Milorad B. Dzodzo

CFD Analysis of PWR Core Top Region: Top Fuel Assembly and Top Nozzle Regions

This paper aims to study the pressure distribution and flow patterns in the top fuel region of the AP1000™ reactor using CFD. This study is being performed as part of a CFD evaluation of the flow in the top fuel and upper plenum regions of a PWR reactor vessel. The flow patterns, including cross flows in the top fuel region, are inter-related with the flow distribution and pressure forces in the reactor vessel upper plenum region. Before detailed computations of the flow in the whole top fuel and upper plenum region are performed, conducting local computations for segments of the domain can provide information about physical aspects of the flow as well as mesh sensitivities. The domain of interest in this paper is the top fuel region including the upper part of the fuel assembly (top grid, fuel rods, top nozzle), upper core plate, and core component hold-down device. The commercial CFD computer code STAR-CCM+ is employed to generate the computational mesh, to solve the Reynolds-averaged Navier-Stokes equations for incompressible flow with a Realizable k-epsilon turbulence model, and to post-process the results. The complicated geometry of the top fuel region needs to be simplified so that the mesh size for the CFD model of the whole upper plenum and top fuel region does not exceed current software and hardware capabilities. In this study, several different trimmed meshes have been generated to study the effects of the geometries of the hold-down device and the lateral flows. Mesh sensitivity studies have been conducted for each individual part, i.e., the top grid, top nozzle, upper core plate, and hold-down device, in order to determine the proper geometrical simplifications. Pressure drop measurement data are compared with the predicted CFD results and act as a guideline for the mesh selection. These studies support the applicability of the geometrically simplified models and chosen mesh size for the CFD model of the full upper plenum and top fuel regions.Copyright

Application of CFD Model for Inlet Flow Region of 17×17 Fuel Assembly

A numerical investigation was performed to study the variation in axial and lateral velocity profiles occurring downstream of the inlet nozzle of a typical Westinghouse 17×17 PWR fuel assembly. A Computational Fluid Dynamic (CFD) model was developed with commercial CFD software. The model comprised the lower region of the fuel assembly, including: the Debris Filter Bottom Nozzle (DFBN), P-grid, Bottom Inconel grid, one and half grid span, as well as the lower core plate hole. The purpose of the study was to obtain insight into the flow redistribution resulting from the interaction of the jet arising from the lower core plate hole and the fuel assembly structure. In particular the axial and lateral velocities before and after the nozzle were studied. The results, axial and lateral velocity contours, streamlines and maximum axial and lateral velocity distributions at various elevations are presented and discussed in relation to the potential risk of high turbulent excitation over the rod and the resulting rod-to-grid fretting-wear damage. The CFD model results indicated that the large jet flows from the lower core plate are effectively dissipated by DFBN nozzle and the grids components of the fuel assembly. The breakup of the large jets in the DFBN and the lower grids helps to reduce the steep velocity gradients and thus the rod vibration and fretting-wear risk in the lower part of the fuel assembly. The presented CFD model is one step towards developing advanced tools that can be used to confirm and evaluate the effect of complex PWR structures on flow distribution. In the future the presented model could be integrated in a larger CFD model involving several fuel assemblies for evaluating the lateral velocities generated due to the non-uniform inlet conditions into the various fuel assemblies.Copyright

Two and Three-Dimensional Simulations of Enhanced Heat Transfer in Nuclear Fuel Rod Bundles

The effects of artificial roughness for the purpose of thermal performance improvement in pressurized water nuclear reactors are investigated. The artificial roughness consists of two-dimensional ribs parallel to the turbulent flow. The fuel rod bundle subchannel is preliminarily modeled as an annulus using the finite element method in ANSYS/FLOTRAN. The Navier-Stokes equations are solved from the SST (Shear Stress Transport) turbulence model for the simulated annulus thermal-flow. The analyses are performed for ribs dimensions and pitch provided by published previous work. It is found that, heat transfer and differential pressure have similar behavior with highest heat transfer occurring at the reattachment point. The finite element model describes well the characteristics of turbulent flow in smooth and rough rod when compared to previous semi-empirical models. Next paper extends the analysis by comparing numerical results with experimental test data and sensitivity analyses for different roughness configurations.Copyright

Application of CFD for Modeling Flows in Feed-Water Pipelines

A numerical investigation was performed to study the flows in several PWR power plant feed-water pipeline configurations. Computational Fluid Dynamic (CFD) models were developed with commercial CFD software. The models comprised sections of the pipeline networks including geometrical configurations like; elbows, expansions and contractions, valves, T and Y branches. The purpose of the study was to obtain insight into the flow redistribution, flow patterns and axial velocity profiles resulting from the interaction between various pipeline branches and position of the geometrical configurations. The results, streamlines, axial velocity contours, lateral velocity vectors and velocity profiles are presented and discussed in relation to the shape of the pipeline network. The CFD models results indicate that in some cases axial velocity profiles and flow patterns can differ from the developed turbulent flows in the straight pipes. The presented CFD models are one step towards developing advanced tools that can be used to evaluate the effects of the pipeline geometry on the flow structure and axial velocity distribution.Copyright

