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[en] Viscosity, shear ratio, bubble injection method, and surface contamination were considered as possible reasons on the discrepancy. Therefore, on the universal understanding of lift force acting on bubble, now it is very unclear state. To solve the problem, experimental basis are crucial due to computational limitations of current CFD method. In this paper, experimental results on bubble shift in shear flow similar to Li et al. are presented. Although similar test condition is tried, this kind of experiment is valuable to do, due to rareness of the experimental data. Shift of air-water bubble in linear shear flow is experimentally investigated. It is observed that similar trend to results of Li et al.(2016) can be well reconstructed from the current experimental setup. Based on the experimental results, it seems that critical bubble diameter for air-water bubble is lower than 2mm like as transitional diameter for wake instability of free rising air-water bubble.
[en] Printed circuit heat exchangers (PCHEs) have been previously investigated regarding their ability to enhance heat transfer rates. The performance of heat exchangers has been evaluated using both experimentation and computational fluid dynamics (CFD) simulation. Prior studies have attributed the observed or estimated changes in heat transfer or pressure to the geometric configuration of the heat exchangers. Other studies considered PCHE performance and resultant heat transfer enhancements on the basis of PCHE shape characteristics. In this study, we evaluated the flow characteristics in a single unit component of a tangled fluid channel using both qualitative and quantitative methods. Analysis using non-dimensional parameters and graphical visualizations were used to qualitatively determine the flow characteristics in a single unit component. Results indicated that the heat transfer rate (expressed as a Nusselt number) changed at a Reynolds number of 200; the slope of a heat transfer rate increased for Reynolds numbers >200. Also at a Reynolds number of 200, the slopes of the two friction factors (f and f = 64/Re) representing pressure loss substantially diverged. These qualitative results suggested turbulent flow for Reynolds numbers >200, which is well below the Reynolds number of 2,300 commonly used to define turbulent flow. In addition, visual assessment of the unit component’s velocity streamlines for different Reynolds numbers indicated mingled flows in the mixing zone for Reynolds numbers >400.
[en] In this study, in order to minimize the error brought by non-uniform heat flux, the spherical heaters are employed as heat source; subsequently, thermal field and heat transfer characteristics of the pebbles are investigated. The thermal field of the pebble surface in PBR is measured with heat source in different shapes. The HTGR design concept exhibits excellent safety features due to the low power density and the large amount of graphite present in the core which gives a large thermal inertia in an accident such as loss of coolant. However, the possible appearance of hot spots in the pebble bed cores of HTGR may affect the integrity of the pebbles, which has drawn the attention of many scientists to investigate the thermal field and to predict the maximum temperature locations in the pebbles using CFD method, Lee et.al has also done some experimental work on measuring the surface temperature of the pebbles as well as visualizing flow patterns of the coolant gas, and it was found that the temperature near the contacting points between pebbles was not higher than the flow stagnation points due to the higher thermal conductivity of the pebble. Certain error of temperature measurement might occur because of not very uniform heat flux in the pebbles since heater in cylindrical shape was utilized as heat source in previous experiment. More uniform heat flux and more complicated thermal profile are found in the result obtained using spherical heaters. The result shows that the temperature in contact point is higher than that in the top point, which is different from the previous results. The complex thermal phenomena observed in the lower-half side-sphere can be explained by the flow pattern near the surface
[en] The flow field and heat transfer characteristics of a pebble bed packed in a face-centered-cubic (FCC) structure have been discussed in other studies, and the hot spots were partly identified and the stagnation areas observed. However, that is not the finishing line because to design a safer pebble bed reactor only marking where the hot spots are is far less than enough, it is better to find a method by which the hot spots can be reduced or controlled. Since the objects will have influences on the flow regime of fluid, which mostly determines the localizations of hot spots, through adjusting the shape, the size, and the position of the objects it is believed that control the appearance of hot spots is practical. This paper presents corresponding simulation results obtained through CFX, even though it just shows the first-stage output, hopefully, insights on how to finally achieve that goal can still be found in this paper and in the following reports. Objects were put in an FCC structured pebble bed to see their influences on the thermal and flow fields. It was found that the overall influence was pretty small; however, the flow separation points moved their positions clearly. Also, turbulence was found formed above the bottom pebble. More simulations need to be conducted in order to actualize the control of hot spots.
