Regulatory Perspectives on the Condition Assessment and Monitoring of Safety Significant Major Reactor Components for Long-Term Operation
An abstract of the technical paper presented at:
IAEA Fourth International Conference on Nuclear Power Plant Life Management
October 23–27, 2017
Canadian Nuclear Safety Commission
In Canada, nuclear power plant operators are required to ensure the fitness for service of safety significant structures, systems and components (SSCs), in accordance with license condition (LC) 7.1 of the power reactor operating licence. The accompanying licence conditions handbook (LCH) provides the compliance criteria for meeting the regulatory requirement and requires periodic inspection of the safety significant SSCs following CNSC-accepted inspection standards. In addition, the LCH mandates a well-structured and effective aging management program (AM program) following the requirements set out in REGDOC-2.6.3, Fitness For Service–Aging Management, for the safety significant SSCs and component-specific plans for the major components like fuel channels, feeder pipes, steam generators, calandria vessels and reactor internal components. These component-specific aging management plans are normally called lifecycle management plans (LCMPs).
Most of the Canadian CANDU nuclear power plants are entering periods of long-term operation (LTO) beyond their initially assumed operating life. The current and predicted condition and proactive management of known or plausible aging issues are of primary regulatory concern when assessing requests for LTO of major components like fuel channels, feeder pipes, steam generators, calandria vessels and reactor internal components.
Previously, as part of the planning for potential refurbishment projects, the CNSC required nuclear power plant (NPP) operators to complete a one-time integrated safety review (ISR) in accordance with CNSC regulatory document RD-360, Life Extension of Nuclear Power Plants. The ISR is similar in scope to the periodic safety reviews (PSRs) described in the IAEA safety standards. The ISR included comprehensive and systematic component condition assessments and the development of an integrated implementation plan (IIP) which identified safety improvements to support post-refurbishment safe operation of an NPP. The main assumption behind an ISR is that during a refurbishment project, some of the major components, like fuel channels, would be replaced. The aim of the ISR was to enhance the level of safety of a refurbished NPP to a level comparable to a modern, newly built NPP when it is practical to do so. The period of time assessed under an ISR encompassed the full period of planned extended operation of an NPP, and licence applicants were required to re-apply for an operating licence at five- year intervals.
It should be noted that in 2009, an Integrated Regulatory Review Service (IRRS) mission to Canada recommended the implementation of PSRs as part of the licensing basis for Canadian NPPs. Further, the implementation of PSRs was identified as a guiding principle by the contracting parties to the 2015 Vienna Declaration on Nuclear Safety (VDNS). In April 2015, the CNSC published REGDOC-2.3.3, Periodic Safety Reviews, which supersedes RD-360 and contains requirements that are consistent with IAEA Specific Safety Guide No. SSG-25, Periodic Safety Review for Nuclear Power Plants. With the publication of REGDOC-2.3.3 and its implementation in the licensing basis for Canadian NPPs, the CNSC now requires the Canadian CANDU NPP operators to perform a detailed PSR to demonstrate continued safe operation of NPPs. Although the ISR was a one-time process applied only to NPPs undergoing refurbishment, the PSR process can be applied to NPPs at any stage of operation and must be repeated at identified intervals referred to as PSR periods in this paper. PSR periods typically encompass a 10-year licence renewal period. In the case of an NPP entering a period of LTO, the PSR should also consider the impact the full period of planned extended operation will have on the safe operation of SSCs when practical to do so.
One of the essential components of the PSR process, like the ISR, is determining the actual condition (condition assessment and monitoring) of safety significant SSCs, including confirming that the applied aging management practices will assure fitness for service (FFS) of these SSCs for the continued operation of the NPP through the entire PSR period. It is expected that the condition assessments, implemented monitoring programs and AM programs will demonstrate that safety significant SSCs are meeting and will continue to meet the design basis and design assumptions for the PSR period and throughout the intended long-term operation. If this objective cannot be met, the applicant is required to develop an IIP containing the required corrective actions and safety enhancements.
The actual condition of these major components is an important factor in any review of plant design safety and performance during the entire PSR period. Hence, it is important to understand and thoroughly document the condition of the major components to determine whether they are capable of and adequate for meeting the design basis, assumptions and requirements throughout the PSR period. Additionally, the assessment should review the ongoing maintenance, surveillance and in-service inspection program(s), as applicable, to support continued operation of the major components.
Understanding active and plausible aging-related degradation is important. Mechanisms that can initiate flaws, hydride blisters, embrittlement and other irradiation or environment-induced material property changes, adverse microstructural and metallurgical changes or changes in the ability of the material to withstand the anticipated loading conditions in the presence of flaws must be understood. Assessments should consider the effectiveness of existing plant inspection programs to identify and characterize aging-related degradation and include any significant findings from national or international R&D activities undertaken to quantify the functional capability of the major components and its materials. Once the actual condition of the major components and its material has been determined, the results should be assessed against the applicable design basis and assumptions to confirm that design basis assumptions have not been significantly challenged and will remain so throughout the PSR period.
The main challenge is to understand and quantify the actual condition of major components and their ability to withstand continuous operation. Another challenge is to evaluate time- and environment-dependent aging degradation and its effects on the safe operation of these major components.
This paper describes the regulatory expectations for the detailed evaluations necessary to determine the actual condition of safety significant major components like fuel channels, feeder pipes, steam generators, calandria vessels and reactor internal components. The paper also presents the regulatory review practices adopted by the CNSC in reviewing the condition assessments, including monitoring programs and the LCMPs. Additionally, some lessons learned from the recent Canadian experience are presented and discussed.
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