Edge Subchannels Research Articles

Experimental study on the flow characteristics of 7-pin hexagonal bundle assembly with grid spacers

Lead-cooled Fast Reactors (LFRs) are of great significance for future nuclear power systems. With the advantages of great neutronic properties and high passive safety, LFR has been selected as the reactor of China initiative Accelerator Driven System (CiADS), which has the potential to transmute long-lived nuclear waste into short-lived ones and enhance the utilization of nuclear fuel. The hexagonal bundle with grid spacers has been identified as a promising design of the CiADS fuel assembly. The new design of grid spacers has the advantages of reliable fixation, small flow resistance, and not easy to cause blockage. Flow characteristics in this fuel assembly model were revealed by the particle image velocimetry (PIV) method. The results showed the presence of cross-flow and high axial velocity areas around the grid spacers. The strongest cross-flow mixing was observed downstream of the grid spacers at a distance of approximately z/Dh = 7.5 (axial height/hydraulic diameter). Although the influence of the grid spacers was weak, it was still detectable downstream at a distance of z/Dh = 32. The axial velocity loss rates were obtained and they increased with Reynolds number. The highest axial velocity loss rate was in the edge subchannel. Measurements of axial pressure drop uncovered the pressure loss characteristics of grid spacers and a new correlation was proposed. This work contributes to the understanding of the flow and pressure drop characteristics of the CiADS fuel assembly and can facilitate the design of more efficient nuclear reactors.

Applicability of RANS models and pressure drop in edge subchannels for 19-pin wire-wrapped fuel bundle channel in CiADS

The accelerator-driven subcritical system has a strong transmutation ability and high inherent safety, and it is internationally recognized as the most promising long-life nuclear waste disposal device. This study involves the construction of a Visual Hydraulic ExperimentaL Platform (VHELP) for the purpose of evaluating the applicability of Reynolds-averaged Navier–Stokes (RANS) models and analyzing the pressure distribution within the fuel bundle channel of China initiative accelerator-driven system (CiADS). Measurements of thirty differential pressures in edge subchannels within a 19-pin wire-wrapped fuel bundle channel were obtained under different conditions using deionized water. The pressure distribution in the fuel bundle channel at Reynolds numbers of 5000, 7500, 10,000, 12,500, and 15,000 was simulated using Fluent. The results show that RANS models obtained accurate results, and the shear stress transport k-ω model provided the most accurate prediction of the pressure distribution. The difference between the results of the Shear stress transport (SST) k-ω model and experimental data was the smallest, and the maximum difference was ±5.57%. Moreover, the error between the experimental data and numerical results of the axial differential pressure was smaller than that of the transverse differential pressure. The pressure periodicity in axial and transverse directions (one pitch) and a relatively three-dimensional pressure measurements were studied. The static pressure fluctuated and decreased periodically as the z-axis coordinate increased. These results can facilitate research on the cross-flow characteristics of liquid metal-cooled fast reactors.

PIV and CFD Study on Crossflow Characteristics in a 7-Pin Wire-Wrapped Bundle Channel

China initiative Accelerator Driven System (CiADS) combines a linac, spallation target and a Lead-cooled Fast Reactor (LFR) together, which is designed to transmute nuclear waste and accelerate the progress of China’s energy technology research towards the goal of carbon neutrality. A LFR uses helical wire-wrap spacers as positioning components to enhance crossflow mixing in the reactor core. To study the velocity distribution and crossflow characteristics in wire-wrapped rod bundle channels, a 2 : 1 magnified scale 7-pin bundle fuel assembly model was fabricated using polymathic methacrylate. Particle image velocimetry (PIV) and computational fluid dynamics (CFD) simulations were used to investigate the velocity distribution in the 7-pin bundle flow channels at Reynolds number of 1250~5000 in the x z plane and Reynolds number of 1500 and 2500 in the x y plane. The deviation between CFD simulation results and PIV experimental data was small, and the Reynolds Average Navier-Stokes model could accurately simulate the flow characteristics of the wire-wrapped fuel rod bundle channels. The maximum crossflow velocity caused by helical wires was about 40% of the axial bulk velocity. The normalized crossflow velocity at the subchannel interface varied approximately sinusoidally with the axial height. As the Reynolds number increased, the velocity distribution trend and the loss rate of axial velocity in flow channels remained essentially constant while the peak value of crossflow velocity increased. The contour images of velocities with different axial heights were obtained from the x y plane, and their velocity distribution had a certain periodicity. The axial velocity loss rate in each subchannel caused by wire-wrap spacer resistance was between 7.35% and 38.51%, and the axial velocity loss rates in inner subchannels were usually higher than those in edge subchannels.

