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Abstract Turbulent mixing of coolant streams can result in an oscillatory mixing phenomenon called thermal striping. These fluctuations have the potential to lead to anticipated thermal fatigue failures in advanced nuclear reactors. To predict thermal striping, robust and computationally affordable modeling tools that are capable of accurately representing complex turbulence are needed. Hybrid turbulence approaches, such as detached-eddy simulation and scale-adaptive simulation, have shown some success in resolving complex unsteady turbulence for massively separated flows, however the applicability of these models to internal flows is limited.A STRUCTure-based (STRUCT) second-generation Unsteady Reynolds-Averaged Navier–Stokes turbulence model was recently proposed at the Massachusetts Institute of Technology to robustly extend the applicability of hybrid closures. In this work, the STRUCT model is evaluated using experimental data taken at the Reactor Cavity Cooling System separate-effects test facility at the University of Michigan. The experiments observed the interaction of parallel symmetric rectangular jets, and include measurements for mean profiles of velocity and Reynolds stresses. In the present work, the simulation results are assessed against mean profiles of velocity and Reynolds stresses, demonstrating the ability to reproduce the unsteadiness of the jets in close agreement with the measurements at considerably reduced computational cost. [ABSTRACT FROM AUTHOR]

Drag force acting on swimming marine mammals is difficult to measure directly. Researchers often use simple modeling and kinematic measurements from animals, or computational fluid dynamics (CFD) simulations to estimate drag. However, studies that compare these methods are lacking. Here, computational simulation and physical experiments were used to estimate drag forces on gliding bottlenose dolphins (Tursiops truncatus). To facilitate comparison, variable drag loading (no‐tag, tag, tag + 4, tag + 8) was used to increase force in both simulations and experiments. During the experiments, two dolphins were trained to perform controlled glides with variable loading. CFD simulations of dolphin/tag geometry in steady flow (1–6 m/s) were used to model drag forces. We expect both techniques will capture relative changes created by experimental conditions, but absolute forces predicted by the methods will differ. CFD estimates were within a calculated 90% confidence interval of the experimental results for all but the tag condition. Relative drag increase predicted by the simulation vs. experiment, respectively, differed by between 21% and 31%: tag, 4% vs. 33%; tag + 4, 47% vs. 68%; and tag + 8, 108% vs. 77%. The results from this work provide a direct comparison of computational and experimental estimates of drag, and provide a framework to quantify uncertainty. [ABSTRACT FROM AUTHOR]

In the present paper we report about the first attempt to demonstrate and assess the ability of state-of-the-art high-fidelity computational tools to reproduce the complex patterns of CRUD deposits found on the surface of operating Pressurized Water Reactors (PWRs) fuel rods. A fuel assembly of the Seabrook Unit 1 PWR was selected as the test problem. During Seabrook Cycle 5, CRUD induced power shift (CIPS) and CRUD induced localized corrosion (CILC) failures were observed. Measurements of the clad oxide thickness on both failed and non-failed rods are available, together with visual observations and the results from CRUD scrapes of peripheral rods. Blind simulations were performed using the Computational Fluid Dynamics (CFD) code STAR-CCM+ coupled to an advanced chemistry code, MAMBA, developed at Los Alamos National Laboratory. The blind simulations were then compared to plant data, which were released after completion of the simulations. [ABSTRACT FROM AUTHOR]

This work presents a sensitivity study of boiling and two phase flow models for thermal hydraulics simulations in nuclear reactors. This study quantifies sources of uncertainty and error in these simulations by computing global sensitivities of figures of merit, or output, to model parameters, inputs, and mesh resolution. Results are obtained for the DEBORA benchmark problem of boiling in a channel driven by a heated wall section. Scalar outputs of interest consist of axial pressure drop, average wall temperature in the heated section, average void fraction at the end of the heated section, and the centroid of the radial distribution of the void fraction at the end of the heated test section. Sensitivities to both individual heat fluxes and to the parameters in the models for these heat fluxes are computed. [ABSTRACT FROM AUTHOR]

