Byeongnam Jo

Experimental Investigation Into Thermal Stratification by Direct Condensation in a Scaled Suppression Pool of Fukushima Daiichi Nuclear Power Plant

Experimental and numerical studies into thermal stratification by direct steam condensation in a torus type suppression pool were carried out to investigate the reactor core isolation cooling in the accidents of Fukushima Daiichi nuclear power plants. The suppression pool was manufactured to be a 1/22 scaled model of a Fukushima Daiichi nuclear power plant. Two different types of spargers were employed to simulate different units of the plants. In a sparger, 132 holes were uniformly drilled on the side of a pipe. However, the other sparger injected steam to the bottom. Flow rate was varied in a wide range to examine the effect on thermal stratification in the suppression pool. The experimental results showed that the sparger type influenced formation of thermal stratification. Moreover, steam flow rate strongly affected the onset time of thermal stratification, and the disappearance of the thermal stratification was affected by subcooling temperature. Computer simulation using a commercial software was conducted and the results show similar temperature profiles to the experimental results. Steam condensation was visualized in a vicinity of the spargers using high speed camera.Copyright

Dynamic visualization of eutectic reaction between boron carbide and stainless steel

ABSTRACT To investigate the eutectic reaction process of control-rod materials in a boiling water reactor (BWR), fundamental tests using boron carbide (B4C) powder inserted between stainless steel (SS) plates were performed and dynamically visualized. The eutectic reaction process near the contact area of the two materials and the behavior of molten material and B4C powder were visualized in real time. The temperature, reaction area, and maximum reaction-layer thickness were obtained. The average temperature range of the test was 1455–1481 K. Through dynamic visualization, some important and previously undiscovered phenomena were observed. The solid part of the SS plate and the strong surface tension of the melt retained the melt inside the specimen, preventing it from flowing out from the surface; the melt then invaded the B4C powder region during the reaction. Diffusion of the B4C powder and migration of the nonreacted B4C powder from the B4C powder region through the retained melt to the SS region were observed. This migration accelerated the local reaction growth rate. The time-resolved observation of these dynamic phenomena offers significant insights to the improvement of numerical calculation codes for severe accident analyses of BWRs, including the Fukushima Daiichi nuclear reactors.

Experimental Investigation Into Buckling Failure of Slender Metal Plates in Severe Accident Conditions

Buckling failure load of stainless steel columns under compressive stress was experimentally measured in severe accident conditions, which addresses the accidents in Fukushima Daiichi nuclear power plants. Firstly, buckling failure load defined as load which causes failure of the column (plastic collapse) was measured in a wide range of temperatures from 25 °C up to 1200 °C. The load values measured in this study were compared to numerical estimations by eigenvalue simulations (for an ideal column) and by nonlinear simulations (for a column with initial bending). Two different methods for measurement of the buckling failure load were employed to examine the effect of thermal history on buckling failure. Different load values were obtained from two methods in high temperature conditions over 800 °C. The difference in the buckling failure load between two methods increased with temperature, which was explained by the effect of creep at high temperatures. Moreover, the influence of asymmetric temperature profiles along a plate column was also explored with regard to the failure mode and the buckling failure load. In present study, all of the buckling processes were visualized by a high speed camera.Copyright

Buckling Behaviors of Metallic Columns Under Compressive Load at Extremely High Temperatures

This study aims to investigate buckling behaviors of a slender stainless steel column under compressive loads in severe accident conditions, which addresses the accidents in Fukushima Daiichi nuclear power plants. Firstly, buckling load, defined a load which generates a failure of the column (plastic collapse) was experimentally measured in a wide range of temperatures from 25 °C up to 1200 °C. The buckling load values measured were compared to numerical estimations for both an ideal column and for a column initially bent. Secondly, creep buckling tests were also performed for extremely high temperatures (800 °C, 900 °C, and 1000 °C). Creep buckling was found to occur very quickly compared to general creep times under tensile stresses. Time to creep buckling was exponentially increased with decrease of loads applied. Lateral deflection of a test column was estimated using captured images by a high speed camera. It was suggested to represent creep buckling behaviors as a time-lateral deflection curve. Moreover, an empirical correlation was developed to predict creep buckling time, based on the Larson-Miller model with experimental results obtained in present study.Copyright