Experimental Investigation of the Subchannel Axial Pressure Drop and Hydraulic Characteristics of a 61-Pin Wire Wrapped Rod Bundle

Abstract Wire-wrapped hexagonal fuel bundles have been extensively investigated due to their enhanced heat transfer and flow characteristics. Experimental measurements are important to study the thermal-hydraulic behavior of such assemblies and to validate and improve the predictive capabilities of specialized correlations and computational tools. Presently, very limited experimental data is available on the local subchannel pressure drop. Experimental measurements of subchannel pressure drop were conducted in a 61-pin wire-wrapped rod bundle replica, for Reynolds numbers between 190 and 22,000. Specialized instrumented rods were utilized to measure the local pressure drop and estimate the subchannels' friction factor. Three interior subchannels, one edge subchannel, and one corner subchannel were selected to study the effects of location and flow regimes on the friction factor and hydraulic behavior. The transition boundaries from laminar to transitions regimes, and from transition to turbulent regimes were estimated for the subchannels analyzed. The results were found to be in agreement with the predictions of the upgraded Cheng and Todreas detailed correlation (UCTD). The results of the experimental campaign provided a better understanding of the hydraulic behavior of the subchannels of wire-wrapped bundles, in relation to its geometrical features.

Applicability of Rans Models and Pressure Drop in Edge Subchannels for Wire-Wrapped Fuel Bundle Channel in CiADS

Applicability of Rans Models and Pressure Drop in Edge Subchannels for Wire-Wrapped Fuel Bundle Channel in CiADS

Analysis of pressure losses and flow distribution in wire-wrapped hexagonal rod bundles for licensing. Part I: The Pacio-Chen-Todreas Detailed model (PCTD)

This two-part paper presents an updated version of the long term MIT-NTHU modeling effort on the hydraulics of hexagonal arrays of wire wrapped fuel pins. It has been stimulated by the emergence of technical licensing considerations from the current development efforts for MYRRHA, the accelerator-driven system cooled by lead–bismuth eutectic being conducted at SCK CEN in Belgium. This added licensing perspective has been introduced into this hexagonal array modeling effort through the recent collaboration of this paper’s lead author with the MIT-NTHU team resulting in the creation of the new model called the Pacio-Chen-Todreas Detailed model (PCTD).Models used for licensing calculations must be able to predict pin bundle hydraulic and thermal behavior within a prescribed upper bound of uncertainty. This must be done for all postulated scenarios with sufficient accuracy to maintain required safety margins. This capability requires a predictive model of sufficient detail to represent the bundle friction and flow distribution behavior. The Upgraded Cheng and Todreas (UCTD) model from 2018 is unique in this ability and was selected as the basis for this effort.The major improvement needed was correction of the consistent over-prediction of flow velocity in the edge subchannels. This was addressed by recognizing and adding an additional physical mechanism of momentum exchange due to the flow mixing effect resulting from the wire rod spacers. While this added momentum exchange has also slightly improved the UCTD bulk friction factor prediction, most importantly it has provided for corrected prediction of subchannel axial velocity. Since the velocities in the corner, edge and interior subchannels differ, this velocity distribution has been characterized as the bundle flow split. Its accurate prediction is essential for prediction of maximum rod cladding temperature the key bundle safety parameter.Part I, here following, summarizes the existing UCTD and then presents the new PCTD model including the governing equations, the constitutive models, the empirical parameters, an example calculation and an associated statistical evaluation. Part II, published separately in this issue, presents the experimental data base from which the empirical parameters in the new model are calibrated. Based on the analysis of these data, it is suggested that future experimental efforts focus on the laminar flow regime (relevant in accidental conditions) and flow-split data. Moreover, numerical simulations are expected to play an increasing role in the safety assessment, and the database collected in this work can be used for a benchmarking exercise to evaluate further model developments.

