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Öğe Analogy experiments and Lattice-Boltzmann simulations of forced convection heat transfer of a packed bed under a constant temperature heating condition(Elsevier France-Editions Scientifiques Medicales Elsevier, 2026) Tiftikci, Ali; Polat, Eyyub; Moon, Je-Young; Chung, Bum-JinForced convective heat transfer from a constant temperature packed bed to fluid was investigated varying the sphere diameter and bed height. Mass transfer experiments were performed using the CuSO4-H2SO4 electroplating system on the basis of the analogy between heat and mass transfers. Lattice-Boltzmann simulations were also conducted. Sphere diameters were varied from 0.004 to 0.010 m and flow velocity from 0.01 to 0.56 m/s, which corresponds to Re-dh values of 13-5409. The measured and the computed Nu(dh)'s agreed and lay within the existing correlations, showing more agreement with recent studies. The numerical investigation showed the local temperature and turbulence characteristics within the packed bed. The flow regime transition occurred at Re-dh similar to 400. During both transitional and turbulent flows, a decrease in sphere diameter impaired the heat transfer through the packed bed given the existence of stagnant and recirculation flows through the narrow and complex flow paths. Heat transfer impairment was observed with the increased bed height only during transitional flow, which it was attributable to the preheating effect owing to the constant temperature heating condition. However, this impairment can be restored by the strong flow mixing at the turbulent flow condition. We also developed the forced convective heat transfer correlations.Öğe CFD modeling of natural convection in pebble bed geometry with finite volume method(Walter De Gruyter Gmbh, 2023) Catalbas, Salih Said; Tiftikci, AliIn this study, we used the finite volume method to computationally model natural convective flow in packed bed geometry. Using the OpenFOAM (R) v2112 code, we performed the computational analysis. We successfully meshed the intricate packed bed flow geometry, which consists of several spheres positioned at random. The spheres have sizes of 0.006 and 0.01 m, and the associated Rayleigh numbers are 1.83 x 10(7) and 8.48 x 10(7) respectively. We used the packed bed heights of H/d = 5, 10, and 20 in the simulations. By comparing the results of the OpenFOAM (R) v2112 simulations of the natural convection flow for all self-heating sphere in a packed bed, we demonstrated that the velocity distributions and Nusselt values are in good agreement with the experimental data. Additionally, it was evident from the velocity and temperature distributions in a packed bed core that there was a major temperature rise at nearby low velocity fields and a minor velocity rise in the intermediate and upper elevations. We showed that increasing the height of the pebble-bed core and correspondingly increasing the quantity of spheres inside it makes the flow more difficult and also generates local hot spots. This study is notable for using the finite volume method to evaluate natural convection flow in all self-heating packed beds and for simulating packed bed flow using a significant number of spheres. These two factors contribute to the originality of this work.Öğe Comprehensive neutronic modeling of slug flow by Monte Carlo technique(Pergamon-Elsevier Science Ltd, 2022) Turkmen, Mehmet; Tiftikci, AliThis study examines modeling uncertainty in slug flow due to the use of a uniform density in neutronic simulations. We created a detailed neutronic model of the slug flow regime, in the form of a Taylor bubble and small bubbles, in a periodic slug unit height. We explored various slug flow representations in a square unit cell geometry of a boiling water reactor and deviations (delta k) in k(infinity) from the uniform approach. We used OPENMC with the ENDF/B-VIII.0 library for neutronic simulations. The results showed that depending on how much the slug flow is detailed, uncertainty in modeling can be as high as 7300 pcm and strongly varies with the overall void fraction, Taylor bubble void fraction, and slug unit height but not with small bubbles size. With the inclusion of the Taylor bubble and small bubbles, the maximum uncertainty is calculated to be about 700 pcm. (C) 2022 Elsevier Ltd. All rights reserved.Öğe INVESTIGATING THE POTENTIAL FUELS FOR DFRm REACTOR CONCEPT(Nuclear Society Slovenia, 2024) Daydas, Semra; Tiftikci, AliThe Dual Fluid Reactor (DFR) is a conceptual design that combines the advantageous properties of two of the selected Gen IV reactor concepts: the Molten Salt Reactor (MSR) and the Lead Cooled Fast Reactor (LFR). In the DFR molten salt is used (from MSR) in a separate circuit and is cooled by the molten lead (from LFR). In the DFR, molten fuel flows inside the SiC fuel tubes while being cooled by lead that flows around these fuel tubes. The advantages of these two separate cycles enable the use of undiluted fuel salts and metallic fuels. Therefore, there are mainly two DFR concepts named DFRs and DFRm which use molten salt and molten metallic fuel respectively. In this study, neutronic analysis has been conducted for the DFRm design by using SERPENT code. Reference DFRm reactor uses U-Cr metallic fuel with the lowest temperature of 860 degrees C with 4.78 wt. % Cr, %12.8 wt.