United States Nuclear Regulatory Commission - Protecting People and the Environment

Post-Test-Analysis and Nodalization Studies of OECD LOFT Experiment LP–02–6 With RELAP5/MOD2 CY36–02 (NUREG/IA–0088, PSI–Bericht Nr. 92)

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Publication Information

Date Published: August 1992

Prepared by:
D. Lübbesmeyer

Paul Scherrer Institute (PSI)
Wurenlingen and Villigen
5232 Villigen PSI
Switzerland

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

Experiment LP-02-6 was conducted on October 3, 1983. It was the first large-break loss-of-coolant accident (LOCA) simulation and the fourth experiment at all conducted in the Loss-Of-Fluid-Test (LOFT) facility at the Idaho National Engineering Laboratory under the auspicies of the OECD. This experiment, which was designed to meet requirements outlined by the USNRC as specified in the OECD LOFT Project Agreement, simulated a double-ended off-set shear of a commercial pressurized water reactor (PWR) main coolant inlet pipe coincident with loss of offsite power. Experiment LP-02-6 addressed the response of a PWR to conditions closely resembling a USNRC "Design Basis Accident" in that prepressurized fuel rods were installed and minimum US emergency coolant injections were used.

This report presents the results and analysis of nine post-test calculations of the experiment LP-02-6 by using the RELAP5/Mod2 cy36-02 computer code with different nodalizations; these calculations have been performed within the International Code Assessment Program (ICAP). Starting with a "standard nodalization" as more or less used by the code developers at EG&G, we have reduced the number of volumes and junctions (especially in the pressurizer, the steam generator secondary side and the intact loop) as well as the number of radial zones in the fuel rods, for different nodalization studies.

Generally, the code has calculated most of the thermohydraulic parameters of the LOFT-experiment within an accuracy of approximately ±20%. For the cladding temperatures, these deviations are sometimes higher, but the code has never underpredicted the peak cladding temperatures significantly. Except for the cladding temperatures, only small discrepancies have been observed for the other main parameters of the results of runs using different nodalizations but reduced numbers of volumes and junctions usually have lead to a decreased running time for the problem.

The time behaviours of the cladding temperatures have been significantly affected by the choosen nodalizations. The most comparable results with the experimental data have been achieved by using a medium number of nodes.

With respect to high mass-flux, early bottom-up rewetting, one of the key-events of experiment LP-02-6 as well as of most of the other LOFT large break experiments, RELAP5/Mod2 was not able to predict this phenomenon except with a certain manipulation by initiating the reflood option.

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