United States Nuclear Regulatory Commission - Protecting People and the Environment

Post-Test-Analysis and Nodalization Studies of OECD LOFT Experiment LP–LB–1 With RELAP5/MOD2 CY36–02 (NUREG/IA–0089, PSI–Bericht Nr. 91)

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Publication Information

Date Published: October 1992

Prepared by:
D. Lübbesmeyer

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

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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Experiment LP-LB-1 was conducted on February 3, 1984, in the Loss-Of-Fluid-Test (LOFT) facility at the Idaho National Engineering Laboratory under the auspicies of the OECD. It simulated a double-ended offset shear of one inlet pipe in a four loop PWR and was initiated from conditions representative of licensing limits in a PWR. Additional boundary conditions for the simulation were loss of offsite power, rapid primary coolant pump coastdown, and UK minimum safeguard emergency core coolant injection rates.

This report presents the results and analysis of ten post-test calculations of the experiment LP-LB-1 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 the "standard nodalization" as more or less used by the code developers at EG&G, for different nodalization studies, we hate 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.

Generally, the code has calculated most of the thermohydraulic parameters of the LOFT-experiment LP-LB-1 within an accuracy of approximately ±20%, but always has underpredicted the cladding temperatures up to a value of 150 K. Except for the cladding temperatures, only small discrepancies have been observed between the results of calculations using different nodalizations. Reduced numbers of volumes and junctions usually have decreased the running time of the problem but in one case, due to numerical instabilities even has prolonged it a little bit. The time behaviours of the cladding temperatures have been significantly affected by the choosen nodalizations but surprisingly, the results for the cases with a reduced number of volumes and junctions seem to be slightly closer to the experimental data.

With respect to top-down rewetting, one of the key-events of experiment LP-LB-1 during the blow-down phase, RELAP5/Mod2 was not at all able to predict this phenomenon.

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