Analysis of pressure losses and flow distribution in wire-wrapped hexagonal rod bundles for licensing. Part II: Evaluation of public experimental data

This two-part paper presents an updated version of the long term MIT-NTHU modeling effort on the hydraulics of hexagonal arrays of wire wrapped fuel pins. It has been stimulated by the emergence of technical licensing considerations from the current development efforts for MYRRHA, the accelerator-driven system cooled by lead–bismuth eutectic being conducted at SCK CEN in Belgium. This added licensing perspective has been introduced into this hexagonal array modeling effort through the recent collaboration of this paper’s lead author with the MIT-NTHU team resulting in the creation of the new model called the PCTL model (PCT). Models used for licensing calculations must be able to predict pin bundle hydraulic and thermal behavior within a prescribed upper bound of uncertainty. This must be done for all postulated scenarios with sufficient accuracy to maintain required safety margins. This capability requires a predictive model of sufficient detail to represent the bundle friction and flow distribution behavior. The Upgraded Cheng and Todreas (UCTD) model from 2018 is unique in this ability and was selected as the basis for this effort. The major improvement needed was correction of the consistent over-prediction of flow velocity in the edge subchannels. This was addressed by recognizing and adding an additional physical mechanism of momentum exchange due to the flow mixing effect resulting from the wire rod spacers. While this added momentum exchange has also slightly improved the UCTD bulk friction factor prediction, most importantly it has provided for corrected prediction of subchannel axial velocity. Since the velocities in the corner, edge and interior subchannels differ, this velocity distribution has been characterized as the bundle flow split. Its accurate prediction is essential for prediction of maximum rod cladding temperature the key bundle safety parameter. Part I, published separately in this issue, summarizes the existing UCTD and then presents the new PCT model including the governing equations, the constitutive models, the empirical parameters, an example calculation and an associated statistical evaluation. Part II, here following, presents the experimental data base from which the empirical parameters in the new model are calibrated. Based on the analysis of these data, it is suggested that future experimental efforts focus on the laminar flow regime (relevant in accidental conditions) and flow-split data. Moreover, numerical simulations are expected to play an increasing role in the safety assessment, and the database collected in this work can be used for a benchmarking exercise to evaluate further model developments.

Turbo DFE Assisted Time-Frequency Packing for Probabilistically Shaped Terabit Superchannels

We present a probabilistically shaped (PS) time-frequency-packing (TFP) wavelength-division multiplexing superchannel system employing higher order modulation (HoM) formats, with an objective to improve the spectral efficiency (SE). However, TFP systems suffer from inter-symbol interference (ISI) and/or inter-carrier interference (ICI). Additionally, the bandwidth limitations of the wavelength selective switches in the fiber link may cause severe ISI for the edge sub-channels (SCs) in a superchannel. Mitigation of such interferences is particularly challenging for HoM systems, since the implementation of the well-known turbo equalization schemes, such as the Bahl-Cocke-Jelinek-Raviv equalizer, is computationally challenging for larger constellations. In this paper, we investigate an expectation propagation based, computationally efficient, turbo decision feedback equalizer for ISI cancellation, in tandem with a parallel interference cancellation based ICI mitigation. By optimizing the parameters in the shaping and the packing dimensions, we show through our numerical results that the proposed PS-TFP superchannels enabling 1.8 Tbps data rates offer up to 1.05 dB packing gain over an unpacked system using the same modulation format, and a 1.15 dB shaping gain over an unshaped system that uses a lower modulation order to achieve the same SE.

Large Eddy Simulation of a 5 × 5 rod bundle: Impacts of a central control rod thimble tube