In order to assess the accuracy and validity of subchannel, system, and computational fluid dynamics codes, the Paul Scherrer Institut has participated in the OECD/NRC PSBT benchmark with the thermal-hydraulic system code TRACE5.0 developed by US NRC, the subchannel code FLICA4 developed by CEA, and the computational fluid dynamic code STAR-CD developed by CD-adapco. The PSBT benchmark consists of a series of void distribution exercises and departure from nucleate boiling exercises. The results reveal that the prediction by the subchannel code FLICA4 agrees with the experimental data reasonably well in both steady-state and transient conditions. The analyses of single-subchannel experiments by means of the computational fluid dynamic code STAR-CD with the CD-adapco boiling model indicate that the prediction of the void fraction has no significant discrepancy from the experiments. The analyses with TRACE point out the necessity to performadditional assessment of the subcooled boiling model and bulk condensation model of TRACE. [ABSTRACT FROM AUTHOR]

Abstract: A computational fluid dynamic (CFD) model for the pressure vessel of the evolutionary pressurized reactor (EPR™) was developed and validated. The aim of this model is the simulation of transients where three-dimensional effects play a strong role, such as boron dilution and main steam line break (MSLB) scenarios. First, a full solid (CAD) model has been built, that includes all details of the reactor pressure vessel (RPV) and the internals which are important for fluid dynamic analyses. The solid model has then been used as basis for the generation of the computational mesh necessary to carry out CFD simulations. Both a hexahedral and a polyhedral mesh have been created. The CFD model has been validated against experimental results of the JULIETTE facility, a 1:5 scaled mock-up of the EPR™ reactor RPV built by AREVA and equipped with advanced instrumentation. The performances of the hexahedral and the polyhedral meshes are investigated in relation to the agreement with experimental data, convergence and CPU requirements. In addition, the effect of the cold-leg swirls on the velocity field inside the RPV is investigated. These swirls mimic the effects of the main coolant recirculation pumps on the flow field at the entrance of the RPV. It is shown that the CFD model is able to capture the shift of the maximum velocity in the downcomer annulus observed in the experimental results. Good qualitative as well as quantitative agreement with the experimental data is achieved. [Copyright &y& Elsevier]

In the present work, two large eddy simulations (LESs) of single isothermal jets discharging into large enclosure facilities are proposed. The geometries and tested flow conditions correspond to scaled experiments of the upper plenum of high-temperature gas-cooled reactors. More specifically, two reference experiments were conducted at Texas A&M University and Michigan University. The objective of the present work is to validate these simulations with their corresponding reference experiments. The proposed LES models are performed with NekRS, a spectral element code with graphics processing unit capabilities developed at Argonne National Laboratory. These simulations were performed on the Summit supercomputer at Oak Ridge National Laboratory. For validation purposes, first- and second-order statistics from the computational fluid dynamics (CFD) calculation are compared with measurements obtained from the experiments. The models proved to be accurate, as these results are in good agreement. Additionally, flow visualization is provided showing that these models are able to retrieve similar effects to what are described in the literature for this type of flow configuration. Finally, the proposed models are part of a broader effort under the current Integrated Research Project of Nuclear Energy Advanced Modeling and Simulation 1.1, whose main objective is to deliver fast-running models to accurately predict complex physical phenomena, including for instance, turbulent mixing and thermal stratification. In this regard, the CFD models proposed here will be used to generate a high-fidelity data set to be applied in conjunction with data-driven methods to improve turbulence modeling closures. [ABSTRACT FROM AUTHOR]

Abstract Hybrid Reynolds-Averaged Navier-Stokes–Large Eddy Simulation was used to reveal detailed flow information and timescales in an isothermal reactor cavity cooling system plenum four-jet configuration. Plenum asymmetry and nonuniformity work together to cause premature jet merging. Bulk stirring in the plenum causes lateral jet vortex shedding, strong jet-jet interactions, swirl, and premature confluence. Two dominant transient modes exist: a jet flow timescale and then a plenum circulation timescale that is nearly three orders of magnitude larger. A primary consequence is that frequencies far less than the presumed 10 Hz threshold for thermal striping are pervasive. A second result is that scale-resolved computational fluid dynamics (CFD) models (as well as experimental rigs) need hundreds of seconds of statistically stationary flow time (tens of thousands of jet timescales) to produce stationary time averages. Fluid typically arrives at positions on the laser sheet in less time than it spends at those positions fluctuating in the streamwise and lateral directions. Also, a previously undocumented, but experimentally confirmed, vortex trap was identified via CFD. Finally, two-point velocity correlation analyses demonstrated a few dozen strong correlations across positions on the laser sheet. Expected close-proximity correlations emerged, but others across larger spaces also were connected. Most of these correlated at timescales close to that of the jet. [ABSTRACT FROM AUTHOR]

