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Implementation of a Spacer Grid Rod Thermal-Hydraulic Reconstruction (ROTHCON) Capability into the Thermal-Hydraulic Subchannel Code CTF

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Nuclear Technology
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The Consortium for Advanced Simulation of Light Water Reactors (CASL) is developing a core simulator capability known as the Virtual Environment for Reactor Applications Core Simulator (VERA-CS) to
address nuclear industry challenge problems such as crud-induced power shift (CIPS). CTF is the thermalhydraulics subchannel code that provides thermal feedback in the coupled neutronics, thermal-hydraulics, crud-chemistry simulation that VERA-CS performs. It has been discovered that the coarse meshing approach used by CTF (in which fuel rods are discretized into four azimuthal segments) can be a source of error in predicting crud growth and boron distribution in VERA-CS CIPS calculations. Spacer grid effects lead to complex rod-to-fluid heat transfer behavior that, when not resolved, can lead to error in the prediction of crud growth and boron deposition. A higher-fidelity computational fluid dynamics (CFD) approach can be used instead of CTF, but this leads to excessive simulation times. This paper presents an approach for using high-fidelity CFD data to create shape functions that are used in CTF to reconstruct rod surface heat transfer behavior as a function of spacer grid geometry. The approach is demonstrated for a 5 x 5 rod bundle facility with five mixing vane grids with a range of operating conditions encountered in nominal pressurized water reactor (PWR) conditions. It is shown that the grid heat transfer maps are successful at introducing a higher fidelity heat transfer modeling capability into CTF.