Uncertainty analysis of sub-channel code calculated ONB wall superheat in rod bundle experiments using the GRS methodology

Rod bundle experiments were performed for prototypical PWR operating conditions in the project "New Experimental Studies of Thermal-Hydraulics of Rod Bundles (NESTOR)". The intent of the project was to improve the understanding of the Axial Offset Anomaly (AOA) through improved modeling of Onset of Nucleate Boiling (ONB) (EPRI, 2008) using sub-channel codes. Skewing of the axial power profile (AOA) is most likely driven by the deposition of boron in the crud layer on nuclear fuel rods, which is caused by boiling on the fuel rod surface (EPRI, 2008). VIPRE-I (Srikantiah, 1992), a sub-channel code, was chosen for the analysis of NESTOR tests and for which uncertainty analysis was performed. NESTOR experimental results were used to optimize grid-loss coefficients, friction-loss coefficients, and a single-phase heat transfer model in the code. By modeling NESTOR ONB tests, the VIPRE-I calculated wall superheat was determined at the experimental ONB locations. This calculated ONB wall superheat could be used as a criterion in VIPRE-I for the prediction of ONB; however, it is important to quantify the uncertainty of this calculated ONB wall superheat in order to know the accuracy of such a criterion. The VIPRE-I model optimization process, however, was a complicated one and involved interaction of both experimental and code modeling uncertainties. The propagation of these uncertainties was treated using the Gesellschaft für Anlagen und Reaktorsicherheit (GRS) methodology; a process which is detailed in this paper.