Two types of analysis - "best estimate" and "partially-conservative" are performed. "Best-estimate" approach is based on the best-estimate codes with realistic boundary and initial conditions plus uncertainty analysis, while "partially-conservative" approach - on the best estimate codes with conservative boundary and initial conditions plus conservative assumptions. The performed comparison of both calculations show that the peak fuel cladding temperatures for "best-estimate" calculation taking into account the uncertainty analysis is slightly lower in comparison with "partially-conservative" calculation. "Partially-conservative" approach requires considerably less computational time. However, when with this methodology obtained results do not meet acceptance criteria (as it was in this particular case), the complete analysis by employing "best-estimate" approach is necessary.

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Experimental study was made on the multidimensional behavior of upward gas-liquid two-phase flow in a vertical pipe with an axisymmetric sudden expansion, which is one of the typical multi- dimensional channel geometries. In this report, the void fraction distributions along the flow direction at the below and above of the sudden expansion were measured for various flow conditions. Based on the void fraction distributions, the cross-sectional averaged void fractions were calculated for various locations in the flow direction. The detailed void fraction data was compared with the flow patterns in the sudden expansion. Furthermore, the prediction of averaged void fraction in the flow direction was carried out using one-dimensional two-fluid model (see Fig. 1). We have discussed whether such void fraction distributions in the sudden expansion can be predicted or not by analytical results. As results, it concretely pointed out that some multi-dimensional modeling or modifications for numerical simulation would be needed for more accurate prediction of two-phase flow. However, they also revealed that the two-phase flow behavior even in the sudden expansion might be predicted to a certain extent using the one-dimensional two-fluid model. if the values of C_O and V_<gi> could be successful and accurately obtained on the basis of experimental data (see Fig. 2).

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Temperature fluctuation of thermal striping are considered in the hot and cold water mixing tees under plant piping systems, for example residual heat removal system of light water reactor (LWR). In order to establish the Japan Society of Mechanical Engineers (JSME) guidelines concerning high-cycle thermal fatigue of piping systems for thermal striping in the mixing tee, experimental studies of thermal striping with scale models of mixing tees with hot and cold water were planned. This paper reports test results of Type A of collision type mixing tee in which water flows from both sides of a main pipe to a branch pipe. As a first step, visualization test with acrylic pipes tests were carried out as in order to understand the flow pattern characteristics and fluid temperature fluctuation. As a second step, temperature fluctuation measurement tests with metal pipes were carried out to measure temperature fluctuations of fluid and pipe wall. Unsteady heat transfer coefficient was also evaluated based on these results. Parameters in the tests were the velocity ratio K, which is the ratio of the velocity in one side of the main pipe to that in the other side of main pipe, and main flow velocity. The ranges of the velocity ratio and the main flow velocity were determined based on the investigation of the piping design conditions of actual plants. The effect of the velocity ratio K was examined. From the visualization test results for K=0.2,large temperature fluctuation occurs near the junction. As the distance downstream from the tee junction increases, the mean temperature approaches the bulk temperature and temperature fluctuating decreases at downstream. From the visualization test results for K=1,boundary surface of hot and cold water is made in the center of the mixing tee. Large temperature fluctuations occurs at near the junction, but temperature fluctuations are comparatively large in comparison with those for K=0.2 at the downstream. The effect of the main flow velocity were examined for K=0.2. Non-dimensional mean temperature distribution and its fluctuation are almost equal for different main flow velocity, the effect of the main flow velocity is considered to be small. The effects of the upstream piping elements were examined. As the upstream piping element, a diffuser, a globe valve with full open condition or a short elbow was installed at twice the length of the main pipe diameter from the center of a tee. From the profiles of the mean and fluctuating temperatures for the diffuser, valve and elbow, temperature fluctuations are decreased in comparison with the base case without upstream piping elements. With the temperature fluctuation measurement of pipe wall, unsteady heat transfer coefficient was evaluated. Unsteady heat transfer coefficient is found to be larger near the junction in the base case K=0.2,the values are two to three times as that of calculated using empirical formula for steady turbulent flow in a pipe.

