Optimal sample size for probability of detection curves

The use of probability of detection curves to quantify the reliability of non-destructive examination(NDE) systems is common in the aeronautical industry, but relatively less so in the nuclear industry, atleast in European countries. Due to the nature of the components being inspected, sample sizes tendto be much lower. This makes the manufacturing of test pieces with representative flaws, in sufficientnumbers, so to draw statistical conclusions on the reliability of the NDT system under investigation,quite costly. The European Network for Inspection and Qualification (ENIQ) has developed an inspectionqualification methodology, referred to as the ENIQ Methodology. It has become widely used in manyEuropean countries and provides assurance on the reliability of NDE systems, but only qualitatively.The need to quantify the output of inspection qualification has become more important as structuralreliability modelling and quantitative risk-informed in-service inspection methodologies become morewidely used. A measure of the NDE reliability is necessary to quantify risk reduction after inspectionand probability of detection (POD) curves provide such a metric. The Joint Research Centre, Petten, The Netherlands supported ENIQ by investigating the question of the sample size required to determine areliable POD curve. As mentioned earlier manufacturing of test pieces with defects that are typically foundin nuclear power plants (NPPs) is usually quite expensive. Thus there is a tendency to reduce sample sizes,which in turn increases the uncertainty associated with the resulting POD curve. The main question inconjunction with POS curves is the appropriate sample size. Not much guidance on appropriate samplesizes can be found in the published literature. Often only qualitative advice is given. The aims of the work summarized in this paper were to develop numerical simulations to determineappropriate and effective inspection target sizes, their number, and distribution to produce valid PODvs. size curves, and to summarize these findings as guidelines for the NDE practitioner in designing anexperiment to assess system inspection effectiveness.JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Managemen