On the Functional Failure and Quantification of Margins
32nd Annual CNS Conference - 2011 June 05-08

Presented at:
32nd Annual CNS Conference
2011 June 05-08
Niagara Falls, Canada
Session Title:
Reactor Safety and Licensing

Dumitru Serghiuta (Canadian Nuclear Safety Commission)


This paper reviews some of the modern approaches in treatment and quantification of uncertainties in the context of quantification of margins and potential benefits offered by the use of “functional failure” concept and application of order-statistics modern techniques in safety assessments, as well as some of main areas for R&D. It presents some observations and suggestions aimed at contributing to the debate related to quantification and qualification of margin for CANDU reactors. The views expressed in this paper are those of the author and do not necessarily reflect those of CNSC, or any part thereof.


Recent NPPs operating experience shows that in some cases operational and design modifications may lead the plant far away from the original design. Aging and operating life extension, power uprates, new fuel designs with increased performance, such as increased burnup, and R&D discovery issues, as well as cumulative effects of simultaneous or subsequent design changes in a plant, which can be larger than the accumulation of the individual effects of each change, can challenge original safety margins while fulfilling all the regulatory requirements. The aspects related to margin quantification have received a considerable amount of attention from utilities, designers, methodology practitioners, and regulators, due to the significant impact on operation and the need to better evaluate and understand the overall level of safety of operating plants. At the root of the debate are two questions: (1) what is an appropriate framework of criteria and limits, and methods and methodologies for quantification of margins and (2) what are the main areas for new research directions and efforts to reduce the current uncertainties for better economics and improved safety of the current reactors and requirements of the new reactors designs.

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