems. Radioactive contamination is impossible to see, smell or taste, and its health effects may take years to show up, but it is still deadly.

Tritium — The emission of large amounts of the radioactive element tritium is unique to the CANDU reactor, because it is produced by the exposure of heavy water to radiation. A 4 Sievert dose of tritium oxide absorbed into the body is lethal to half of those exposed. This dose is caused by about 200 gigabecquerels (i.e. 200 X 109 becquerels, or about 5.4 curies) of tritium oxide. From 1989 to 1992, the eight Ontario Hydro CANDU reactors at Bruce released on average over 4,500 terabecquerels (TBq) (i.e. 4,500 X 1012 becquerels) of tritium oxide to air and water per year (about 570 TBq per year, per reactor). However, accidental tritium releases can be very large — an accident at Pickering in August 1992 resulted in a leak of 2,300 terabecquerels (i.e. 2,300 X 1012 becquerels) into Lake Ontario.

Low-level radioactive waste — Uranium mines in the Canadian provinces of Ontario and Saskatchewan have left a deadly legacy of over 200 million tonnes of radioactive and acidic tailings. The tailings release the hazardous radioactive elements radium and radon (a gas). Radioactive wastes are also created by the uranium refining and conversion processes. The best method to clean up this radioactive waste is very controversial and very expensive.

High Level Radioactive Waste — High level radioactive waste (used reactor fuel) is a problem that lasts virtually forever. In Canada, that waste is currently being stored in water-filled pools, or in dry canisters at reactor sites. The Canadian government is holding an environmental assessment on a nuclear industry proposal to bury the waste in rock formations of the Canadian north. Environmentalists strongly oppose the underground disposal concept, instead supporting above-ground storage. AECL has often suggested that high-level radioactive waste could eventually be returned to Canada by CANDU buyers. However, this would face strong public opposition and is not an approved policy.

CANDU Reactors:

Building the Bomb

Through its international trade in uranium, heavy water, tritium and nuclear reactors, Canada has contributed significantly to the proliferation of nuclear weapons. Until 1962, Canada supplied plutonium for American nuclear weapons. In 1974, India detonated a nuclear bomb using plutonium manufactured in a reactor given to them by Canada.

The CANDU reactor can aid proliferation in several ways. CANDUs possess on-line refuelling capability — the reactor continues to operate while fuel is being removed and inserted. This makes it much more difficult to determine if spent fuel is being removed to make plutonium for nuclear weapons. Because CANDU uses natural uranium, fuel enrichment is not required. Since uranium enrichment is difficult and expensive, this may make it easier for a CANDU owner to build a bomb.

CANDU Reactors:

Unsustainable Development

CANDU reactors will inevitably worsen economic and social problems.

CANDU reactors have a very high capital cost, and produce very few permanent jobs. This is a gross mis-match for most developing countries, which tend to lack capital, but have abundant labour.

The high level of borrowing required for nuclear reactors increases debt problems. Huge cost overruns are typical, and the cost of decommissioning and waste management are usually underestimated.

Because of its high cost, investment in nuclear power precludes investment in truly sustainable energy alternatives: conservation measures and renewable energy.

Large reactors require extensive infrastructure, including: a large electricity grid; technical and regulatory staff; uranium mining, heavy water and uranium fuel capability; and waste management facilities.

Because of the connection, or potential connection to nuclear weapons development, nuclear power can become an important factor in regional military conflicts.

CANDU Reactors:

Small Is Not Beautiful

AECL has designed a smaller 450 MW reactor, “CANDU-3”, targeting countries with smaller energy needs. However, attempts to construct a prototype reactor in two Canadian provinces (New Brunswick and Saskatchewan) have failed. An attempt to have the reactor licensed in the United States has been put on hold after the U.S. Nuclear Regulatory Commission requested better documentation.

AECL also designed a smaller reactor, the 10 MW “Slowpoke Energy System”, or “Mega-Slowpoke”, intended for district heating and radioisotope production. Between 1985 and 1990, AECL’s offer to build a Mega-Slowpoke for free was turned down by four different communities across Canada because of safety concerns. This program has since been cancelled.


1. Bruce Reactor #2 was shut down in October 1995. Bruce 2 was an 848 MWe (net) reactor that began commercial operation in 1977.

2. Canada also sold three smaller CANDU prototypes, one to Pakistan, and two to India. As of October 31, 1994, there were 436 “operable” reactors, and 48 reactors under construction, for a total of 484. See: World Nuclear Industry Handbook 1995, Nuclear Engineering International, p. 9.

3. Charles Komanoff, Capital Cost Escalation at Ontario Hydro CANDU Plants: What Should be Expected in the Future? Coalition of Environmental Groups, December 1992.

4. Charles Komanoff, Performance Reliability of Ontario Hydro CANDU Plants: What Should be Expected in the Future? Coalition of Environmental Groups, November 1992, p. 12.

