Taku Nagatake

Research of Seawater Effects on Thermal-Hydraulic Behavior at Severe Accident: 1 — Research Plan and Results of Preliminary Experiments

The development of analytical method to predict and assess the current state of the reactor cores of Fukushima Daiichi nuclear power plant is required. Experimental researches are also required to obtain data to verify the analytical results and to understand the phenomena that are important to the accident progress evaluation.In the Fukushima Daiichi nuclear power plant accident, seawater was injected into the reactors to cool down nuclear fuels. Core cooling with seawater has never been assumed and the effect of seawater on heat transfer in core is not clear. Then, effects of seawater on thermal-hydraulic behavior must be investigated to understand the phenomena occurred in the accident and to evaluate current state of the reactor cores.In this series of research work, the effects of seawater on thermal-hydraulic behavior before and after degradation of the cores will be researched experimentally. Experimental results will be incorporated to numerical simulation codes to evaluate effects of seawater on the Fukushima Daiichi nuclear power plant accident. In the experimental research part, we have a plan performing two heat transfer experiments to evaluate thermal hydraulic performance of sea water and effects of salt precipitation. A precipitation state confirmation experiment is performed to obtain basic information required for the experiments with salt precipitation. In this paper, outline of the research plan is explained and the results of the precipitation state confirmation experiment is shown.Copyright

Development of Prediction Technology of Two-Phase Flow Dynamics Under Earthquake Acceleration: (14) Numerical Simulation of Two-Phase Flow in Subchannels Under Accelerating Condition

An earthquake is one of the most serious phenomena to consider for the safety of a nuclear reactor in Japan. Therefore, structural safety of nuclear reactors has been studied and nuclear reactors were contracting with structural safety for a big earthquake. However, it is not enough for safety operation of nuclear reactors because thermal-fluid safety is not confirmed under the earthquake. For instance, behavior of gas-liquid two-phase flow is unknown in seismic conditions. Especially, fluctuation of void fraction is an important factor for the safety operation of the nuclear reactor. In previous work, fluctuation of void faction in bubbly flow was studied experimentally and theoretically to investigate the stability of the bubbly flow. In such studies, flow rate or void fraction fluctuations were given to the steady bubbly flow. In case of the earthquake, the fluctuation is not only the flow rate, but also a body force on the two-phase flow and shear force through the pipe wall. Interactions of gas and liquid through their interface also act on the behavior of the two-phase flow. The fluctuation of the void fraction is not clear for such complicated situation during the earthquake.Therefore, the behavior of gas-liquid two-phase flow is investigated experimentally and numerically in a series of studies. In this study, to develop the predictive technology of two-phase flow dynamics under earthquake acceleration, a detailed two-phase flow simulation code with an advanced interface tracking method TPFIT (Two-Phase Flow simulation code with Interface Tracking) was expanded to two-phase flow simulation in seismic conditions. In a previous study, we performed a numerical simulation of a two-phase bubbly flow in a horizontal pipe and a vertical bubble motion in a water tank in seismic conditions. And it was confirmed that the modified TPFIT can be applicable to the bubbly flow in seismic conditions.In this paper, the two-phase bubbly flow in a simulated single-subchannel excited by oscillation acceleration was simulated by using the expanded TPFIT. A calculation domain used in this simulation was a simplified subchannel in a BWR core. And time-series of void fraction distributions were evaluated based on predicted bubble distributions. When no oscillation acceleration was added, void fraction concentrated in a region near the wall. When oscillation acceleration was added, void fraction distribution was changed by time. And coalesces of bubbles occurred in the numerical simulation, and bubbles with relatively large diameter were observed. In the results, complicated void fraction distribution was observed, because the response of void fraction distribution on the oscillation acceleration was dependent on not only imposed acceleration, but also the bubble diameter.Copyright

Development of Prediction Technology of Two-Phase Flow Dynamics Under Earthquake Acceleration: 10 — Numerical Prediction of Velocity Profile Around Bubble Under Accelerating Condition

