A successful treatment of model uncertainty results in an expression of uncertainty that includes the true value at some stated level of confidence. Code structure uncertainties (model uncertainty) are a crucial source of uncertainty quantification for thermal-hydraulics (TH) system codes such as RELAP5, TRAC, and recently consolidated TRACE code. These codes are an assembly of models and correlations for simulation of physical phenomena and behavior of system component. In some cases there are alternative sub-models, or several different correlations for calculation of a single phenomenon of interest. There are also “user options” for choosing one of several models or correlations in performing a specific code computation. Dynamic characteristics of TH add more complexity to the code calculation, meaning for example, that specific code models and correlations invoked are sequence-dependent, and based certain (dynamic) conditions being satisfied. This paper discusses the techniques developed in the Integrated Methodology for Thermal-Hydraulics Uncertainty Analysis (IMTHUA), specifically for the treatment of uncertainties due to code structure and models. The methodology comprehensively covers various aspects of complex code uncertainty assessments for important accident transients. It considers the TH code structural uncertainties explicitly by treating internal sub-model uncertainties, and by propagating such model uncertainties in the code calculations (including uncertainties about input parameters.) Structural uncertainty assessment (model uncertainty) for a single model will be discussed by considering “correction factor”, “bias”, and also through sub-model output updating with available experimental evidence. In case of multiple alternative models, techniques of dynamic model switching, user controlled model selection, model mixing, and model maximization/minimization will be discussed. Examples from different applications including, Marviken test facility blowdown, LOFT LBLOCA and a typical PWR LOCA scenario calculations will be provided for greater clarification of the proposed techniques.

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