% U-235 and 82.42 wt.% U-238 composition. To increase the operation temperature margin U-Ni and U-Fe fuels are proposed. U-Ni eutectic metallic fuel reaches its lowest melting point at 740 degrees C with 11 wt.% nickel and 89 wt.% uranium composition. As for U- Fe fuel, fuel composition at the eutectic point is 10.2 wt.% Fe and 89.8 wt.% uranium at 725 degrees C. It is shown that both U-Ni and U-Fe fuels have similar k(eff) trends with the reference U-Cr fuel with lower k(eff) values. This is because proposed fuel compositions have lower uranium content. To reach the same k(eff) values U-235 content in the fuel must be higher for both U-Ni and U-Fe fuels. Thus, with these fuels in the DFRm core, it is possible to reduce the fuel freezing risk and by this, the safety would be increased and ensure a wider operating temperature range.Öğe Investigation of natural convection heat transfer of self-heating packed beds(Pergamon-Elsevier Science Ltd, 2023) Tiftikci, Ali; Catalbas, Salih Said; Polat, Eyyub; Ahn, Hyun-Ha; Han, Jeong-Won; Chung, Bum- JinThis study investigated heat transfer characteristics of natural convective flow in the packed bed geometry experimentally and numerically. The natural convection experiments for self-heating condition were performed using the copper sulfate electroplating system based on the analogy between heat and mass transfers. Also, the same conditions were simulated with Lattice-Boltzmann Method (LBM) and the results were compared. The spheres diameters were d = 4, 6 and 10 mm and the corresponding Rayleigh numbers were 5.43 x 106, 1.83 x 107 and 8.48 x 107 respectively. The packed bed height to sphere diameter ratios, H/d were 5, 10 and 20. The Nusselt numbers (Nud), velocity and shear stress distributions of both analyses were in good agreement. Both results showed the same order (10-2 m/s) of irregular velocity distributions resulting from the random arrangement of the spheres. In spite of the laminar flow condition, wake and vortex generated by the packed bed geometry increased the turbulent kinetic energies to one-or two-order higher values than those at the entrance. The temperature profiles of flow inside packed bed were approximately uniform for d = 4 mm cases but there were temperature gradients in case of d = 10 mm. This revealed that the use of smaller fuel sphere can prevent from unwanted hot spots inside the packed bed. The Nud values from LBM results and newly developed empirical correlation for natural convective flow in packed bed were consistent.Öğe Monte Carlo model of annular flow in boiling water reactors(Pergamon-Elsevier Science Ltd, 2020) Tiftikci, Ali; Turkmen, MehmetAnnular flow regime consists of a liquid film layer, a wave layer attached on liquid film layer and droplets nonuniformly dispersed within vapor core. Each has a great importance from the viewpoint of nuclear reactor safety calculations as annular flow occupies about two-thirds of total length of a BWR fuel rod. For this purpose, the annular flow in unit cell geometry was modeled by Monte Carlo method for various void distributions. Effect of each distribution was investigated to calculate the magnitude of the deviation in infinite multiplication factor due to modeling. Since the phase structures present in annular flow are added step-by-step (starting from homogeneous model approach (HEM)) in this work, delta k is calculated by taking the difference from the previous case. The results show that k(infinity) is strongly dependent on liquid film thickness, existence of wave, shape of wave and droplets. Liquid film thickness leads to increase k(infinity) whereas wave and droplets have a negative impact on v. Wave shape has double-sided effect on k(infinity). Although maximum deviation is about 400 pcm for liquid film layer, 300 pcm for wave layer, 150 pcm for wave shape and 300 pcm for droplets, the combined effect is at most 400 pcm.Öğe Neutronic examination of the U-Mo accident tolerant fuel for VVER-1200 reactors(Korean Nuclear Soc, 2024) Daydas, Semra; Tiftikci, AliIn this study, we investigated the possibility of employing accident tolerant fuel (ATF) in VVER-1200/V491 assembly without gadolinium-containing fuel rods using the Monte Carlo code Serpent 1.1.7 with ENDF/B-VII cross-section library. The analysis involves assembly design with reflective boundary conditions. To compare the neutronic performances, U-5Mo, U-7.5Mo, U-10Mo, and U-15Mo fuels were chosen in addition to ordinary UO2 2 fuel. The concentration of 135 Xe, 149 Sm, fissile and fertile isotopes with burnup, reactivity feedback with fuel temperature variation, and beta eff values were calculated. The results indicate that the fuel cycle length increases by 54.27% for U-5Mo, 32.6% for U-7.5Mo, and 13.8% for U-10Mo, while it decreases by 16.4% for U-15Mo fuel. Additionally, the effect of 95 Mo content in natural Mo was investigated by reducing the 95 Mo concentration. According to the results, each proposed fuel's fuel cycle length extended when the depletion ratio of 95 Mo increased. Additionally, the calculations for reactivity feedback guarantee safe operating conditions for all UxMo fuels.