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[en] In the recent past, several discoveries stemming from analytical and research work have lead to a realization that safety margins previously demonstrated in the Safety Reports of some CANDU plants may have been overestimated. The new analyses indeed had shown that to preserve the safety margins some restrictions needed to be applied to operational limits. In some cases these restrictions included limiting the maximum allowable power. In particular, Bruce B reactors have been limited in the recent years to 90 % full power operation due to concerns related to consequences of large break loss of coolant accidents. The Canadian Nuclear Safety Commission (CNSC) has requested all licensees of power reactors to implement measures to restore and improve safety margins. Those utilities that had power deratings imposed on operation of their reactors are also being driven in their margin improvement efforts by the economic incentive to have their licensed power limit restored to 100% full power. Approaches adopted by different utilities vary depending on the specific situation and may include: - optimization of operational limits and conditions; - refinement of safety analysis tools and methods (in particular, development of a best estimate and uncertainty methodology); - further experimental investigation to better validate tools used in accident analysis; - implementation of design changes (the most significant being the new fuel design); - development of an integrated risk informed licensing methodology to demonstrate that the overall risk is little unaffected by the recent discoveries; - implementation of accident management strategies as a toll to address residual risks. Many of these activities require significant advance effort and are novel in licensing application. The CNSC staff is involved in these efforts through identification and development of regulatory requirements and expectations. A concise outline of major activities in the area of safety margins and power up-rates in Canada is given in this presentation. In the past, in Canada there have been more power deratings than increases in the licensed reactor power, a situation caused directly by the erosion of safety margins. Deficiencies in earlier safety analyses, new knowledge gained from experimental research, as well as ageing effects, have led to the need of restoration of margins in order to maintain high power operation. Recently, some of the licensees have embarked on ambitious programs to improve safety and economic performance of their plants; such programs include implementation of significant design changes and may lead to applications for power increase. (author)
[en] To summarize: • Canadian requirements and practices evolve in response to modern expectations and arising challenges. • Operating NPPs have implemented various physical and procedural enhancements and will continue seek further improvements. • Some of these enhancements are expected to be standard features in advanced future NPP. • CNSC remains focused on protecting safety of workers, public and environment
[en] Safety analysis is one of the components of the overall safety assessment required to demonstrate that a proposed nuclear power plant, once constructed, would operate safely, without posing unreasonable risks to the public, workers and the environment. It is also one of the so-called safety programs utilized by the CNSC in the on-going evaluations of safety performance of the operating plants. This presentation will explore why, after decades of safe nuclear power plant operation, the safety analysis remains to be an area of significant regulatory attention, both in general terms as well as from the Canadian perspective. With regard to the latter, the paper will touch upon specifics and evolution of the Canadian regulatory framework and some of the recent 'discovery issues'. The current trends, such as introductions of novel complex methods, ever-increasing attention to consideration of uncertainties, and prioritization based on safety significance will also be explored. Finally, this presentation will venture to consider potential future developments and expectations that may shape the safety analysis for nuclear power plants in the future. (author)
[en] Implementing accident management counter measures or actions to mitigate consequences of a severe accident is essential to reduce radiological risks to the public and environment. Station-specific severe accident management guidelines (SAMGs) have been developed and implemented at all Canadian nuclear power plants. Following the Fukushima Daiichi nuclear accident certain enhancements were introduced to the SAMG, namely consideration of multi-units accidents, events involving spent fuel pools, incorporation of capability offered by the portable emergency mitigating equipment, and so on. To evaluate the adequacy and usability of the SAMGs, CNSC staff initiated a number of activities including a desktop review of SAMG documentation, evaluation of SAMG implementation through exercises and interviews with station staff, and independent verification of SAMG action effectiveness. This paper focuses on the verification of SAMG actions through analytical simulations. The objectives of the work are two-folds: (a) to understand the effectiveness of SAMG-specified mitigation actions in addressing the safety challenges and (b) to check for potential negative effects of the action. Some sensitivity calculations were performed to help understanding of the impact from actions that rely on the partially effective equipment or limited material resources. The severe accident computer code MAAP4-CANDU is used as a tool in this verification. This paper will describe the methodology used in the verification of SAMG actions and some results obtained from simulations. (author)
