Discrepancy has long existed about nozzle diameter effect on critical flow rate in two-phase critical flow analysis. The issue is of great importance because it involves whether the scaling test can accurately simulate the actual process during loss of coolant accident. A series of transient critical flow experiments has been performed in China Institute of Atomic Energy (CIAE), which aims to study the nozzle diameter effect. The diameter of adopted test nozzle is 5mm, 10mm and 15mm, respectively. The experiment result shows that the discharge mass flow rate decreases as nozzle diameter increases. This is contradictory with the results obtained from steady state critical flow experiment. Comprehensive analysis shows that the diameter effect observed in transient critical flow experiment is transient effect caused by system pressure change, and the mechanism is the dynamic unbalance in the transient period. It is presumed that for the same test section with a certain diameter, if the pressure change velocity is different, the measured critical flow rate will be different. The conclusion is validated in the pressurization and depressurization blowdown test. It| is arrived that nozzle diameter has no direct effect on critical flow rate, and thus the discrepancy on diameter effect is clarified.

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