Design Extension Conditions: Concept and Its Application to Operating Reactor in Canada

Abstract

The Canadian Nuclear Safety Commission (CNSC) formally introduced the term Design Extension Conditions (DEC) with the publication of regulatory document REGDOC-2.5.2, “Design of Reactor Facilities: Nuclear Power Plants.” The primary drivers for this development were the desire to maintain alignment with the equivalent International Atomic Energy Agency (IAEA) safety standard and to reflect lessons learned from the Fukushima Daiichi accident. This REGDOC establishes design requirements and guidance for new Nuclear Power Plants (NPPs), including those pertaining to DEC. Other regulatory documents in the Canadian regulatory framework provide requirements for safety analysis and accident management as well as other aspects relevant to DEC. The concept of DEC is reflective of international best practices and allows further strengthening of “Defence in Depth”; however, it requires further elaboration, particularly with respect to currently operating reactors. The currently available guidance specific to DEC is not comprehensive, in particular, regarding the interface with the plant design basis, its role in the Defence-in-Depth, selection of requirements, impact on operating limits and conditions. Nevertheless, the practices begin to emerge, given that the topic of DEC is being advanced rapidly both nationally and internationally, in particular in the framework of the IAEA. The CNSC and Canadian stakeholders are actively discussing how the current requirements and guidance are to be applied, and the emerging consensus will be captured in a new Canadian standard. This paper will provide an overview of the CNSC’s recent deliberations on the subject and an outline of the challenges that we still have to address. With this in mind, this paper does not aim to provide a final established position, but rather to stimulate international discussion on the subject of DEC, in particular its application to the older nuclear facilities. The paper will provide the definition of DEC as currently used in Canada, describe interfaces with the other fundamental safety concepts such as Defence-in-Depth, explain the approach for identification of DECs and the underlying principles associated with design, analysis, operational, and procedural requirements.