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In both a boiling water reactor and an advanced type of pressurized water reactor under construction in Korea named APR1400,when a pressure relieving system is in operation, water, air and steam discharge successively into a sub-cooled water pool through spargers. Among the phenomena occurring during the discharging processes, the air bubble clouds with a low-frequency and high-amplitude oscillation may result in significant damage to the submerged structures if the resonance between the bubble clouds and structures occur. The phenomena involved are so complicated that most predictions of frequency and pressure loads have resorted to experimental work and computational approach has been precluded. This paper deals with a numerical simulation on the behavior of air bubble discharging into a sub-cooled water pool through a sparger without LRR (Load Reduction Ring) by using a commercial CFD code, FLUENT. The simulation includes both the transient flows of water, air and steam in the pipe and the oscillation of the air bubble subsequently formed in the pool. The VOF (Volume Of Fluid) model is selected to simulate the multi-flows of water, air and steam. It precludes a modeling of steam condensation in the pool since the resultant loads of steam condensation are relatively lower than those from the air bubble oscillation. The simulation shows that one large air bubble which is formed at the sparger head oscillates and moves upward to the water surface subsequently. Comparison of the cases with and without LRR (Note that the authors had subsequently. Comparison of the cases with and without LRR (Note that the authors had previously performed a simulation on the behavior of air bubble with LRR.) shows that the maximum dynamic pressures at the wall without LRR are much larger than those with LRR, which implies that LRR has a great impact on reducing the dynamic pressures. The frequency of the case with LRR is slightly lower due to the larger size of air bubble cloud formed only at the sparger head. The pressure and temperature distributions obtained in the computation seem to be reasonable from a physical viewpoint. It seems that the change of initial air mass in the computation range does not have an impact on the dynamic pressures at the wall. Whereas, the frequency increases as the air mass decreases, which is consistent with the well-known trend that the frequency of a large air bubble is inversely proportional to the air bubble radius. The dynamic pressures and the frequency at the wall increase as the inlet boundary pressure increases, which is consistent with the previous investigations.

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This paper describes the characteristics of the cavity flow which appears in a pipe with a closed end, and the development of the evaluation method of a penetration length of cavity flow. The penetration length of a cavity flow in a branch pipe with a closed end was clarified experimentally under the condition that there was a constant flow in the main loop which was connected with a branch pipe, which consist of a vertical and a horizontal pipe. The experiment was carried out for that branch pipe arrangement. In this experiment, the effect of horizontal branch pipe length and diameter, temperature, and fluid velocity in the main loop were clarified. The evaluation method of penetration length of cavity flow into a vertical, L_sh, was developed based on the experimental results. This method was consists of two model, i.e. the analysis model for cavity flow and the natural convection which appears in down stream of thermal stratification layer. It was confirmed that the method predicted penetration length of cavity flow well.

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An experimental study of the external vibration effect on the heat transfer characteristics of single and two-phase flows in an annular tube is carried out. An experimental set-up was constructed to study the heat transfer in a stationary, as well as, in oscillating annular tube. The annular tube was heated electrically through the inner surface, which is a stainless steel tube (St 304) 13 mm outer diameter, while the outer tube, of 3.7 cm inner diameter, made from a glass. The experimental set-up was equipped with a vibrating system to excite the annular tube in the frequency range of 0 up to 134 Hz. Several sensors for measuring wall and fluid temperatures, heat fluxes and volume flow rates of both phases were used. The obtained results show that the heat transfer coefficient can be significantly increased by vibration of the test section.

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Downward two-phase flow was studied considering its significance in view of Light Water Reactor Accidents (LWR) such as Loss of Heat Sink (LOHS) by feed water loss or secondary pipe break. The flow studied, was an adiabatic, air-water, co-current, vertically downward two-phase flow. The experimental test sections had internal hydraulic diameters of 25.4 mm and 50.8 mm. Flow regime map was obtained using the characteristic signals obtained from an impedance void meter, employing neural network based identification methodology to minimize the subjective judgment in determining the flow regimes. A four sensor conductivity probe was used to measure the local two phase flow parameters, which characterize the interfacial structures. The local time averaged two-local two phase flow parameters, which characterize the interfacial structures. The local time averaged two-phase flow parameters measured were : void fraction (α), interfacial area concentration (a_i), bubble velocity (v_g), and Sauter mean diameter (D_sm). The flow conditions were from the bubbly flow regime. The local profiles of these parameters as well as their axial development revealed the nature of the interfacial structures and the bubble interaction mechanisms occurring in the flow. Furthermore, this study provided a good database for the development of the interfacial area transport equation, which dynamically models the changes in the interfacial area along the flow field. An interfacial area transport equation was developed for downward flow based on that developed for the upward flow, with certain modifications in the bubble interaction terms. The area averaged values of the interfacial area concentration were compared with those predicted by the interfacial area transport model. In most of the bubbly flow conditions the one-group interfacial area transport model worked resonably well.

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The integral measures employed in the present study show that the strength of the swirling flow differs among the four subchannels examined in the present study. The lateral velocities in subchannel 6 are larger than in subchannels 5,7,and 10. In addition, the angular momentum in subchannel 6 has the slowest spatial decay. As the flow develops in the streamwise direction, the vortices in subchannels 5,6,and 7 migrate toward the southeast rod. The radial profiles of azimuthal velocity in subchannels 5,6,and 7 are modeled using the analytical form of the azimuthal velocity for a Lamb-Oseen vortex.