An integrated effort by the Versatile Test Reactor (VTR) Gas-Cooled Fast Reactor (GFR) Team to develop an experiment vehicle or extended-length test assembly for the VTR experiments is led by the Idaho National Laboratory and supported by an industrial partner, General Atomics, and university partners, including Texas A&M University, University of Michigan, Oregon State University, University of Houston, and University of Idaho. The overall focus of the effort is to design a helium gas-cooled cartridge loop (GCL) to assist with the testing of fuels, materials, and instrumentation to further support development of advanced reactor systems. This study is divided into two parts. Part I provides the GCL functional requirements and critical irradiation data needs for advancing GFR technologies. Part II includes the measurement techniques developed to measure the thermophysical properties of the different materials in the GCL, as well as the functionality and efficacy of these instrumentation and control systems within the GCL. This paper, Part I, describes the overall preliminary conceptual design of the VTR helium cartridge loop, the design of a fission product venting system, the thermal-hydraulic effects of flow direction, and gamma-heating generation in the cartridge. This paper also describes a three-dimensional computational fluid dynamics study that was carried out to examine the effects of the helium flow direction in the GCL on its thermal-hydraulic characteristics, engineering feasibility, and in-VTR experiment design. Both steady-state operation and a transient scenario (pressurized loss of forced circulation) were analyzed for the upward and downward helium flow options in the test article section in the GCL to provide quantitative data for selection of the helium flow direction. Additional analyses and development, as well as integrated out-of-pile testing, are planned to demonstrate and verify the performance of the GCL prior to insertion into the VTR. [ABSTRACT FROM AUTHOR]

• A high-resolution gamma tomography system (HRGTS) with an Ir-192 source is described. • Simulations and experiments thermal-hydraulic research facilty mock-ups. • Subchannel void fraction accuracy was found to be 1 The High-Resolution Gamma Tomography System (HRGTS) facility is under development at the University of Michigan. This system is planned generally for application to two-phase flow loops representing nuclear fuel bundles. These measurements should provide insight into fluid dynamics phenomena and high resolution data for validation of computational fluid dynamics (CFD) codes. The HRGTS is planned to be deployed in the Michigan Advanced Rod Bundle fLow Experiment (MARBLE). an adiabatic test loop consisting of an 8x8 square lattice arrangement. Simulations modeling MARBLE were performed to study how the counting statistics of the imaging process propagate into void fraction uncertainty. The simulation results showed that an accuracy of around 1% is expected with imaging times of 1 min. Computed tomography measurements were performed on partial mockups of the bundle geometry, which confirmed that the technique can be used to obtain accurate subchannel void fraction data in complicated and challenging measurement scenarios. [ABSTRACT FROM AUTHOR]

A hybrid multiscale coupling methodology based on a domain overlapping approach has been developed for coupling System Thermal Hydraulics (STH) and Computational Fluid Dynamics (CFD) codes. The method has been implemented between the modern STH code SAM and the CFD code NekRS, using the coupling tool Cardinal. The coupling aims to extend the STH code's applicability to scenarios where local momentum and energy transfers are important yet difficult for STH codes to capture, such as three-dimensional mixing. Two coupling strategies are implemented and compared: a hybrid domain overlapping method and the conventional domain decomposition method. The strategies are applied to two closed-loop applications, and the present method shows superior stability behavior when compared to the domain decomposition method. Then, the present coupling method is validated against experimental data from a double T-junction experiment. The present STH/CFD coupling shows improved agreement with experimental data when compared to STH standalone simulations. • A hybrid domain overlapping coupling method is presented. • Numerical stability is compared against a domain decomposition method using two loop configurations. • Proposed method found to exhibit more favorable stability behavior. • Proposed method is validated against experimental data considering three-dimensional scalar mixing. [ABSTRACT FROM AUTHOR]