5. William Marcus, The Cost of Nuclear Plant Capital Modifications: A Statistical Analysis, IPPSO, April 1992.

6. Ontario Hydro, Interrogatory Response 4c.15.17, Ontario Energy Board Hearing HR 22, May 19, 1994.

7. Charles Komanoff, OM&A and Capital Modifications Costs at Ontario Hydro CANDU Plants: What Should be Expected in Future? Coalition of Environmental Groups, 1993, p. 2.

8. Ontario Hydro, Interrogatory Response 4c.45.4, Ontario Energy Board Hearing HR 22, May 19, 1994.

9. Ontario Hydro, A Journalist’s Guide to Nuclear Power, 1988, p. 2.

Box 1

CANDU: What is it?

CANDU stands for CANadian Deuterium Uranium reactor — playing on the North American boast of capability, “can do”. The CANDU is a Pressurized Heavy Water Reactor (PHWR) using heavy water (deuterium) as both a moderator and coolant. The CANDU reactor core is a horizontal cylinder known as a “calandria”.

Through the calandria run hundreds of horizontal tubes, inside which are pressure tubes containing fuel bundles. U.S. Light Water Reactors require enriched uranium fuel at about 2% to 4% uranium235 and use a relatively poor moderator (ordinary “light” water); whereas CANDU reactors use natural uranium at about 0.7% uranium235, but have a very good moderator (heavy water). Heavy water is very expensive and difficult to manufacture, making CANDU more expensive than other reactor designs.

In 1993, Ontario Hydro and AECL sold heavy water to South Korea for over $300 (CDN) per kg (about $225 US/kg). CANDUs require about one (metric) tonne of heavy water for every megawatt (MW) of capacity.

Heavy water moderator and coolant slow down neutrons to sustain a chain reaction. In addition, heavy water coolant flows through the pressure tubes past the fuel bundles, to transfer heat to the steam generators.

box 2

Retubing of Pickering “A” — CANDU reactors must be retubed — virtually rebuilt — after 15 to 20 years of operation. This is the single largest expense for CANDU reactors after construction — more than the original capital cost of the reactor. Pickering “A” was retubed after a massive pressure tube rupture at Pickering reactor #2 in 1983. CANDU tubes become brittle and subject to breakage after irradiation and the absorption of hydrogen.

The retubing of Pickering reactor #2 began in August 1983, after just 12 years of operation, and was followed by retubing of the other three reactors at the station. The cost of retubing the Pickering “A” reactors was $935 million — $219 million more than the original $716 million cost of the four reactors!

box 3

CANDU Power Reactors Worldwide *

CountryNumber of Station Names


Canada 21Pickering (8)

Bruce (7)

Darlington(4) Gentilly (1)

Point Lepreau (1)

South Korea 4Wolsong

Argentina 1Cordoba

Romania 1Cernavoda

* Smaller prototype and research reactors, sometimes referred to as “CANDU” reactors exist in India, Pakistan and Taiwan

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6 Responses to CANDU Reactors – buyer beware

  1. Thomas Whitmore says:

    A substantial proportion of your comments abut accidents, hazards, and plutonium production relate specifically to the NRX and NRU reactors — and specifically not the CANDU.

    As a result of these errors, probably 40% of your negatives, attributed to the CANDU, are incorrect. Many of these are the most serious charges, related to the NRX and NRU — eg both serious fuel failures in the 50’s, and the Indian plutonium.

    You are mis-stating the case against the CANDU, which afaik actually appears to be one of the better & safer reactor designs.

    Tritium from CANDU is not primarily a proliferation problem – weapons programs produce their tritium (minor component) from different sources, entirely.

  2. chemfood99 says:

    There is the argument that NO reactor is safe until it is figured out what to do with spent fuel. Please visit and share at Nuclear News Now

    • THE WU says:

      THERES WORSE THINGS SON! look at ANY other industry. Can they tell you where any waste is at exactly any time? That’s what I thought… for now its the safest and most economical thing going. Other than natural disasters like Fukushima which the entire industry learns from and makes corrections accordingly. You like many others have a bad view on Nuclear and what it really is and does.

  3. RW says:

    Much of what you have sited as evidence that CANDU reactors are unsafe is/are related to maintenance, human error, and questionable materials grade. The decision to sell by-products is not related to the core issue of safety. I would well imagine all designs become more expensive as they age. This is a reality of anything mechanical. Why would “any” reactor be any different?

  4. Kieran Wall says:

    Something that should be edited: Tritium isn’t an element, it’s an isotope of hydrogen.

  5. Manir says:

    It can be categorized as “Fake News”.
    The information are unsubstantiated. I checked couple of them such as accident in India, it is exaggerated.
    Every nuclear reactor has its inherent risks. One thing is apparent, even if CANDU is described as so bad, it did not have any “boom” in the past. Whereas, other types of reactors have multiple “booms” e.g Chernobyl and Fukushima.

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