Rod bundle flows are prevalent in nuclear engineering for both light water reactors (LWR) and advanced reactor concepts. Unlike canonical channel flow, the flow in rod bundles presents some unique characteristics, notably due to the inhomogeneous cross section which can present different local conditions of turbulence as well as localized effects characteristic of external flows. Despite the ubiquity of rod-bundle flows and the decades of experimental and numerical knowledge acquired in this field, high-fidelity datasets of multiple-pin configurations are still relatively sparse due to the computational cost involved in generating them at relevant Reynolds numbers. These datasets are of great value as they allow for assessment of the reliability of various turbulence models, as well as allow for a deeper understanding of the flow physics. We present Large Eddy Simulations (LES) of the flow at Reynolds number 19,000 in a square 5 × 5 rod bundle representative of LWR fuel generated by the flow solver Nek5000. We consider standard configurations as well as configurations where the central pin is replaced with a control rod guide thimble, and analyze the flow fields in detail. The accuracy of the LES of the base 5 × 5 configuration is established by comparing against Direct Numerical Simulation (DNS) results. We compare the LES results from Nek5000 with an advanced Reynolds Stress model from STAR-CCM+, which shows good agreement in the wide gaps with larger discrepancies in the narrow gaps. In particular, evidence of gap vortex streets and changes in the turbulence structure are seen in the edge subchannels and in the narrow thimble gaps in LES, but are not predicted by STAR-CCM+.

Experimental Study on the Effect of Localized Blockages on the Friction Factor of a 61-Pin Wire-Wrapped Bundle

Abstract The thermal-hydraulic behavior of the flow in rod bundles has motivated numerous experimental and computational investigations. Previous studies have identified potential for accumulation of debris within the small subchannels of typical wire-wrapped assemblies with subsequent total or partial blockage of subchannel coolant flow. A test campaign was conducted to study the effects of localized blockages on the bundle averaged friction factor of a tightly packed wire-wrapped rod bundle. Blockages were installed within the bundle, and fluid pressure drop was measured across one wire pitch for a Reynolds number range of 500–17,200. The Darcy–Weisbach friction factor of the perturbed rod bundle geometry was compared with that of the unblocked bundle, as well as with the predictions of a well-established friction factor correlation. Differing effects based on blockage size and location for various flow regimes were studied. A number of conclusions can be made about the effects of the blockages on the friction factor, such as an increasing effect of the blockage on friction factor with an increase in Reynolds number, a change in flow behavior in the turbulent transition flow regime near Reynolds number 3000, differences in effect on friction factor for different types of subchannel blockage, and a nonlinear trend in friction factor variation with flow area impeded for edge subchannels. To this end, all data and quantified uncertainty produced in this study are made available for comparison and validation of advanced computational tools.

Steady-State Thermal-Hydraulic Model for Fluoride-Salt-Cooled Small Modular High-Temperature Reactors

A whole-core, steady-state, thermal-hydraulic model for the cylindrical pin-type fluoride-salt-cooled small modular advanced high-temperature reactor (SmAHTR) is developed. In this preconceptual reactor design initially proposed by Oak Ridge National Laboratory, each fuel assembly in the graphite-moderated core has the FLiBe coolant flowing parallel to a hexagonal array of fuel and moderator pins. The present study considers a slightly modified fuel assembly design with a hexagonal inner housing compared to the original cylindrical housing. Burnable poison pins and control rods are also included in the fuel assembly considered here. The thermal-hydraulic model employs finite volumes to solve three-dimensional conduction in the pins and the hexagonal graphite reflector regions in the core. Heat transfer between the fuel assemblies is also addressed. The finite volumes in the fluid region are modeled using a subchannel approach in which the fluid is discretized into edge, corner, and interior subchannels and the resulting mass, momentum, and energy equations are systematically solved. The subchannel model also includes the transport between adjacent subchannels both due to radial pressure gradient–driven cross flow and turbulent mixing. Appropriate closure models from the literature are used to quantify axial and lateral flow resistances, heat transfer from solid to fluid, and turbulent mixing. The resulting thermal-hydraulic model provides detailed temperature and flow information for the entire core at a modest computational cost. Preliminary verification studies are also performed and reported. Whole-core, steady-state results are presented for this SmAHTR core configuration for different power profiles. The effect of grid refinement and total mass flow rate into the core on the peak fuel temperature is also investigated. Fuel temperatures from a preliminary analysis with pin power distributions from a neutronic model are also included. The peak fuel temperature of ~1229°C in this illustrative case is below the steady-state operation limit for the SmAHTR.