An earthquake is one of the most serious phenomena for the safety of a nuclear reactor in Japan. Therefore, structural safety of nuclear reactors has been studied and nuclear reactors ware contracted with structural safety for a big earthquake. However, it is not enough for safety operation of nuclear reactors because thermal-fluid safety is not confirmed under the earthquake. For instance, behavior of gas-liquid two-phase flow is unknown under the earthquake conditions. Especially, fluctuation of void fraction is an important factor for the safety operation of the nuclear reactor. In the previous work, fluctuation of void faction in bubbly flow was studied experimentally and theoretically investigate the stability of the bubbly flow. In such studies, flow rate or void fraction fluctuations were given to the steady bubbly flow. In case of the earthquake, the fluctuation is not only the flow rate, but also a body force on the two-phase flow and shear force through the pipe wall. Interactions of gas and liquid through their interface also act on the behavior of the two-phase flow. The fluctuation of the void fraction is not clear for such complicated situation under the earthquake.Therefore, the behavior of gas-liquid two-phase flow is investigated experimentally and numerically in the series of study. In this study, to develop the predictive technology of two-phase flow dynamics under earthquake acceleration, a detailed two-phase flow simulation code with an advanced interface tracking method TPFIT was expanded to two-phase flow simulation under earthquake conditions. In this paper, the bubbly flow in a horizontal pipe excited by oscillation acceleration and under the fluctuation of the liquid flow was simulated by using the expanded TPFIT. Predicted time series of velocity profiles around the bubbles and shapes of bubbles were compared with measured results under flow rate fluctuation and structural vibration. Predicted results were almost same as measured results qualitatively. And it was concluded that the expanded TPFIT can be applied to qualitative analysis of bubbly flow under accelerating conditions.Copyright

Evaluation of Thermal Stress Distribution With Elasticoluminescent Materials

A new elasticoluminescent material, which is a kind of mechanoluminescence, has been developed by National Institute of Advanced Industrial Science and Technology (AIST), Japan. The material has the characteristic that can produce luminescence repeatedly even at very small elastic deformation. By coated to the surface of a structure, the material can visualize the distribution of stress and can make a diagnosis to the soundness of the structure. So far, this has been carried out for building, bridge, etc, under normal temperature condition. Because this material has a characteristic that luminescence not only with the change of stress, but also with the change of the temperature and even the temperature changing rate, it is considered being difficult to be used under high temperature condition.In this paper, the elasticoluminescent materials are used under high temperature condition to seek the possibility of the visualization of the distribution of thermal stress. Test section was designed to be able to generate thermal stress. Luminescence data from the elasticoluminescent materials, strain data and temperature distribution data were derived. The comparison between the luminescence data and the strain data show the elasticoluminescent material can measure the distribution of thermal stress qualitative.Copyright

Development of Prediction Technology of Two-Phase Flow Dynamics Under Earthquake Acceleration (6) Numerical Simulation of Bubble Deformation Near Wall Under Accelerating Condition

Earthquake is one of the most serious phenomena for safety of a nuclear reactor in Japan. Therefore, structural safety of nuclear reactors has been studied and nuclear reactors were contracted with structural safety for a big earthquake. However, it is not enough for safety operation of nuclear reactors because thermal-fluid safety is not confirmed under the earthquake. For instance, behavior of gas-liquid two-phase flow is unknown under the earthquake conditions. Especially, fluctuation of void faction is an important factor for the safety operation of the nuclear reactor. In the previous work, fluctuation of void faction in bubbly flow was studied experimentally and theoretically to investigate the stability of the bubbly flow. In such studies, flow rate or void fraction fluctuations were given to the steady bubbly flow. In case of the earthquake, the fluctuation is not only the flow rate, but also body force on the two-phase flow and shear force through a pipe wall. Interactions of gas and liquid through their interface also act on the behavior of the two-phase flow. The fluctuation of the void fraction is not clear for such complicated situation under the earthquake. Therefore, the behavior of gas-liquid two-phase flow is investigated experimentally and numerically in a series of study. In this study, to develop the prediction technology of two-phase flow dynamics under earthquake acceleration, a detailed two-phase flow simulation code with an advanced interface tracking method TPFIT was expanded to two-phase flow simulation under earthquake conditions. In this paper, outline of expansion of the TPFIT to simulate detailed two-phase flow behavior under the earthquake condition was shown. And the bubbly flow in a horizontal pipe excited by oscillation acceleration and under the fluctuation of the liquid flow was simulated by using expanded TPFIT. Predicted deformation of bubbles near wall was compared with measured results under flow rate fluctuation and structural vibration.Copyright