ICONE11-36616 EXPERIMENTAL STUDY ON THERMO-HYDRAULIC INSTABILITY ON REDUCED-MODERATION NATURAL CIRCULATION BWR CONCEPT

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Reduced-moderation natural circulation BWR has been promoted to solve the recent challenges in BWR nuclear power technology problems as one of advanced small and medium-sized reactors equipped with the passive safety features in conformity with the natural law. However, the elimination of recirculation pumps and high-density core due to the increase of conversion ratio could cause various thermo-hydraulic instabilities especially during the start-up stage. The occurrences of the thermo-hydraulic instabilities especially during the start-up stage. The occurrences of the thermo-hydraulic instabilities are not desirable and it is one of the main challenges in establishing reduced-moderation natural circulation BWR as a commercial reactor. The purpose of this present study is to experimentally investigate the driving mechanism of the thermo-hydraulic instabilities and the effect of system pressure on the unstable flow patterns. Hence, as the fundamental research for this study, a natural circulation loop that carries boiling fluid with parallel boiling channel has been constructed. Channel gap that has been set at 2mm in order to simulate reduced-moderation reactor core. Pressure ranges of 0.1 up to 0.7MPa, input heat flux range of 0 up to 577kW/m^2,and inlet subcooling temperatures of 5,10,and 15K respectively, are imposed in the experiments. This experiment clarifies that changes in unstable flow patterns with increase in heat flux can be classified into two in response to system pressure range. In case of atmospheric pressure, unstable flow patters has been classified in beyond order, (1) in-phase geysering, , (2) transition oscillation combined with both features of in-phase geysering and natural circulation oscillation, (3) natural circulation oscillation induced by hydrostatic head fluctuation, (4) density wave oscillation, and finally (5) stable boiling two-phase flow. On the other hand, in the system pressure range from 0.2 to 0.7MPa, unstable patters have been dramatically changed in the following order (1) out-of-phase geysering, (2) density wave oscillation and lastly (3) stable boiling two-phase flow. Transport mechanism of the thermo-hydraulic instabilities and changes in the flow patterns with increment in heat flux are almost equal to the results obtained in the previous natural circulation loop equipped with 5mm channel gap, except for in-phase geysering. Considering in-phase geysering, it is one of new kind of the instabilities since it had never been observed in the loop with wider channel gap. This instability will be explained in this paper on the transport mechanism in detail.
社団法人日本機械学会の論文

ICONE11-36616 EXPERIMENTAL STUDY ON THERMO-HYDRAULIC INSTABILITY ON REDUCED-MODERATION NATURAL CIRCULATION BWR CONCEPT

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