Nuclear Technology / Volume 164 / Number 2 / November 2008 / Pages 265-277
Technical Paper / Thermal Hydraulics
Different flow patterns of steam forward flow and nitrogen reverse flow in U-tubes were observed in the reflux condensation experiments using the Bethsy facility with 34 U-tubes. In this study, the behavior was calculated using RELAP5/MOD3.2 with two and three flow channels of U-tubes. By the modification of the weighting factor for the calculation of friction coefficients, the nitrogen reverse flow was successfully calculated. In the calculations changing the flow area ratio of two flow channels, the number of active U-tubes with steam forward flow was predicted using the assumption that flow was most stable in the case with the maximum nitrogen recirculation flow rate, and it agreed rather well with the observed number of active U-tubes (19 to 24 U-tubes) within the difference of 4 U-tubes. In the calculations with three flow channels, without the assumption, the average of the ratios of active U-tubes in several calculations (four cases in this study) with different flow area ratios of the three flow channels gave good prediction of the ratio of active U-tubes. The results indicate the validity of the assumption that the flow with the maximum nitrogen recirculation flow rate may be the most stable and appear most probably among different numbers of active U-tubes.