Probabilistic Evaluation of the Impact of Inspection Uncertainties on the Estimation of Flaw Growth Rate

Abstract of the technical presentation presented at:
5th International Symposium on Probabilistic Methodologies for Nuclear Applications (ISPMNA)
Tokyo, Japan
October 7-9, 2024

Prepared by:
M. Pandey
Professor, UNENE Chair, University of Waterloo

B. Wasiluk and J. Riznic
Canadian Nuclear Safety Commission

Abstract:

Steam generators (SGs) fitness for service in Canada is assured through periodic and supplementary inspections, periodic metallurgical examination of surveillance tubes and satisfying applicable acceptance standards. SG tubing examination results should satisfy general acceptance standards included in Clause 14 of CSA N285.4 Periodic Inspection of CANDU Nuclear Plant Components otherwise are dispositioned with detailed methodologies and acceptance criteria from Fitness-for-Service Guidelines for Steam Generator and Preheater Tubes. In addition, required aging management program should satisfy CNSC regulatory document REGDOC-2.6.3 Aging Management and consider outlined guidelines. CNSC staff review and monitor inspection activities and dispositioning of degradation findings as part of the regulatory oversight exercised by the CNSC.

Steam generator tubing is inspected during planned maintenance outages to detect any new or previously reported flaws. Using flaw population sizing data collected during the outage, condition and operational assessments are performed in support of the fitness for service of SG tubing. The estimated flaw growth rate is an important parameter used in the operational assessment of SG tubing.

SG tubing inspections primarily use eddy current probes. The detection and sizing uncertainties associated with the inspection probe are represented by the probability of detection (POD) and sizing error curves, respectively. POD and sizing errors are greatly affected by internal deposits, variations in SG tube wall thickness, interference from support structures and other factors. Because of the sizing error, measured flaw size is not identical to actual flaw size, and this makes it challenging to estimate the flaw growth rate.

A probabilistic framework has been introduced to filter out the confounding influence of inspection uncertainties on the flaw growth rate estimate. In this study, a maximum likelihood function is derived that accounts for the effect of POD and noise in the examination signal. The presented developments are expected to outline a systematic framework for more detailed estimation of flaw growth rate distribution, explicitly accounting for involved uncertainties. The investigation findings should provide more technical insights and quantify embedded conservatism in present operational assessments of SG tubing. Ultimately, it should assist CNSC staff with technical reviews of fitness for service evaluations of SG tubing, including those operated in life extension.

To obtain a copy of the abstract’s document, please contact us at cnsc.info.ccsn@cnsc-ccsn.gc.ca or call 613-995-5894 or 1-800-668-5284 (in Canada). When contacting us, please provide the title and date of the abstract.