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, an application in the Structural Integrity Assessments 
Modular – OCI (SIAM-OCI™) framework. Oakridge Consulting International (OCI) performed the 
comparative assessment via its role as a supporting international member of the ATLAS+ consortium. 
Benchmarking studies were also carried out comparing the LOCI™ results with the XPIPE™ code 
developed by Materialprüfungsanstalt Universität Stuttgart (MPA) and a new leakage-rate code 
developed by Jacobs Engineering, Cheshire, UK.In the context of an LBB assessment where emphasis is placed on the detection of small leakage rates, the 
distinction between conservative and best-estimate models could 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, e.g., the 
deterministic approach in the German standard KTA 3206. 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 the 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 under-represented. However, conservatism can lead to over-estimates of the probability of leakage or 
rupture and, thereby, invalidate the model. Moreover, conservatism at a subsystem (i.e., model) level can 
potentially become non-conservative when implemented at the system or framework level.

Comparative Assessments of Different Leakage-Rate Models for ATLAS+

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

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