Canadian licensing framework of CANDU reactors and safety analysis challenges in the allocation of Emergency Mitigating Equipment.

This paper summarizes the existing framework of safety analysis and licensing of Canada Deuterium Uranium (CANDU) reactors in Canada, established by the Canadian Nuclear Safety Commission (CNSC) and discusses challenges with implementing the lessons learned from the Fukushima accident, in the nuclear safety analysis conducted for the existing older and new CANDU 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 has published regulatory documents and technical papers outlining the challenges to be addressed with respect to implementations of the Design Extension Conditions (DEC). This paper explores the current requirements and the principles of the regulatory framework in Canada, and discusses practical approaches to determine where in every accident sequence, crediting an Emergency Mitigating Equipment (EME)would be most beneficial, for example, using a portable EME for mitigating the consequences of a Severe Accident, or preventing them. A systematic examination of CANDU Probabilistic Safety Assessment (PSA) results shows that crediting mobile EME in Level 1 PSA accident sequences have challenges and may not be the most beneficial use of EME. While adding another layer of redundancy to backup support systems (electrical power, cooling water or instrument air) further reduces the frequency of a core damage accident scenario, portable EME can only have a short-term mission time, and the main benefit of using them in Level 1 PSA, is to delay the consequences and releases. This paper proposes a thorough review of both Level 1 and Level 2 PSA models and conducting sensitivity analysis results, before making decision on where to allocate EME (i.e., in Abnormal Incident Manuals (AIM), Emergency Operating Procedures (EOP) or Severe Accident Management Guidelines (SAMG). It is crucial to balance between prevention and mitigation of severe core damage and large release frequencies to maximize the safety of the plant and minimize the impact on public safety. Instead of focusing on reducing the probability of occurrence (or frequency) of accidents by allocating every available EME to Level 1 PSA, an alternative approach would be determining where to allocate them to maintain a balance between prevention and mitigation of accidents. • Illustrative process flow-diagrams of pre and post Fukushima CANDU safety and licensing framework in Canada. • Using Probabilistic Safety Assessment (PSA) results for identification of BDBA and Design Extension Condition (DEC). • Most likely BDBA (DEC) in CANDU design are Local and Widespread Fuel Damage, with no regulatory requirement limit in Canada. • Using PSA to find balance between core-damage prevention and mitigation for allocating Emergency Mitigating Equipment (EME). • Challenges for crediting EME in CANDU's safety analysis and focusing on core-damage prevention. [ABSTRACT FROM AUTHOR]