Canadian Regulatory Perspective on the Application of Break Exclusion Zone

SMiRT28 Conference, Toronto, August 10-15, 2025

Authors: H. Mazhar, S. Eom, K. Kirkhope, M. Hornof

The safety and reliability of piping components, particularly those forming part of high-energy lines in a nuclear power plant, are paramount. The Canadian Nuclear Safety Commission (CNSC) regulatory framework provides that Structures, Systems and Components (SSCs) important to safety must be designed and located in a manner that minimizes the probability and effects of hazards (e.g., fires and explosions) caused by external or internal events. A High-Energy Line (HEL) break inside the Reactor Building (RB) is a typical internal hazard. CNSC REGDOC-2.5.2, Design of Reactor Facilities, requires that the design of SSCs include protection against postulated pipe ruptures, unless otherwise justified. For the Canadian operating nuclear fleet, this was achieved through the application of postulated pipe failure and pipe whip jet impingement analysis (PWJIA) as per a systematic approach that assumes the failure at every pipe weld connection. The consequence of piping failure on adjacent safety systems needed for safety functions including reactor shutdown, fuel cooling, and the containment boundary has been evaluated by means of PWJIA. Whip restraints and supports are used to restrain and limit the consequences of these postulated pipe failures. Whip restraints and supports may increase access limitations for inspection and maintenance activities within containment and the RB; however, they enhance safety by design. The Break Exclusion Zone (BEZ) concept has been applied in the United States as a method to screen out portions of high-energy piping near the containment boundary from requiring further assessments for protection against dynamic effects of postulated pipe breaks (pipe whip, jet impingement). This approach is accepted by the US Nuclear Regulatory Commission according to  BTP (Branch Technical Position) 3-4, Postulated Rupture Locations in Fluid System Piping Inside and Outside Containment. However, the application of the BEZ approach is restricted by a set of conditions in BTP 3-4. Similarly, international practices recommend that the BEZ concept be applied only under specific restrictions and conditions. This paper will discuss the Canadian perspective regarding the BEZ concept as compared with other international practices, and the conditions for considering the BEZ approach based on international best practices.

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