Rigorous assessment of system components is a cornerstone of reliable nuclear power plant operations and key to the long-term sustainability of the industry. A three-year IAEA Coordinated Research Project (CRP) examining techniques for evaluating the frequency of pipe failures in advanced water cooled reactors (WCRs) has produced an analytical framework to enhance consistency in this critical area. The project, which concluded in 2021, collated expert knowledge and developed a novel approach for analyzing pipe reliability, supporting the reliable operation of some currently deployed reactors as well as planned new build projects.
Piping integrity is critical to ensuring the safety and reliability of nuclear power plants (NPPs). Pipes play numerous vital roles in NPPs, such as transporting cooling water to both the reactor vessel and the spent fuel pool, carrying steam to the electricity-generating turbines and delivering hydrogen to cool generator components. Early efforts in the 1960s, highlighted the importance of understanding and addressing potential failures in primary coolant systems.
These findings initiated a continuous process of inspection and advancement in piping and its reliability analysis aimed at enhancing the safety standards of nuclear installations. Over the years, this analysis has led to the development of robust methodologies and practices for assessing piping reliability, laying the groundwork for ensuring the integrity of piping systems in NPPs.
Today, with the emergence of advanced WCRs, the need for comprehensive analysis of piping reliability has come into sharper focus. The unique designs, materials, and operating environments of these reactors present new challenges for assessing and mitigating potential risks associated with piping failures. Despite significant progress in methodology development, the need for more relevant operating experience data specific to advanced WCRs remains a notable challenge. This lack of data poses a significant obstacle in accurately assessing the reliability of piping systems and necessitates innovative approaches to address this gap.
The selection of piping materials and design approaches for advanced WCRs inherently prioritize higher levels of structural reliability within pressure boundaries. It is anticipated that probabilistic pipe failure rates for advanced WCRs will be notably lower, potentially by an order of magnitude. However, due to the absence of operational experience data for these systems, uncertainties in reliability assessments are expected to be considerable. Addressing this technical challenge requires meticulous documentation and validation of model inputs and outputs in new piping reliability analyses.
CRP Results
The results of this CRP are relevant for the planning and implementation of piping reliability analysis for advanced WCRs. The CRP produced a seven-step analytical framework to promote consistency in how an analysis is organized, executed and documented.
A central aspect of the analysis framework is the importance of using consistent terminology, especially with respect to (a) the definition of what constitutes a pipe failure, (b) the minimum requirements to be placed on a state-of-the-art piping reliability model, (c) differentiation between pipe failure rate, pipe failure frequency and pipe failure probability, and (d) the different structural integrity management processes and how these could be addressed analytically.
A benchmark of the different methodologies applied drew the following conclusions on the methodology selection and implementation:
- All implementations required extensive input data preparation and some form of computer code implementation to facilitate the quantification processes, including the uncertainty analyses;
- Implementations of the methods necessitated pre-planning of the computations and post-processing of results;
- Regardless of the selected methodology, successful implementation depended on the experiences that had been obtained by the analysts from previously performed practical applications; and
- The value and importance of operating experience data cannot be understated. All methods benefited from and required information on material performance and pipe failures.
Impact and relevance
In completing the CRP, participants documented their accomplishments in an IAEA Technical Document as well as a Nuclear Energy Series publication:
- “Technical Insights from Benchmarking Different Methods for Predicting Pipe Failure Rates in Water Cooled Reactors” provides valuable technical insights and recommendations for establishing piping reliability parameters, and
- “Methodologies for Assessing Pipe Failure Rates in Advanced Water Cooled Reactors” includes information on evaluating different methodologies for assessing pipe failures in advanced WCR. These publications serve as a comprehensive resource for countries, offering guidance on applying suitable methodologies for pipe failure rate analysis in advanced WCRs and ensuring the continued safety and reliability of nuclear installations worldwide.
Nine research contract and agreement holders from Canada, Germany, the Republic of Korea, Lithuania, Malaysia, the United States of America and Tunisia participated in this CRP.