Research on the characteristics of thermal hydraulics in Reduced-Moderation Natural Circulation BWR during startup stage

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With an aim of clarifying the thermal hydraulic characteristics in the reduced-moderation natural circulation BWR during start-up stage, fundamental experiments with regard to thermal hydraulic instability has been experimentally and analytically investigated with a natural circulation parallel boiling channels, which simulates the fundamental and representative flow characteristics around the core. The new findings are as follows.(1)Some thermal hydraulic instabilities that might be occur during the start-up stage due to rapid condensation of steam bubbles or flashing phenomena, such as geysering phenomena and density wave oscillation, have been experimentally investigated under several input thermal conditions and the loop geometries.(2)Bubble size is one of the important factors to determine the flow characteristics. A bubble generator is implemented to change the bubble size. Using bubble generator newly developed by this study, the results confirmed that the bubble generator could … More control the bubbles size. In order to clarify the effect of bubble size on flow structures, measurements of bubbly flow in vertical pipe were performed using ultrasonic Doppler method(UDM), wire mesh tomography(WMT) and digital image processing(DIP). Combination of the three techniques could clarify not only liquid information, but also bubbles' information.(3)We developed measurement system to sent and received pulse ultrasound and developed the software to process received echo signal. And these systems are applied to pattern recognition of the flow pattern from bubbly flow to annular flow.(4)Multidimensional flow analysis code based on the particle-fluid model applied to the experimental results of measured development of bubbly flow in vertical rectangular tube and vertical circular tube. And physical phenomenon to influence a bubbly flow was investigated.(5)Based on the mentioned above results of study, we suggested stability and rational start-up procedure as well as about the necessity of chimney divide. Less