Numerical investigation on vortex behavior in wire-wrapped fuel assembly for a sodium fast reactor

Abstract The wire-wrapped fuel bundle is an assembly design in a sodium-cooled fast reactor. A wire spacer is used to maintain a constant gap between rods and to enhance the mixing of coolants. The wire makes the flow complicated by creating a sweeping flow and vortex flow. The vortex affects the flow field and heat transfer inside the subchannels. However, studies on vortices in this geometry are limited. The purpose of this research is to investigate the vortex flow created in the wire-wrapped fuel bundle. For analysis, a RANS-based numerical analysis was conducted for a 37-pin geometry. The sensitivity study shows that simulation with the shear stress transport model is appropriate. For the case of Re of 37,100, the mechanisms of onset, periodicity, and rotational direction were analyzed. The vortex structures were reconstructed in a three-dimensional space. Vortices were periodically created in the interior subchannel three times for one wire rotation. In the edge subchannel, the largest vortex occurred. This large vortex structure blocked the swirl flow in the peripheral region. The small vortex formed in the corner subchannel was negligible. The results can help in understanding the flow field inside subchannels with sweeping flow and vortex structures.

CFD investigation on thermal-hydraulic behaviors of a wire-wrapped fuel subassembly for sodium-cooled fast reactor

Different from the square-arrayed fuel rod bundles in pressurized water reactors, the fuel subassembly for sodium-cooled fast reactors features with wire spacers helically wrapped on the fuel rod. This intricate geometry consequently imposes a significant challenge in numerical simulation. In the present study, a new marked and separated meshing method is developed to reduce the required mesh and modeling time. Instead of directly generating the complex geometry and mesh, the hexahedral wire wrapper mesh was dealt with locus equations. First, the numerical modeling method was validated by combining the experimental data of a 19-pin wire-wrapped rod bundle and the simplified Cheng and Todreas correlation. Furthermore, a comparative study of different meshing methods was conducted. Then, an investigation on the three-dimensional flow phenomenon and heat transfer characteristics of a 61-pin wire-wrapped fuel subassembly for the China Experimental Fast Reactor was carried out. Finally, complex axial and transverse flow behaviors and temperature field in different types of subchannels were quantitatively analyzed. Results indicate that the axial velocity exhibits periodic oscillation with a maximum normalized value of 1.54. The flow oscillation in the interior subchannels presents a smaller amplitude and higher frequency than that in the corner and edge subchannels. Local hot spots occur in the gaps beneath the spacer wires. Sweeping and cross flow patterns exist in the peripheral and interior subchannels, respectively. In addition, the secondary transverse flow induced by the spacer wires is stronger in the corner subchannel. The cross flow strength reaches a peak value equal to 30% of the average axial velocity, which promotes the thermal mixing.

Stereoscopic PIV measurements of near-wall flow in a tightly packed rod bundle with wire spacers

In this study, we experimentally investigated the flow field characteristics in the complex geometry of a tightly packed 61-rod bundle with wire spacers by employing matched-index-of-refraction (MIR) and stereoscopic particle image velocimetry (SPIV) techniques. SPIV measurements were performed on a cross-flow plane at Reynolds numbers of 500, 2500, and 6300, and the measurement area covered the interior and edge sub-channels that were located near the enclosure wall. The SPIV results showed that peaks of the velocity magnitude occurred in the edge sub-channel, indicating the wall effects on the flow symmetry in the cross-flow plane. Mean velocities at the central areas of the edge and interior sub-channels showed flat profiles for Re=500 and local peaks around 0.15<y/Drod<0.3, due to the wire’s presence and near-wall location of the edge sub-channel. The rms fluctuating profiles velocities showed that local maximal peaks linearly shifted from y/Drod=-0.44 to −0.36 and −0.28 as Re increased. Taylor’s hypothesis was applied to the SPIV velocity vector fields for reconstructing quasi-instantaneous three-dimensional (3D) vortical structures in the wire-wrapped rod bundle. The visualization of 3D vorticity iso-surfaces showed that vortical structures were generated from shear layers of neighboring rods and traveled in the streamwise direction. Finally, proper orthogonal decomposition (POD) analysis was applied to SPIV velocity snapshots taken at the edge and interior sub-channels to extract the most statistically dominant flow structures. The POD velocity decomposition revealed coherent structures with sizes and shapes comparable to instantaneous vortical structures identified by iso-surfaces. Energy fractions of the first POD modes were found to reduce from 46% and 31% to 26% and 18% when Reynolds numbers increased from Re=500 to 6300, respectively. On the other hand, kinetic energy levels of low-order POD modes (modes 2, 3, 4, etc.) increased when the Reynolds number increased.