Natural Convection in Cubic and Rhomb-Shaped Enclosures

Laminar natural convection in cubic and rhomb–shaped enclosures (rhomb angles 59°, 44° and 28.2°) with two opposite vertical walls kept at different temperatures was investigated experimentally and numerically. The enclosures were filled with glycerol and the Rayleigh (Ra) and Prandtl (Pr) numbers ranged from 2,000<Ra<369,000 and 2,680<Pr<7,000. The visualization of the velocity and temperature fields was obtained by using Plexiglass and liquid crystal particles as tracers. The finite volume method based on the finite difference approach was applied for numerical analysis. The velocity and temperature fields and average Nusselt numbers were found as a function of the Reyleigh and Prandtl numbers. Comparison of the average Nusselt numbers for cubic and rhomb-shaped enclosures indicates decrease of heat transfer for the cases when the lower and upper vertical walls of the rhomb-shaped enclosures are at lower and higher temperatures, respectively. This is due to the tendency of fluid stratification in the lower and upper corners.Copyright

CFD Analysis of PWR Reactor Vessel Upper Plenum Sections: Flow Simulation in Control Rods Guide Tubes

The AP1000™ PWR reactor vessel upper plenum contains numerous control rod guide tubes and support columns. Below the upper plenum are the upper core plate and the top core region of the fuel assemblies. Before detailed CFD simulations of the flow in the entire upper plenum and top core regions are performed, conducting local simulations for smaller sections of the domain can provide crucial and detailed physical aspects of the flow. These sub-domain models can also be used to perform mesh sensitivities and to assess what geometrical details may be eliminated from the larger model in order to reduce mesh size and computational requirements. The study discussed in this paper focuses on the sections of the domain related to the control rod guide tubes. The commercial CFD code STAR-CCM+ is employed to generate the computational mesh, to solve the Reynolds-averaged Navier-Stokes equations for incompressible flow with a Realizable k-epsilon turbulence model, and to post-process the results. Two sub-domains are modeled and analyzed: (1) a 1/4 section of one control rod guide tube by itself and (2) a representative unit cell containing two sections of adjacent control rod guide tubes and one 1/4 section of a neighboring support column. For the 1/4 guide tube model (sub-domain 1), trimmed meshes of up to 16 million cells are generated to compute the flow and pressure fields in both complete and simplified (without chamfers and narrow gaps) models. Comparisons of the results lead to the conclusion that the simplified geometry model might be used when developing larger domain models in the future. The representative unit cell (sub-domain 2) is assumed to be positioned in the center of the upper plenum where the global lateral flow effects are minimal. At this position, the lateral flows are generated mainly by the flow as it exits the guide tubes. After flow enters the unit cell from the bottom, there are three potential locations for flow to leave the unit cell: (1) lower locations near the support column and the upper core plate, (2) side windows in the lower portion of the guide tubes, and (3) upper locations near the guide plates positioned inside the guide tubes. Both trimmed and polyhedral meshes are generated as part of the mesh sensitivity studies. Predicted flow rates at each of the outflow locations in conjunction with results from the mesh sensitivity studies provide guidance on (1) what geometry to preserve or remove, (2) what geometry can be simplified to reduce the required mesh, and (3) an estimate of the total mesh required to model the control rod guide tube and support column geometry in the much larger simulation of the entire upper plenum and top fuel domain.Copyright

Comparison of CFD Model Velocity Profiles to Test Data for Inlet Nozzle Region of Nuclear PWR Fuel Assemblies

A numerical investigation was performed to study the variation in axial velocity profiles occurring downstream of the inlet nozzle region of Nuclear PWR fuel assemblies. Computational Fluid Dynamic (CFD) models were prepared for the inlet nozzle region of a section of fuel assembly, simulating the lower support plate located under the fuel assembly, the inlet nozzle of the fuel and the downstream fuel region. Two different nozzle designs were modeled to study how each nozzle impacts the dissipation of the jet velocity profiles occurring downstream of the nozzle. The two different nozzle designs included a standard round chamfered hole flow plate and a chamfered slotted flow plate. The evaluation of the axial velocity profiles occurring downstream of the nozzle flow plate is critical in understanding the fuel rod vibration and rod fretting in the first grid span. Excessive rod vibration in this region can occur due to high axial jet velocities and steep axial velocity gradients generated from the holes in the lower support plate. The excessive rod vibration can lead to fuel rod wear and fuel failure. Axial velocity profiles were predicted for the different nozzle designs using the CFX code. These velocity profiles were compared to air test velocity measurements for the same nozzle designs. Velocity measurements were made in a 3.763/1 over-scale air test section simulating a 6×6 rod array of the inlet nozzle region and downstream fuel region. Reasonable agreement was observed between the velocity measurements and CFD model predictions. The results also indicate that nozzle flow plate geometry can have a significant affect on the dissipation of the jet axial velocity profiles and the steepness of the axial velocity gradients downstream on the inlet nozzle. The application of CFD tools can be used to optimize the inlet nozzle geometry to better dissipate jets and reduce axial velocity gradients downstream of the nozzle at a minimal increase in pressure drop. This will help reduce fuel rod vibration and rod fretting.Copyright