Guidelines for Implementing Design Extension Conditions Due to Seismic Hazards in CANDU Nuclear Power Plants

Abstract

One of the recent developments in defining safety requirements for the design of nuclear power plants, stemming from the lessons learned and recommendations from the 2011 earthquake at Fukushima, Japan, is the inclusion of 'Design Extension Conditions' or DECs in the design. Design extension conditions represent one of the categories used to define the different plant states on the basis of their frequency of occurrence. One of the main objectives for defining a set of design extension conditions is enhancing the plant's capabilities to withstand, without unacceptable radiological consequences, accidents that are more severe than design basis accidents. The set of DECs are derived based on engineering judgement, deterministic assessments and probabilistic assessments of the plant and are considered a subset of the Beyond Design Basis Accident (BDBA) conditions.The Canadian Nuclear Safety Commission (CNSC) adopted the guidelines of International Atomic Energy Agency (IAEA) in extending the plant design envelope to include DECs. In addition, the CNSC expects operating nuclear power plants to be seismically evaluated using updated site-specific seismic hazard. From a safety analysis prospective, the updated seismic hazard at the site of a nuclear power plant exceeding its original design basis level could be considered a design extension condition that would have potential to damage SSCs important to safety.This paper provides a review of the Canadian regulatory requirements as well as a proposed design methodology to be followed in addressing the higher demands of seismic design extension conditions for existing CANDU nuclear power plants. The proposed design methodology builds on the current guidelines and requirements of applicable codes and standards for structures, systems and components (SSCs) but at the same time recommends more relaxed acceptance criteria and application of material and sectional overstrength factors, ensuring that the safety requirements for the design extension condition are met.