Öğe Neutronic performance analysis of U-Mn fuel and MgO-BeO tube material in the dual fluid reactor(Korean Nuclear Soc, 2026) Daydas, Semra; Tiftikci, AliThis study investigates the feasibility of using a Uranium-Manganese (U-Mn) alloy as an alternative fuel for the Dual Fluid Reactor (DFRm) concept to increase the temperature margin, along with a Magnesium Oxide-Beryllium Oxide (MgO-BeO) ceramic fuel tube. Neutronic analyses were performed using the SERPENT 1.1.7 Monte Carlo code with the ENDF/B-VII cross-section library to evaluate fuel performance, reactivity behavior, and safety margins. The results indicate that the U-Mn fuel exhibits lower keff values and shorter fuel cycle length to those of the U-Cr fuel, while providing the advantage of a lower eutectic temperature. Reactivity coefficients found to be negative for both fuel and coolant with the SiC fuel tube, ensuring inherent safety during temperature excursions. However, for the MgO-BeO configuration, the reactivity coefficients for MgO-BeO were found to be positive, which represents a critical drawback of this material; hence, further geometrical optimization is required. Consequently, although U-Mn fuel maintains negative temperature feedback under SiC-based configurations, alternative tube materials such as MgO-BeO require further optimization to ensure stable and inherently safe reactivity behavior throughout the fuel cycle. Future research could focus on optimizing reactor geometry to enhance the utilization potential of U-Mn fuel.Öğe Neutronics modeling of bubbles in bubbly flow regime in boiling water reactors(Korean Nuclear Soc, 2019) Turkmen, Mehmet; Tiftikci, AliThis study mainly focused on the neutronics modeling of bubbles in bubbly flow in boiling water reactors. The bubble, ring and homogenous models were used for radial void fraction distribution. Effect of the bubble and ring models on the infinite multiplication factor and two-group flux distribution was investigated by comparing with the homogenous model. Square pitch unit cell geometry was used in the calculations. In the bubble model, spherical and non-spherical bubbles at random positions, sizes and shapes were produced by Monte Carlo method. The results show that there are significant differences among the proposed models from the viewpoint of physical interaction mechanism. For the fully-developed bubbly flow, k(inf) is overestimated in the ring model by about 720 +/- 6 pcm with respect to homogeneous model whereas underestimated in the bubble model by about -65 +/- 9 pcm with a standard deviation of 15 pcm. In addition, the ring model shows that the coolant must be separated into regions to properly represent the radial void distribution. Deviations in flux distributions principally occur in certain regions, such as corners. As a result, the bubble model in modeling the void fraction can be used in nuclear engineering calculations. (C) 2019 Korean Nuclear Society, Published by Elsevier Korea LLC.Öğe Possibility of YH1.85 and MgO-BeO as a moderator in high temperature reactors(Pergamon-Elsevier Science Ltd, 2024) Tiftikci, AliThis study compared the neutronic behaviors of the graphite material utilized in standard HTGR reactors to those of yttrium hydride and MgO-BeO moderator materials using the OpenMC and Serpent Monte Carlo codes. keff values for various fuel/moderator ratios in a TRISO particle-embedded moderator matrix were computed. The effect of packing fraction was then investigated, and the keff values for different packing fractions were compared for the MgO-BeO, YH1.85, and C (graphite) moderator matrices in the geometry layout containing randomly placed TRISO particles. In contrast to graphite and MgO-BeO moderators, the keff values for YH1.85 moderator indicated that a very small moderator volume is adequate for thermalization. We also investigated how temperature reactivity feedback impacts different packing fractions. In the temperature reactivity feedback analysis, the overall temperature coefficient was determined to be negative for all moderators. The temperature feedback coefficient for the fuel with the YH1.85 moderator was found to be significantly lower in magnitude than for the system with graphite and MgO-BeO. The burnup findings revealed that graphite and MgO-BeO behaved similarly; however, YH1.85 was shown to be useful only at very low moderator/fuel ratios. With typical HTGR power density, the fuel cycle time for YH1.85 was found to be around 516 days. Also, while the YH1.85 moderator system reduces the risk of proliferation by producing small amounts of 239Pu and 241Pu, it was determined that it is more suitable for small modular and microreactor designs for high-temperature reactors with low moderator/fuel ratio requirements.