CFD investigation of three-dimensional flow phenomena in a JAEA 127-pin wire-wrapped fuel assembly

This paper presents a RANS (Reynolds Averaged Navier-Stokes simulation) based CFD (Computational Fluid Dynamics) investigation of the three-dimensional flow phenomena in a JAEA (Japan Atomic Energy Agency) 127-pin wire-wrapped fuel assembly. Complicated and vortical flow phenomena in the wire-wrapped fuel bundles have been elucidated by a CFD analysis with a high resolution scheme and a shear stress transport (SST) turbulence model, and by a vortex structure identification technique based on the critical point theory. The CFD results show a good agreement with the JAEA experiment results with the 127-pin wire-wrapped fuel assembly, which was implemented using water for validating the pressure drop formulas in the JAEA sub-channel analysis code, ASFRE. The generation and demise of the vortex structures in the edge, corner, and interior sub-channels were periodically synchronized with the angular positions of the wire spacers. The axial and tangential flows in the edge and corner sub-channels are much stronger than those in the interior sub-channels. The vortex structures in the edge sub-channels have higher axial and tangential velocities than those in the corner sub-channels, interior sub-channels, and the wake region generated by wire spacers. The vortex structures in the edge sub-channels are a type of longitudinal vortex, and have a larger scale than the other sub-channels. The vortex structures in the corner sub-channels have lower axial and tangential velocities than those in the edge and interior sub-channels. The turbulence intensity in the sub-channels is remarkably dominated by the magnitude of the axial velocity. The vortex structures in the edge, corner, and interior sub-channels have a higher turbulence intensity and lower vorticity than the small-scale wakes near the wire spacers. The driving forces on the wire in the X-, Y-, and Z-directions are not only dependent on the axial velocity, but are also significantly dependent on the relative angular position between the wire spacer and rod, and the relative position between the wire spacer and duct wall.

CFD analysis of heat transfer in hexagonal subchannels of super fast reactor in upward flow

AbstractA super fast reactor is a fast spectrum, supercritical, water‐cooled reactor. This paper represents CFD analysis of heat transfer in hexagonal subchannels of super fast reactor using FLUENT in ANSYS. The numerical simulation of grid stability was done by considering different mesh sizes and the turbulence model for heat transfer of supercritical water was also carried out and compared with the experimental data. RNG k‐ϵ turbulence model with enhanced wall treatment was considered for simulations. Heat transfer and heat generation rate analysis of the outer surface rod wall is carried out with different subchannels by changing various parameters like boundary conditions and pitch‐to‐diameter ratio. The analyses reveal that the outer surface of the rod wall temperature decreases with increase in pitch‐to‐diameter ratio. Maximum coolant temperature rises in edge subchannels more than corner subchannels. Further analysis is carried out with different mass fluxes. Increases in mass flux has minimal effect on the maximum rod wall surface temperature. Maximum cladding surface temperature for the corner subchannel is less compared to the edge subchannel. Heat generation rate also decreases with increase in pitch‐to‐diameter ratio. This paper also investigates the buoyancy effect on subchannels with varying heat flux as boundary conditions considering constant mass flux.

Thermal-hydraulic effect of wire spacer in a wire-wrapped fuel bundles for SFR

The computational fluid dynamics (CFD) investigation is carried out over the full-scale experimental facility of SIENA’s 7-pin fuel assembly using commercial CFD code, CFX. The purpose of this research is to understand the effect of wire spacer on flow and temperature fields in fuel bundle by comparing the CFD analysis with wire spacer and without wire spacer. Thermal-hydraulic phenomena in a fuel bundle have been investigated by Reynolds Averaged Navier-Stokes (RANS) flow simulation based on the shear stress transport (SST) turbulence model. Calculated pressure drop in the fuel bundle agrees well with Cheng & Todreas model. When the turbulent Prandtl number in the CFD analysis is adopted with 0.02, Nusselt number of the CFD analysis results has a good agreement with Mikityuk model, Graber and Rieger model, and Borishanski et al. model. Based on the CFD investigation, the wire spacers locally induce a tangential flow by up to about 13% of the axial velocity. The tangential flow in the corner and edge sub-channels is much stronger than that in the interior sub-channels. The flow with a high tangential velocity is periodically rotating in a period of wire lead pitch. Pressure drop value without wire spacer is about 6% less than that with wire spacer over the various range of Reynolds number. The cross flow due to the wire spacer can achieve to enhance heat transfer characteristics up to about 50%. Furthermore, the wake regions due to helically wrapped wire spacers are developed nearby the counter rotating position of wire spacers. These wake regions have the peak temperatures in the sub-channels. In this analysis case, we found that the temperature uncertainty of sub-channel is about 5% of temperature difference between inlet and outlet of the test section.

