The TRAC/RELAP Advanced Computational Engine (TRACE) thermal-hydraulics code is currently under development by the United States Nuclear Regulatory Commission (NRC). TRACE is used for safety analyses of both conventional and advanced light water reactors. NRC assessed the prediction accuracy of the code by quantifying the axial void distribution in a rod bundle under low-pressure (0.16 to 0.44 Pa) and low-flow conditions (0.015 to 0.20 kg/s), using data obtained from the Rod Bundle Heat Transfer (RBHT) facility at Pennsylvania State University. NRC simulated 73 steady-state experiments (assessment cases) with variations in the total rod power, inlet subcooling, system pressure, and injection flow rate. Comparisons between TRACE calculations and RBHT data showed reasonable agreement. TRACE was found to over predict the bundle-exit void fraction by 13.3% with a linear goodness-of-fit (R2) of 0.87 and over predict the local void fraction by 10.1% with an R2 of 0.91. This paper discusses the models and correlations used in the TRACE calculation of mixture level swell, RBHT experimental results, modeling of the RBHT facility, and comparisons between data and code calculations.

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