Yazar "Turkmen, Mehmet" seçeneğine göre listele

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    Comprehensive neutronic modeling of slug flow by Monte Carlo technique
    (Pergamon-Elsevier Science Ltd, 2022) Turkmen, Mehmet; Tiftikci, Ali
    This 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.
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    Monte Carlo model of annular flow in boiling water reactors
    (Pergamon-Elsevier Science Ltd, 2020) Tiftikci, Ali; Turkmen, Mehmet
    Annular 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.
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    Neutronics modeling of bubbles in bubbly flow regime in boiling water reactors
    (Korean Nuclear Soc, 2019) Turkmen, Mehmet; Tiftikci, Ali
    This 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.