Three-dimensional flow phenomena in a wire-wrapped 37-pin fuel bundle for SFR

Abstract Three-dimensional flow phenomena in a wire-wrapped 37-pin fuel assembly mock-up of a Japanese loop-type sodium-cooled fast reactor, Monju, were investigated with a numerical analysis using a general-purpose commercial computational fluid dynamics code, CFX. Complicated and vortical flow phenomena in the wire-wrapped 37-pin fuel assembly were captured by a Reynolds-averaged Navier–Stokes flow simulation using a shear stress transport turbulence model. The main purpose of the current study is to understand the three-dimensional complex flow phenomena in a wire-wrapped fuel assembly to support the license issue for the core design. Computational fluid dynamics results show good agreement with friction factor correlation models. The secondary flow in the corner and edge subchannels is much stronger than that in an interior subchannel. The axial velocity averaged in the corner and edge subchannels is higher than that averaged in the interior subchannels. Three-dimensional multiscale vortex structures start to be formed by an interaction between secondary flows around each wire-wrapped pin. Behavior of the large-scale vortex structures in the corner and edge subchannels is closely related to the relative position between the hexagonal duct wall and the helically wrapped wire spacer. The small-scale vortex is axially developed in the interior subchannels. Furthermore, a driving force on each wire spacer surface is closely related to the relative position between the hexagonal duct wall and the wire spacer.

Laser Doppler measurement and CFD validation in 3 × 3 bundle flow

The five-beam three-component laser Doppler system is operated in the three-beam two-component mode to measure the 3×3 bundle flow with simple grid spacer. Experiment has been conducted at Re=15,200 and 29,900. According to the experiment result, the root mean square (RMS) of axial velocity fluctuation shows large value in the gap and the near-wall region of the edge sub-channel which is induced by the axial velocity gradient. Significant intensity of lateral velocity fluctuation is observed which indicates the strong lateral mixing in a 3×3 rod bundle. Through the correlation analysis coherent structures have been observed in the gap region. The spectral analysis shows that the LDV measurement complies to the Komogorov spectrum law, f−5/3, well. The low-frequency peak spectral density of the axial velocity fluctuation has been observed in the gap region connecting sub-channels with velocity difference. The performance of the SSG model and the baseline Reynolds stress model are investigated based on the experiment result. The models predict higher axial velocity in the interior sub-channel and lower in the edge and corner ones than the experiment result. Large discrepancy between the calculated and measured axial flow velocity is resulted from failure in calculating the strong negative u′w′¯ in the gap region connecting different sub-channels.

CFD analyses in tight-lattice subchannels and seven-rod bundle geometries of a Super Fast Reactor

This paper presents CFD analyses in heat unsymmetric subchannels and heat symmetric seven-rod bundle geometries of a Super Fast Reactor (Super FR) fuel assembly using STAR-CD. The purpose of CFD analyses in heat unsymmetric subchannels is to evaluate the effect of the power differences on the heat transfer in subchannels of the Super Fast Reactor. For heat symmetric seven-rod bundles, the effects of the gap clearance between the fuel rod and the assembly wall and the displacement of the fuel rod on the circumferential temperature distributions and Maximum Cladding Surface Temperature (MCST) are analyzed. The results show that larger power difference between fuel rods gives larger circumferential temperature difference of the hottest fuel rods. Considering cross flow between edge and ordinary subchannels, 1 mm gap between the fuel rod and the assembly wall is better for small MCST although the circumferential temperature difference in edge subchannel is large. MCST increases exponentially with the displacement. The relative error of displacement should be less than 1% if the allowable increment of MCST due to displacement is less than 6 °C.