In support of the Euratom research and training program Advanced Structural Integrity Assessment Tools for Safe Long Term Operation (ATLAS+), this paper presents a comparative assessment of both conservative and best-estimate leakage-rate models carried out using the recently-developed Leak Analysis of Piping – OCI (LOCI™) code. Studies are reported comparing the LOCI™ results with the XPIPE™ code developed by Materialprüfungsanstalt (MPA) Universität Stuttgart and a new leakage-rate code developed by Jacobs. All participants in this study are members of the ATLAS+ consortium.

In the context of a leak-before-break (LBB) assessment where emphasis is placed on the detection of small leakage rates, the distinction between conservative and best-estimate models can be very important. This classification can provide guidance on where these models can be appropriately applied in terms of understanding margins and uncertainties. Country-specific regulatory requirements for an LBB assessment can also play an important role in model selection. A conservative leakage-rate model is required to consistently underestimate the actual leakage rate and is typically applied in deterministic procedures. A best-estimate leakage-rate model places greater emphasis on predictive accuracy. In a probabilistic framework, the model’s estimated solutions can serve as a measure of central tendency (mean, median, or mode) for a prescribed statistical distribution characterizing the uncertainties in leakage-rate predictions.

In a more general probabilistic analysis, model inputs and their associated statistical distributions are sometimes proposed which have conservative values for parameters to ensure that some extreme cases are not underrepresented. However, conservatism can lead to overestimates of the probability of leakage or rupture and, thereby, invalidate the model. Moreover, conservatism at a subsystem level could potentially become non-conservative when implemented at the system or framework level.

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