Canadian regulatory approach towards ageing degradation and in-service surveillance at Canadian CANDU nuclear power plants

Effective in-service surveillance of key safety-related structures, systems, and components (SSC) is an essential aspect for ensuring the long-term safety and reliability of nuclear power plants (NPP). This paper presents the Canadian Nuclear Safety Commission's (CNSC) approach towards ensuring that licensees operate and maintain their NPPs in a safe condition. It describes the processes and requirements in place that ensure prompt notification is given to the regulator following the discovery of previously unconsidered ageing phenomena through in-service and periodic inspections or through in-service failures. The paper goes on to describe the regulatory response towards these events, which involves requiring the licensee to investigate the cause of the failure, re-assess the safety of the facility, adjust the controls on plant operation and surveillance, and implement measures to monitor the condition of the SSC. The paper also briefly discusses the known degradation mechanisms of key CANDU SSCs including dimensional and material changes of CANDU fuel channels, delayed hydride cracking of CANDU zirconium alloy pressure tubes, wall thinning and cracking of carbon steel feeder piping, degradation of CANDU steam generators, and degradation of CANDU containment systems and structures, and describes the requirements in place to ensure licensees sufficiently monitor the condition of these SSCs and appropriately disposition the results of these inspections. Finally, the paper describes the current and planned initiatives to improve the Canadian regulatory requirements and oversight for the surveillance of critical NPP SSCs and discusses the need to increase these efforts in order to account for the increasing effects of degradation mechanisms as Canada's power reactors approach their end of design life