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Numerical and experimental investigations on transient behaviors in a low-pressure natural circulation system   WANG Qiang1, GAO Puzhen2, WANG Zhongyi1, CHEN Xianbing1 and HUANG Ying1   1. Fundamental Science on Nuclear Safety and Simulation Technology Laboratory， Harbin Engineering University， Harbin， Heilongjiang, 150001，China (wangqiang1508@foxmail.com) 2. Fundamental Science on Nuclear Safety and Simulation Technology Laboratory， Harbin Engineering University， Harbin，Heilongjiang, 150001，China (gaopuzhen@sina.com)   Abstract: During the start-up of natural circulation BWRs, the system need to operate under low-pressure condition. However, some abnormal phenomena might occur such as flashing-induced instabilities, lag increment of flowrate, transient burn-out caused by flow oscillations. The objective of this work is to investigate the transient behaviors in low-pressure natural circulation system by numerical and experimental research methods. Two types of experiments were conducted, including a transient process with step disturbance of given power signal and a self-sustaining flow oscillations process. RELAP5 code is adopted in the simulation. The calculated results are validated against the experimental data and show a good agreement with experimental results. Time lag caused by material and fluid properties is hard to simulate by RELAP5 original code. The variation of driving forces plays a dominant role in transient natural circulation. A series of flow instabilities experiments, which related to flashing, are analyzed with the help of numerical code. It is a good method to illustrate the flow instability mechanism. Keyword: natural circulation; transient behaviors; flow instability; RELAP5
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