Design Extension Conditions Concept and its Application to Operating Reactors in Canada

Abstract

CNSC formallyintroduced the term Design Extension Conditions (DEC) with the issue ofregulatory document REGDOC-2.5.2, “Design of Reactor Facilities: Nuclear PowerPlantsâ€. The primary drivers for this development were the desire to maintainalignment with the equivalent International Atomic Energy Agency (IAEA) safetystandard and to reflect lessons learned from the Fukushima Daiichi accident.The Canadianregulatory document for NPP design establishes high level design requirementsand expectations for new Nuclear Power Plants (NPPs), including those pertainingto DEC. Other regulatory documents provide requirements for safety analysis andaccident management as well as other aspect relevant to DEC. In the short timesince this concept of DEC was made part of the regulatory framework in Canada,it has become apparent that it is reflective of the international bestpractices and will allow further strengthening of defence in depth but alsorequires further elaboration, in particular with respect to application tocurrently operating reactors.The currentlyavailable guidance specific to DEC is not comprehensive, in particular,regarding the interface with the plant design basis, its role in theDefence-in-Depth, selection of requirements, impact on operating limits andconditions. Nevertheless, the practices begin to emerge, given that the topicof DEC is being advanced rapidly both nationally and internationally, inparticular in the framework of IAEA. CNSC and Canadian stakeholders areactively discussing how the high level requirements and expectations are to beapplied, and the emerging consensus will be captured in a new Canadianstandard. This paper will provide an overview of recent deliberations by CNSCstaff on the subject and an outline of the challenges that we still have toaddress. With this in mind, this paper does not aim to provide a finalestablished position, but rather to stimulate international discussion on thesubject of DEC, in particular its application to the older nuclear facilities.The paper willprovide the definition of DEC as currently used in Canada, describe interfaceswith the other fundamental safety concepts such as Defence-in-Depth, explainthe approach for identification of DECs and the underlying principlesassociated with design, analysis, operational and procedural requirements.