Assessment of RELAP5/MOD3.2 for Reflux Condensation Experiment (NUREG/IA-0181)

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Publication Information

Date Published: April 2000

Prepared by:
Y.M. Moon, H. C. No, KAIST
H.S. Park, KAERI
Y.S. Bang, KINS

Korea Advanced Institute of Science and Technology
373–1, Gusung-Dong
Yusung, Taejon
305-701 Korea
Korea Atomic Energy Research Institute
P.O. Box 105
Yusung, Taejon
305–600 Korea

Korea Institute of Nuclear Safety PO. Box 114
Yusung, Taejon
305–600, Korea

Prepared as part of:
The Agreement on Research Participation and Technical Exchange
under the International Code Application and Maintenance Program (CAMP)

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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This report describes the experimental works and the assessment of the predictability of RELAP5/MOD3.2 for the reflux condensation experiment in the presence of noncondensible gases in a vertical tube having the same outer diameter of the U-tube riser in Korean standard nuclear power plant (KSNPP). The reflux condensation experiment is performed in conditions of the low pressure, low flow and the high mass fraction of noncondensible gas representing the situation of the loss-of-residual-heat-removal (LORHR) accident during mid-loop operation.

The test facility is composed of the mixture gas generation part and the reflux condensation part. The test section in the latter part is a vertical tube with 19.05mm diameter and 2.4m length surrounded by the coolant block. Reflux condensation occurs in the range of very small flow rates because of the flooding limit. Therefore, the injected steam is completely condensed in the vertical tube.

The flooding as the upper limit of reflux condensation occurs in lower mixture upward flow rate than that of Wallis' correlation. The heat transfer coefficients near the tube inlet increase as the inlet steam flow rate and the system pressure increase. In the presence of noncondensible gas, the heat transfer capability is dramatically decreased. An empirical correlation is developed using the local data of heat transfer coefficients. The degradation factor in the correlation is expressed with four nondimensional parameters. It turns out that the Jacob number and the film Reynolds number are dominant parameters.

A non-iterative condensation model is developed to predict the steam condensation heat transfer in the presence of the noncondensible gases, which is different from the existing iterative model in the RELAP5/MOD3.2 code. The existing models, default model (Colburn-Hagen model) and alternative model (Chato-UCB model), in RELAP5/MOD3.2 are assessed with the reflux condensation data. The heat transfer coefficients estimated by the present non-iterative model and by the existing models of the standard RELAP5/MOD3.2 code are compared with the present experimental data. The default model and the alternative model under-predicts and over-predicts, respectively. The non-iterative model better predicts than the default and alternative models.

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