[en] Flow rate could be also main factor to be proven because it is in charge of a role which takes thermal balance through heat transfer in inner side of fuel assembly. Some problems about a reliability of MELCOR results could be posed in the 2"n"d technical report of NSRC project. In order to confirm whether MELCOR results are dependable, experimental data of Sandia Fuel Project 1 phase were used to be compared to be a reference. In Spent Fuel Pool (SFP) severe accident, especially in case of boil-off, partial loss of coolant accident, and complete loss of coolant accident; heat source and flow rate could be main points to analyze the MELCOR results. Heat source might be composed as decay heat and oxidation heat. Because heat source makes it possible to lead a zirconium fire situation if it is satisfied that heat accumulates in spent fuel rod and then cladding temperature could be raised continuously to be generated an oxidation heat, this might be a main factor to be confirmed. This work was proposed to investigate reliability of MELCOR results in order to confirm physical phenomena if SFP severe accident is occurred. Almost results showed that MELCOR results were significantly different by minute change of main parameter in identical condition. Therefore it could be necessary that oxidation coefficients have to be chosen as value to delineate real phenomena as possible
[en] The pebble bed reactor (PBR) is a candidate reactor type for the very high temperature reactor (VHTR), which is one of the Generation-IV reactor types. The HTGR design concept exhibits excellent safety features due to the low power density and the large amount of graphite present in the core which gives a large thermal inertia in an accident such as loss of coolant. The conclusions are made and may contribute to a better design of a PBR core and a closer inspection of the local hot spots to avoid destruction of pebbles from happening. Thermal field of a PBR core is investigated in this study. Specifically, experiments on measuring the pebbles' surface temperature are performed. It is found that the upper pebble has an overall higher temperature profile than the other pebbles and the stagnation zone under does not increase its surface's temperature. In addition, the temperature profile of the side pebble shows a concave form and it keeps decreasing from the contact point to the vertex in the lower pebble. Lastly, the maximum temperature difference among these points is 5.83 deg. C. These findings above are validated by CFX simulations under two different turbulence models (k-e, SST) and two contact areas (diameter of 6mm and 3.5mm). By contrasting the temperature variation trends of all simulation cases, it is concluded that SST turbulence model with 20% intensity shows a better agreement with the experiment result, nevertheless, slightly deviation is also found in terms of total temperature difference and the peak appears in position 17-19 in experiments
[en] To compare between chemical and physical absorption performance and characteristics of methyl iodide, Ag+-Y and H+-Y zeolites were used. The experiments were conducted by teflon tube packed with each zeolites and constant methyl iodide flow rate at room temperature. There were three conclusion through this study. The characteristics and performance of chemical absorption, which was identified by Ag+-Y zeolite, were that the delayed time until observed the remarkable change concentration of methyl iodide was around 1000min and the concentration was started to increase rapidly and reached the maximum concentration 1073ppm at about 1500min. After reached that point, it was decreased monotonically and observed 154ppm when the experiment was finished. The visual observation, which can identify the color change rate, also showed a good agreement with the observed methyl iodide concentration as a function of time. In respect of delaying the observed remarkable concentration change of methyl iodide, H+-Y zeolite showed more good performance compared with Ag+-Y zeolite(Ag+-Y: around 1000min, H+-Y: around 1300min). However, in terms of reducing the final maximum observed concentration, Ag+-Y zeolite overwhelmed H+-Y zeolite(Ag+-Y: around 1073ppm, H+-Y: around 1493min). But it should be checked the concentration decreasing phenomenon, occurred during Ag+-Y zeolite experiment, for evaluating each zeolites performance properly.
[en] Present study conducted MELCOR code calculation by assuming the situation of complete loss of coolant accident (LOCA) scenario in a PWR 17 x 17 single spent fuel assembly, not an overall spent fuel pool. Present study conducted MELCOR analysis focusing on the heat source (decay heat, oxidation heat) and analysis object was PWR 17x17 spent nuclear fuel assembly. By changing the decay heat and oxidation coefficient, variation of peak cladding temperature to the time of the fuel assembly was analyzed. Different oxidation coefficient results the difference on the initiating time and existence of zirconium fire. From the result of MELCOR analysis, we found the criteria of decay heat that MELCOR simulation result showed the disagreement of peak cladding temperature, about 5.0 kW - 10.0 kW range
[en] Air ingress scenarios could occur in reactor and spent fuel pool accidents. For example, in reactor sequences air could be admitted through the broken reactor vessel at the late phase of a severe accident after the reactor vessel has been breached. In addition, spent fuel assemblies could be directly exposed to air in a postulated complete loss of coolant accident in a spent fuel pool. Several separate effect tests have been performed at KIT and IRSN to study the air oxidation of zirconium alloy cladding. In this study, air oxidation of zircaloy-4 cladding was conducted in 600-1600°C to investigate the breakaway kinetics. The breakaway phenomenon was observed in the temperature range of 600-1000℃ by the phase transformation from the tetragonal to monoclinic ZrO2 at the interface. Above 1050℃, there was no phase transformation and no breakaway kinetics was observed. For the future work, the nitrogen-assisted breakaway phenomenon will be investigated to identify the role of nitrogen in the breakaway kinetics.
[en] This source term could severely affect the environment with a severe radioactive potentially, which has a high radioactivity and a high volatility in a high temperature. Difficultly caught and considered to give sustained damage on thyroid, the iodine source term is protected from releasing to the environment. Especially in the over-high temperature and pressure out of the limit of containment design during a sever accident, the research and experiments to improve the retention efficiency of iodine species in a containment and a FCVS(Filtered Containment Venting System) which vents containment atmosphere in order to stabilize the temperature and the pressure have been conducted. Iodine is produced in and released from fuel rod and converted to other species in a sump water bottom of the containment, which pass thorough a pool in a FCVS during venting. In this paper, studies of phase and species conversion of iodine have been reviewed in order to fine the efficient retention conditions in a containment and a FCVS. In a sump water, high pH decreases the iodine's volatility and the nuclide's emission into the atmosphere. In a high temperature, the iodine volatility significantly decreases, but still it is required to assess the dependence of hydrolysis equilibrium reaction constant on the temperature. Molecular iodine that converted from an iodine ion species is reduced more easily when it is in the I- solution, so the reaction has the higher rate constant. Organic materials affect iodine volatility by the radiolysis into carboxylic acid and carbon dioxide that decrease the sump water's pH. It is considerable to use buffer solutions into the sump water to remain the pH high, but there is an uncertainty about the pH of the sump water which decide the utility of alkaline buffer