Canadian Nuclear Safety Commission
CNSC response to the letter entitled “Neuf questions auxquelles M. Michael Binder devrait répondre” [Nine questions Mr. Michael Binder should answer] published on April 4, 2010, in Le Bulletin régional of Saguenay-Lac-Saint-Jean
In response to the questions of Mr. Duguay and Mr. Giroul
To the editor,
In response to Mr. Duguay’s and Mr. Giroul’s letter containing nine questions directed to the Canadian Nuclear Safety Commission (CNSC), I would like to provide a few facts and correct some of the inaccuracies in their note that may be misleading or confusing.
In exercising its mandate, the CNSC has developed and implemented a stringent regulatory framework for ensuring that the highest level of safety is maintained at all facilities it licenses. The framework includes clear safety goals that a nuclear power plant design must meet to minimize any significant additional risk to the public in comparison with other risks to which the public is normally exposed.
While the CNSC conducts studies as part of its extremely rigorous regulatory approach and decision-making process, issues raised in the Application of the CNSC Risk-informed Decision Making Process to Category 3 CANDU Safety Issues report should not be viewed as questioning the safety of existing reactors which have attained a very high operational safety record, but rather as areas where uncertainty in knowledge exists, where the safety assessment has been based on conservative assumptions, and where regulatory decisions are needed, or will need to be confirmed.
Assessment of CANDU safety issues
The Risk-Informed Decision Making (RIDM) process, used to assess the potential risk associated with safety issues and developed by CNSC staff on the basis of the Canadian Standard Association document on risk management (CSA Q-850), provides the decision makers with information on the risk environment, and recommends corrective measures.
The application of the RIDM process led to the development of risk-informed regulatory positions, including risk control measures for resolution of each safety issue. In accordance with the internationally-followed principle of defense-in-depth, the risk assessment covered all possible combinations of events that could potentially lead to fuel damage, adverse effects on the workers, the public or the environment, or any combination thereof.
None of the scenarios studied and presented in the report yielded a risk significance level that required immediate corrective action. For each issue, the licensees are nevertheless required to take appropriate corrective actions intended to lower the risk and thus minimize any additional risk to the public. In implementing the corrective measures, licensees are required to apply state-of-the-art technology, and modern national and international standards.
Consequences of a postulated large loss-of-coolant accident
Given the overall safety framework for nuclear power plants in Canada, it is irresponsible to draw and publish an analogy between CANDU reactors and a Chernobyl-type accident. CANDU nuclear power plants (NPPs) are equipped with many safety features that make such an accident in Canada practically impossible. These features include:
- two reliable, diverse, fully independent, fully effective fast acting shutdown systems
- a containment structure designed to withstand any credible accident
- an emergency core-cooling system designed to cool the reactor in the postulated event of a loss-of-cooling of the fuel
- full independence between the control system and the shutdown systems
The Chernobyl reactor had none of those features. Furthermore, unlikely combinations of events, such as a Loss-of-Coolant Accident (LOCA) with a consequent loss of Emergency Coolant Injection, were included in the design of the CANDU reactors; this approach has led to a robust CANDU reactor design and assurance of high reliability of reactor process and safety systems. It is grossly misleading to assume that a failure in the reactor will immediately lead to radioactivity releases to the environment. Each CANDU reactor is enclosed in a massive concrete containment structure designed to contain releases due to such potential failures or to any credible accident scenarios. As a result, the postulated scenario of a large LOCA, leading to significant releases to the environment, can be considered practically impossible.
With respect to a possible “power pulse” following a postulated large LOCA, many studies have been undertaken to address such a scenario and found it acceptable. Nevertheless, the CNSC continues to consider this event in the design of CANDU reactors, specifically to assess the effectiveness of the shutdown and emergency cooling systems under accident conditions recognizing that the event is of low probability.
Finally, although the positive void reactivity coefficient is sometimes referred to as a “design weakness”, it enables the shutdown systems to be especially sensitive to slight local temperature or power perturbations, thus ensuring the triggering of two fast shutdown systems for a spectrum of postulated initiating events. This allows CANDUs to have a robust safety response for postulated accidents, not just for large LOCA.
Radioactive waste disposal in Canada
The Government of Canada has put in place a structure of policies, legislation and responsible organizations that govern the management of radioactive wastes in Canada. Under that framework, waste producers and owners are responsible for the funding, management and operation of waste management facilities and other facilities required for their wastes. The CNSC regulates and monitors operation of Canada’s waste management facilities and works together with its federal and international partners to
implement policies, strategies and plans in an effort to ensure that waste owners, and those possessing radioactive wastes, treat, handle, manage and store these wastes in a safe and secure manner. All radioactive waste is currently safely stored at licensed facilities.
Security issues relating to nuclear waste
In response to your reference to Robert Alvarez’s 2003 misleading article on the possibility of terrorist attacks on radioactive waste, please note the CNSC has aggressively ensured that such an event remains very improbable. The nuclear power plants in Canada are subject to the CNSC order issued on October 19, 2001. Following the order, rigorous security measures were put in place, including specialized detection systems and armed forces with the capacity to intervene efficiently. The CNSC verifies the efficiency of these measures on an on-going basis.
Refurbishment of CANDU reactors: Pickering and Gentilly-2
In Canada, all currently operating reactors will be expected to either refurbish or close within a few years since they are approaching the end of the nominal design life of some major components like the pressure tubes. Changes made to operating reactors throughout their life are either to improve safety or to ensure fitness-for-service of major components.
To support operation beyond the nominal design life of these components, the CNSC requires that licensees demonstrate that the technical scope of the refurbishment project is appropriately determined through the conduct of an Integrated Safety Review (ISR) and an Environmental Assessment (EA), and that the project is appropriately executed. With regards to the specific case of Gentilly-2, while preparations are underway for the refurbishment project of that plant, Hydro-Québec (HQ) is following the steps given in CNSC document RD-360, and has produced the ISR basis document. The ISR document provides a list of modern codes, standards, and practices that form the basis for a comprehensive and systematic assessment of the plant (over one hundred modern standards are considered, including applicable international documents). The CNSC has initiated the review of the ISR basis document and, as usual, is prepared to provide clarifications as required.
Conclusion
CANDU reactor design, maintenance and operation have always been in accordance with the highest national and international standards. CANDU reactors have attained a very high operational safety record, as demonstrated by hundred reactor-years of CANDU operational history. Most importantly, the CNSC will continue to make sure that all licensed Canadian nuclear facilities operate safely.
Despite the extremely low risk associated with the operation of NPPs, both CNSC and licensees continue to exercise due diligence in pursuing research and studies which could lead to further design improvements. A responsible regulatory agency such as the CNSC considers and analyzes every conceivable event, even the most unlikely, to minimize any potential risk to the public. The CNSC would not issue a licence to any facility if its safe operation were in doubt.
For more information on all of these issues, I recommend you visit the CNSC Web site at www.nuclearsafety.gc.ca.
Michael Binder
President
Canadian Nuclear Safety Commission
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