Recent Developments in Ensuring Seismic Safety of NPPs Post-Fukushima: A Canadian Perspective

Abstract of the technical paper/presentation presented at:
Structural Mechanics in Reactor Technology (SMiRT) 25 Conference
Aug 5-9, 2019

Prepared by:
Tarek Aziz¹ and George Stoyanov²
1) Independent consultant, TS Aziz Consulting Inc., Mississauga, Ontario, Canada
2) Technical Specialist, Canadian Nuclear Safety Commission

Abstract

This paper focuses on the developments in ensuring seismic safety of Canadian nuclear power plants in the aftermath of the Fukushima Daiichi nuclear accident. These include immediate actions in the aftermath of the accident, but also the implementation of enhancements in seismic requirements.

Nuclear power plants (NPPs) built in Canada and Canadian-designed NPPs overseas are all CANDU design. These unique plants were designed and built to stringent safety and seismic design requirements. These requirements, which govern the seismic design, as well as beyond-design-basis considerations, have evolved over the years, consistent with state-of-the-art practices in the earthquake engineering field. Seismic qualification requirements for NPPs in Canada are provided by a set of five CSA standards. Two specific CSA standards that provide detailed requirements for seismic design and evaluation in Canada are CSA N289.1 “General Requirements for Seismic Design and Seismic Qualification of Nuclear Power Plants”, and CSA N289.3 “Design Procedures for Seismic Qualification of Nuclear Power Plants”, the latest edition of which is to be published in 2019. This paper includes a summary of the unique features of these two CSA documents (which are in use by the nuclear industry) and the associated seismic safeguard requirements with a focus on developments after 2011.

In response to the Fukushima Daiichi nuclear accident, the Canadian Nuclear Safety Commission issued an integrated action plan in 2013 in which two action items involving re-evaluation of the seismic hazard and its impact on Canadian NPPs were specified. In parallel, the Canadian nuclear industry continued to further develop both the seismic probabilistic safety assessment and a seismic assessment for beyond-design-basis earthquakes, in accordance with CSA N289.1 and CSA N289.3 requirements.

The CNSC has requested all NPP licensees to complete a review of the basis for external events against modern state-of-the-art practices for evaluating natural external hazards’ magnitudes and relevant design capacity for these events, including but not limited to: earthquakes, floods, tornadoes and fire. This assessment has also considered an element of human and operational performance under accident conditions.

Improvement of safety against natural hazards, and seismic hazards in particular, in Canada is the result of a detailed action plan by the CNSC and its implementation by the Canadian nuclear industry. This detailed action plan and response, combined with evolving CNSC, CSA and international standards requirements, has resulted in an augmentation of defense in depth for all NPPs.

The Canadian utilities have established an approach to consider beyond-design-basis earthquakes which first defines the design-basis earthquake and then performs the evaluation required to calculate the seismic capacity for beyond the design basis. Based on this evaluation, a plant-level parameter is calculated (i.e. high-confidence-of-low-probability-of-failure). CSA standards are being updated to align with CNSC regulatory documents and to establish detailed requirements related to seismic risk. These requirements will also account for plant aging.

Conclusions

Seismic safety has been addressed by Canadian NPP utilities since the time of the original design. Developments after March 2011 have led to improvements in seismic requirements in Canadian seismic codes and strengthening of the protection against seismic hazard at NPP sites. Together with CNSC actions, this has improved the seismic safety of Canadian NPPs. Periodic review of external hazards to continuously ensure that the plant has sufficient margin above the design basis is mandated by the CNSC.

References:

1. Canadian Nuclear Safety Commission (2013), “CNSC Integrated Action Plan On the Lessons Learned From the Fukushima Daiichi Nuclear Accident”, Ottawa, Canada
2. CSA N289.1-18, “General requirements for seismic design, qualification, and evaluation of nuclear power plants” (2018), Canadian Standards Association, Mississauga, ON, Canada.
3. CSA N289.3-10,”Design procedures for seismic qualification of nuclear power plants” (R2015), Canadian Standards Association, Mississauga, ON, Canada.

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