RELAP5 Assessment Using Semiscale SBLOCA Test S–NH–1 (NUREG/IA-0099)

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Publication Information

Date Published: June 1993

Prepared by:
Euy-Joon Lee, Bub-Dong Chung, Hho-Jung Kim

Safety Analysis Department
Korea Institute of Nuclear Safety
P. O. Box 16, Daeduk Danji
Taejon, Korea

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

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

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Abstract

2-inch cold leg break test S-NH-1, conducted at the 1/1705 volume scaled facility Semiscale, was analyzed using RELAP5/MOD2 Cycle 36.04 and MOD3 Version 5m5. Loss of HPIS was assumed, and reactor trip occurred on a low PZR pressure signal (13.1 MPa), and pumps began an unpowered coastdown on SI signal (12.5 MPa). The system was recovered by opening ADV's when the PCT became higher than 811 K. Accumulator was finally injected into the system when the primary system pressure was less than 4.0 MPa. The experiment was terminated when the pressure reached the LPIS actuation set point.

RELAP5/MOD2 analysis demonstrated its capability to predict, with a sufficient accuracy, the main phenomena occurring in the depressurization transient, both from a qualitative and quantitative points of view. Nevertheless, several differences were noted regarding the break flow rate and inventory distribution due to deficiencies in two-phase choked flow model, horizontal stratification interfacial drag, and a CCFL model. The main reason for the core to remain nearly fully covered with the liquid was the under-prediction of the break flow by the code.

Several sensitivity calculations were tried using the MOD2 to improve the results by using the different options of break flow modeling(downward, homogeneous, and area increase). The break area compensating concept based on "the integrated break flow matching" gave the best results than downward junction and homogeneous options. And the MOD3 showed improvement in predicting a CCFL in SG and a heatup in the core.