United States Nuclear Regulatory Commission - Protecting People and the Environment

Assessment of RELAP5/MOD3.2 With the Semiscale Natural Circulation Experiment, S–NC–8B (NUREG/IA-0144)

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Publication Information

Date Published: August 1998

Prepared by:
Y. S. Bang, K. W.Seul, S. Lee, H. J. Kim

Korea Institute of Nuclear Safety
Advanced Reactor Dept.
P.O. Box 114
Yusung, Taejon
305–600 Korea

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

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

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Abstract

The predictability of RELAP5/MOD3.2 code is assessed for the natural circulation induced by small break loss of coolant accident in the pressurized water reactor by using Semiscale experiment S-NC-8B. The major thermal-hydraulic phenomena observed in the experiment are investigated to evaluate the code predictability for SBLOCA specific thermal-hydraulic phenomena. The Semiscale Mod-2A facility is modeled, as a base case, by using single core channel model. The base case calculation is executed, the result is compared with the experiment data and code predictability on the important thermal-hydraulic phenomena is discussed.

Sensitivity calculations are attempted to figure out the problems in base case prediction, and to find out the effects of two core channel model and ECCMIX component model on the improvement code predictability. The important thermal-hydraulic phenomena include system depressurization, break flow in saturated and stratified conditions, natural circulation in two-phase mode and reflux mode, loop seal behavior at crossover legs, and accumulator injection behavior. The base calculation shows the RELAP5/MOD3.2 can predict the overall thermal-hydraulic behavior such as system depressurizarion, with the exception of underprediction of saturated break flow, deviation of loop seal behavior, and resultant discrepancy in core thermal response. Two core channel model can improve the predictability on loop seal behavior.

ECCMIX component can improve an early accumulator injection behavior and core thermal response. However, discontinuous accumulator injection is one of the problems. in two core channel model calculation. To resolve the accumulator injection problem, the extensive modeling study and/or code model improvement are needed.

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