Quick Facts: Hazards of the CANDU-6

Page - February 2, 2009


  • Atomic Energy of Canada Limited (AECL) developed the Candu-6 reactor in the early 1970s. The CANDU-6 is the only reactor AECL has sold.
  • Nine CANDU-6s have been built internationally in China (2), Argentina (1), South Korea (4) and Romania (2). Two were built in Canada - Point Lepreau in New Brunswick and Gentilly-2 in Quebec.
  • Despite its current promotion of the prototype Advanced CANDU Reactor (ACR), the CANDU-6 remains central to AECL's business plans. AECL hopes to sell additional CANDUs to Argentina, Romania and Turkey etc.
  • AECL's biggest business opportunity is refurbishing existing CANDU-6 reactors, such as Quebec's Gentilly-2 station.
  • Ontario abandoned its plan to build a CANDU-6 reactor in 2006 because the design did not meet modern safety requirements.
  • The CANDU-6 poses unique nuclear safety and proliferation risks that should call into question Canada's continued support of AECL's marketing of this antiquated design and the refurbishment of operating CANDU's in Canada.

A CANDU Design Flaw: Positive Reactivity

  • The world's first significant international nuclear accident occurred in 1952 when Atomic Energy of Canada Limited's (AECL) NRX reactor experienced a power pulse, causing a hydrogen explosion and melting of fuel assemblies.
  • A central cause of the NRX accident was the reactor's "positive reactivity," which refers to the tendency of the reactor's power to increase, potentially in an explosive pulse (p. 19).
  • Following the NRX accident most international regulators and vendors decided to shun reactors with positive reactivity due to the inherent hazard of design. The Canadian nuclear industry and its regulator, however, decided to tolerate this design flaw in order to accommodate the CANDU design (pp. 19 - 20).
  • AECL's continued existence depends on Canada's nuclear regulator continuing to accept positive reactivity, which is contrary to the direction taken by other international regulators (p. 29).
  • In 1972, AECL started up a prototype reactor called Gentilly-1 near Trois-Rivières Quebec. The magnitude of positive reactivity exhibited by Gentilly-1 was so great it could not operate stably.
  • There was concern that the reactor's containment would not withstand an explosive power pulse arising from failure of the emergency shut down system. Gentilly-1 was permanently shut down in 1977 (pg. 20).
  • Despite evidence following the Gentilly-1 fiasco that a mainstream CANDU could experience an explosive power pulse, Canada's nuclear regulator did not prohibit positive reactivity. Instead the regulator required all new CANDU reactors to have two independent emergency shutdown systems, which diverged from the approach taken by most other international regulators (p. 21).
  • The ability of CANDU shutdown systems to operate under accident conditions has not been confirmed by test or experience (p. 21). Confidence in the estimated effectiveness of CANDU shutdown systems in accident situations is low because of the significant uncertainties in modeling such situations (p. 23).
  • The CANDU and Chernobyl RBMK reactor designs both exhibit positive reactivity. A significant contributor to the 1986 Chernobyl accident was positive reactivity (p.38).
  • The Chernobyl accident spurred Canada's nuclear regulator to reassess its assumptions regarding the hazards posed by positive reactivity in CANDU reactors. Work proceeded very slowly. Studies eventually showed a high degree of uncertainty in the assumptions underlying safety assessments for CANDU reactors (p. 20).
  • Anticipating applications for new reactors, the CNSC proposed a new regulatory framework for licensing reactors based on international safety standards in 2005, which "prioritized" reactors with negative reactivity (p. 29).
  • AECL complained that the application of international standards would have negative impacts on the marketing prospects of the CANDU-6 internationally and would reflect badly on operating CANDUs in Canada (p. 29).
  • If modern international safety standards were strictly applied, a reactor with positive reactivity such as the CANDU-6 could not be built (p. 29).
  • In 2008, AECL was forced to abandon the commissioning of two small MAPLE reactors at Chalk River because they exhibited uncontrollable positive reactivity (p. 7).
  • In 2001, AECL began a marketing push in Canada, the United States and the United Kingdom for its prototype Advanced Canada Reactor (ACR). Unlike the CANDU-6, the ACR is intended to have negative reactivity in order to meet modern licensing requirements (p. 7). To do this, it uses slightly enriched uranium instead of natural uranium, and light-water cooling.

A CANDU-6 Vulnerability: Terrorism

The CANDU-6 is a pre-September 11th design and was not designed to resist a terrorist attack.

In 2006, Ontario abandoned its plan to build a new CANDU-6 because of the design changesrequired to meet post-September 11th safety requirements (p. 9).

While requirements for reactors to be more robust against terrorist attacks continue to evolve sinceSeptember 11th, it is clear that the CANDU-6 would not meet current standards if they wererigorously applied (p. 36).

Proliferation Arms Proliferation and the CANDU-6

  • The CANDU-6's use of natural uranium makes it attractive to countries hoping to acquire fissile material (plutonium or high-enriched uranium) for use in nuclear weapons without the need for enrichment facilities.
  • The CANDU-6 practice of online re-fueling makes it difficult to detect and prevent the diversion of used nuclear fuel for the possible use in atomic weapons (p. 24)
  • India produced plutonium for its 1974 nuclear weapons test in its Canadian supplied CIRUS reactor, which used natural uranium fuel (p. 23).
  • It is suspected that Pakistan has used its Canadian supplied KANUPP reactor to produce military plutonium (p. 23-24).
  • Three to four kilograms of plutonium is sufficient to produce an atomic bomb. (p. 13)
  • Canadian reactors will have produced 170 thousand kilograms of plutonium through 2010. (p. 13)
  • AECL is interested in selling additional CANDU-6 reactors to countries such as Turkey, India and Jordan that may be interested in acquiring a ready option for diverting spent reactor fuel for production of nuclear weapons.

CANDU Life-Extension and a Weak Regulator

  • The economic case for re-building and extending the life of CANDU reactors, such as Quebec's Gentilly-2 nuclear station, is weak and dependent on the rigour with which the CNSC imposes modern regulatory requirements and upgrades to the reactors (p. 33).
  • The Canadian Nuclear Safety Commission has significantly weakened its modernized safety requirements to accommodate the design flaws of operating designs in Canada since they were first drafted in 2005 (p. 28-29).
  • The CNSC's imposition of international safety standards to the pre-licensing of the CANDU-6 in 2006 created a tension between the CNSC and the federal government because of the negative impact it had on AECL's ability to retain market share in Ontario (p. 9).
  • CNSC president Linda Keen was subsequently fired by the Harper government for her handling of the socalled radio-isotope crisis.
  • Hydro-Quebec has decided to proceed with the life-extension of the Gentilly-2 nuclear station before it has completed the safety reviews required by the CNSC. This leaves the estimated cost for rebuilding the Gentilly-2 nuclear station open to considerable "regulatory risk" if the CNSC applies modern regulatory requirements stringently (p. 32).
  • The CNSC's approach to life-extension has been improvised and dependent on secretive and ad hoc negotiations between it and reactor operators on the rigour with which modern regulatory requirements will be imposed. (p. 32- 34). In such an environment, the CNSC may be overly apt to prioritize the business interest of nuclear operators over the stringent application of safety standards.