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Scaling criteria for Tehran Research Reactor, a Material Testing Reactor, under natural as well as forced circulation is derived based on fluid-to-fluid modeling. Using these criteria, the basic characteristics of scale model are determined. The aforementional criteria are such as power density, temperature rise, mass flux, and size of the model. In general, numbers of scaling laws are less than the basic unknown characteristics variables of the model and consequently, many scale models can be established for the system under consideration. The choice of model designer has a significant effect on final model and its distortion. In Tehran Research Reactor (TRR), the thickness of fuel plate and the width of coolant flow rate are very thin. For increasing these two parameters that may lead to an efficient and a better model ; two strategies are employed Either, using different clad materials from those that are used in the prototype or, changing the type of coolant fluid.

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The occurrence of flow instability during passive heat removal for Tehran Research Reactor (TRR) has been analyzed. Pressure drop-flow rate characteristics in the general case are determined upon a developed code for this purpose. The code takes into account different friction factors due to different powers as well as different core inlet temperatures. The analysis revealed the fact that the instability can actually occur in the natural convection for a range of power per fuel plates at a predetermined inlet temperature with fixed geometry of the core.

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The droplet behavior in coolant is one of the key phenomena that govern the dynamics and energetics of molten fuel-coolant interactions (MFCI) during a postulated severe accident in a nuclear reactor. The violent interaction between droplet and coolant may lead to a steam explosion, which may contribute early failure of reactor vessel or containment. Phenomenology of steam explosion is very complex, involving intense thermal-hydrodynamics. With the high temperature melt droplet falling into the volatile liquid coolant, the droplet surface experiences intensive film boiling. The formation of vapor film on the droplet surface mitigates the deformation of the droplet because of the distinct properties. In one of hypothetical mechanisms of steam explosion, for example, it is believed that the collapse of the vapor film, triggered by a shock wave, enables liquid-liquid contact and potential for coolant to penetrate into the melt droplet. This phenomenon may cause excessive steam production and droplet fragmentation. Interest in modelling of MFCI has motivated experimental and analytical investigation on steam explosion. But steam explosion is so fast and complex phenomenon, it is very difficult (if not impossible) to capture every detail of the process. The recent development of computational fluid dynamics may improve significantly the understanding of the mechanism of the steam explosion during MFCI. In our previous works, an advanced front-capturing algorithm, called the level set method, was successfully developed for the simulation of two-phase (bubble or droplet) and three-phase (droplet with vapor film in coolant) flows. The interfaces were implicitly captured by smooth level set functions. This method leaves only one set of conservation equations to be solved for all the multi phases. A high order solver, named cubic-interpolated pseudo-particle (CIP) method, is used to solve conservation equations. In the present work, a shock wave is introduced to the multiphase system. For the purpose of comparison, the revolutions of both single bubble and single droplet due to shock wave are simulated, as well as behavior of a droplet covered by a vapor film under shock wave. Due to the complexity of the problem, only hydrodynamic process under isothermal conditions is studied. Under isothermal condition, the vapor film is introduced initially. A two-dimensional model was chosen. The shock wave is introduced at the bottom as the case in most single droplet steam explosion experiment. The other boundaries are set to Neumann (open) conditions. Through the simulation, further understanding on the mechanism of melt droplet evolution due to shock wave is obtained. Due to the high viscosity and density of droplet, the effect of shock wave on droplet deformation is less significant than that on bubble deformation. However, for the droplet with vapor film in a water pool, existence of the vapor film mitigates the droplet deformation properties due to shock wave. The collapse of vapor film and the penetration of water into the droplet are observed under certain conditions.

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On a design and a safety numerical analysis for light water reactors (LWRs), Two-fluid model and Three-fluid model have been used. As for the numerical analysis, the information about the steam bubbles, which are interfacial area concentration, interfacial heat transfer and interfacial shear stress, were based on the experimental data of a single steam bubble flowing upward in a stagnant liquid condition. Therefore, the difference of the result between the numerical analysis and the experimental data in a forced convection flow was occurred. In other word, the characteristics of the steam bubbles in a subcooled boiling flow have not been inspected enough. Since the behavior of the steam bubbles in a subcooled boiling flow was complicated, it was difficult to measure the motion of the steam bubbles experimentally. Consequently, it has been important to clarify the characteristics of steam bubble in a subcooled boiling flow. On the other hand, the development of measurement method was also important for clarifying the steam bubbles motion in a subcooled boiling flow. In present work, the characteristics of steam bubbles in a subcooled boiling flow under an vertical circular channel were investigated by applying a digital image processing method assembling a digital high-speed video camera, a stroboscope, and a personal computer. As a result, the profiles of void fraction, interfacial area concentration and interfacial heat transfer coefficient along the flow direction could be measured. For the steam bubbles whose diameter were less than 8mm, the interfacial area concentration and the mean bubble diameter had a correlation with void fraction despite the variation of liquid flow rate and subcooling. In case the collapse of steam bubble occurred due to an irregular bubble condensation, the interfacial heat transfer coefficient with the bubble collapse was about twice of that without a bubble collapse. And the interfacial heat transfer coefficient without bubble collapse showed a good agreement with the correlation supposed by Akiyama. In addition, the supposed image processing method could be applied to the present experimental condition.

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