[en] Hydrogen is generated, in small amounts, in various systems and containment of nuclear power plants (NPPs) during normal operation. In the course of an accident in water-cooled NPPs, more appreciable amounts of hydrogen can be generated and released into the containment. Hydrogen combustion has long been recognized as a safety concern for NPPs. The hydrogen produced in large quantities may create flammable, and even explosive, gas mixtures. A number of hydrogen mitigation measures have already been developed and implemented in many NPPs since the early 90’s. In Canada, CNSC required the nuclear industry to develop an understanding of hydrogen behaviour, its impact on the safety of CANDU reactors and on this basis implement practical measures to reduce the risk posed by hydrogen in accidents. In response, the nuclear industry in Canada conducted a large research and development (R and D) program to address various aspects of hydrogen behaviour. The collaborative R and D activities by Atomic Energy of Canada Limited (AECL), CANDU Owners Group (COG), and international organizations, resulted in development of a solid knowledge and technology base to the current approach for addressing issues of hydrogen risk. The key outcomes of these activities were the development of analytical tools and codes to predict hydrogen behaviour in containment, and of hydrogen risk mitigation provisions that have been implemented in Canadian NPPs. The purpose of this paper is to describe the recent safety improvements and current focus in hydrogen risk assessment and management measures that are specific to severe accident scenarios at CANDU nuclear power plants and are being taken by the licensees and regulator in Canada
[en] Objective of the meeting: • To provide a forum for the exchange of information and views on the methodologies and tools used in source term evaluation (STE) for severe accidents; - To share state-of-the-art STE practices; - To discuss future activities arising from lessons learned from the Fukushima Daiichi accident so that an appropriate evaluation of the source term can be made, - Ultimately, to facilitate an accurate prediction of the total radiological releases from the Fukushima Daiichi NPP.
[en] This paper explores the overarching safety principles that will likely guide the safety design of advanced reactor technologies. As will be shown, the already established safety framework provides a solid foundation for the safety design of future nuclear power plants. As a specific example, the principle of 'proven technology' is presented in greater detail and its implications for a novel technology are discussed. Research, modeling and prototyping are shown to be components in satisfying this principle. While the fundamental safety principles are in place, their interpretation may depend both on the considered technology as well as the national context. Thus, the regulatory authority will need to be engaged, at an appropriate stage of the technology development, in specifying the regulatory requirements that will have to be met for a specific reactor design. (author)
[en] • Good response to the meeting invitation - Continued interest = importance of subject; • ST means different things to different people and organizations: - Diverse uses of the ST: - Risk and EI assessment; - Validation of AM; - Emergency planning; - Appropriate response to challenges through design or procedure optimization.
[en] A comprehensive technology demonstration program is seen as an important component of the overall safety case, especially for a novel technology. The objective of such a program is defined as providing objective and auditable evidence that the technology will meet or exceed the relevant requirements. Various aspects of such a program are identified and then discussed in some details in this presentation. We will show how the need for such a program is anchored in fundamental safety principles. Attributes of the program, means of achieving its objective, roles of participants, as well as key steps are all elaborated. It will be argued that to prove a novel technology, the designer will have to combine several activities such as the use of operational experience, prototyping of the technology elements, conduct of experiments and tests under representative conditions, as well as modeling and analysis. Importance of availability of experimental facilities and qualified scientific and technical staff is emphasized. A solid technology demonstration program will facilitate and speed up regulatory evaluations of licensing applications. (author)
[en] Safety analysis is one of the components of the overall safety assessment required to demonstrate that a proposed nuclear power plant, once constructed, would operate safely, without posing unreasonable risks to the public, workers and the environment. It is also one of the so-called safety programs utilized by the CNSC in the on-going evaluations of safety performance of the operating plants. This presentation will explore why, after decades of safe nuclear power plant operation, the safety analysis remains to be an area of significant regulatory attention, both in general terms as well as from the Canadian perspective. With regard to the latter, the paper will touch upon specifics and evolution of the Canadian regulatory framework and some of the recent 'discovery issues'. The current trends, such as introductions of novel complex methods, ever-increasing attention to consideration of uncertainties, and prioritization based on safety significance will also be explored. Finally, this presentation will venture to consider potential future developments and expectations that may shape the safety analyses for nuclear power plants in the future. (author)