The falling liquid flow rate under flooding conditions is limited at a square top end of a vertical pipe in the pressurizer surge line with the diameter of about 300 mm that consists of a vertical pipe, a vertical elbow, and a slightly inclined pipe with elbows. In this study, therefore, we evaluated effects of diameters on countercurrent flow limitation (CCFL) at the square top end in vertical pipes by using existing air-water data in the diameter range of D = 19-250 mm. As a result, we found that there was a strong relationship between the constant CK and the slope m in the Wallis-type correlation where the Kutateladze parameters were used for the dimensionless gas and liquid velocities. The constant CK and the slope m increased when the water level is increased in the upper tank h. CCFL at the square top end of the vertical pipes could be expressed by the Kutateladze parameters with CK = 1.53±0.11 and m = 0.97 for D ≥ 30 mm. The CK values were smaller for D = 19-25 mm than those for D ≥ 30 mm.
In order to improve the reliability of using the accident management procedure “low pressure injection by intentional depressurization of the steam generator secondary side” in a small-break loss-of-coolant accident (SBLOCA) with high pressure injection (HPI) failure, the statistical safety evaluation method is applied to the thermal hydraulic analysis of this event. The cornerstone of this method is the development of the phenomena identification ranking table (PIRT). In this paper, the PIRT was developed for the SBLOCA with HPI failure in a conventional PWR plant. The safety evaluation parameter was the peak cladding temperature (PCT). In the PIRT development, the phenomena identification was conducted by dividing the event progression into five phases: blowdown, natural circulation, loop seal, boil-off, and core recovery. As a result, 24 important phenomena were extracted in the PIRT, and the reasons used for ranking of these phenomena were described extensively. In addition, sensitivity analyses using the RELAP5/MOD3.2 code were performed for the most important phenomena. Uncertainty of the analytical models of the three phenomena: “uncovered core heat transfer”, “CCFL at the inlet of the U-tubes”, and “critical flow of the break” had a large influence on the predicted PCT. Therefore it is important to investigate separate effect tests for these phenomena thoroughly in order to prevent the model uncertainty distributions from being extended excessively.
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