This report documents a literature survey of so-called "Model Assisted Probability of Detection" (MAPOD) approaches that may be useful in determining the probability of detection (POD), a metric for quantifying the performance of nondestructive evaluation (NDE) methods. The objective of this report is to summarize MAPOD concepts that have been proposed to date in order to assess specific approaches that may be appropriate for application to improve estimates of POD for field NDE of nuclear power plant components. The limitations of laboratory-based studies to replicate actual field conditions are well-recognized and not limited to the nuclear power industry. Probability of detection estimates based on laboratory studies generally provide ideal environments for performing examinations as compared field settings, which typically will result in non-conservative estimates of POD for field applications. As a consequence of this effort, the authors conclude that use of MAPOD concepts to improve estimates of field NDE performance may require access to certain field data, or alternatively, may require significant laboratory studies to assess the influence of human and environmental shaping factors. If the necessary field data cannot be made available, it is proposed that laboratory efforts will have greater chance of success by focusing on a specific examination technique and component application. The authors also find that MAPOD concepts may have a more immediate contribution to the nuclear power industry through their use in extending personnel and procedure qualifications beyond established limits. Finally, another basis was identified that may be used to adjust POD curves generated in previous reliability studies and performance demonstrations. Many of the previous studies calculated POD as the average of performance data which may be inappropriate for many applications, as the calculation may be nonconservative. A more conservative approach to estimating POD would include basing the calculation on a lower statistical quantile of the data set. v
Several pressurized water reactors have experienced primary coolant leaks as a result of degradation in the tubes and J-groove welds of reactor vessel head penetrations. Leakage has been attributed to primary water stress corrosion cracking (PWSCC) of the Alloy 600 nozzle material and Alloy 182/82 weld materials. More recently, other Alloy 600 components including reactor vessel bottom mounted instrumentation nozzles, dissimilar metal pipe welds, hot leg instrument penetrations, and pressurizer heater sleeves have exhibited degradation. Westinghouse has been actively involved in the development of a comprehensive Alloy 600 degradation management program including investigation of root cause, establishing a safety position, and developing inspection and repair/replacement strategies to address Alloy 600 degradation issues. Personnel from Germany, Sweden and the United States have cooperatively developed equipment and nondestructive examination technologies for identification and characterization of degradation that might exist in these Alloy 600 components and attachment welds. These developments represent significant enhancements to technologies and equipment previously available in the industry and are driven by the need to meet new inspection applications and industry requirements which have continued to evolve over the last several years. This paper describes the inspection capabilities available to support Alloy 600 degradation management programs, field experience with those inspection technologies, and the status of ongoing NDE development efforts to enhance future Alloy 600 inspection programs.
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