Licensing Framework of CANDU Reactors in Canada, and Alternatives for Crediting EME in Safety Analysis

Abstract

This paper summarizes the existing framework of safety analysis and licensing of CANDU reactors in Canada, established by the Canadian Nuclear Safety Commission (CNSC), and explores enhancements to better capture and implement lessons learned from the Fukushima accident, in conducting nuclear safety analysis for the existing and new reactors. This paper compares the Canadian regulatory requirements with other international nuclear regulatory guidelines, in terms of dealing with Beyond Design Basis Accidents (BDBA) and Severe Accidents, and also reviews the past and present event classification principles in safety analysis, set by the CNSC. The CNSC have published regulatory documents and technical papers outlining the challenges to be addressed with respect to implementations of the Design Extension Conditions (DEC). The DEC concept has interfaces with the other safety concepts such as Defence-in-Depth in Deterministic Safety Analysis (DSA) of the Design Basis Accidents (DBA), and Best-Estimation in Probabilistic Safety Assessment (PSA) of Beyond Design Basis Accidents (BDBA). This paper explores the current requirements and the principles of the regulatory framework in Canada, and compare it with alternative practical solutions, such as using portable/mobile Emergency Mitigating Equipment (EME) primarily for mitigation of Severe Accidents consequences, versus preventing them. Systematic examination of CANDU PSA results shows, crediting mobile EME in Level 1 PSA accident sequences have challenges. Although, adding another layer of redundancy to backup support systems (electrical power, cooling water and air) further reduces the frequency of Severe Core Damage (SCD), portable EME can only have a short-term mission time, and the main benefit in using crediting them in Level 1 PSA, is to delay the consequences. This paper proposes a thorough review of PSA before allocating EME to AIM/EOP or SAMG, with the objective of maintaining a balance between prevention and mitigation of severe core damage and large release. Maintaining this balance is paramount and instrumental to public safety. Instead of focusing on reducing the “probability” of accidents by crediting every available EME in Level 1 PSA, an alternative would present a better balance between prevention and mitigation of accidents.