LBLOCA Analysis in a Westinghouse PWR 3-Loop Design Using RELAP5/MOD3 (NUREG/IA-0195)

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Publication Information

Date Published: January 2001

Prepared by:
J.I. Sánchez, C.A. Lage, T. Núñez

Empresa Nacional del Uranio S.A.
Santiago Rusinol, 12
28040 Madrid

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 documents the analysis of a postulated Large Break Loss-of-Coolant Accident (LBLOCA) in a Westinghouse 3-Loop PWR design analysed using the "Best Estimate" code RELAP5/MOD3. This LBLOCA calculation represents ENUSA's contribution to the "Code Assessment and Maintenance Program" (CAMP).

The code used for this analysis is RELAP5/MOD3.2 - the latest CAMP version that was available to ENUSA when this study was performed. Nevertheless, since this version lacked of a reflood axial mesh renoding model, a developmental version was also used to analyse the reflood portion of the accident. This developmental version is RELAP5/MOD3.2 fg.

Five calculations were analysed, and the results of these were compared. The first case described in this document compares two runs made, using the same input deck, on two different platforms: the CRAY-YMP, and the ALPHA SERVER 4100. Both calculations were done with a basic nodalization: downcomer modelled with one 1-D component (collapsed downcomer), without the gap conductance model, and without the reflood model. Both cases were run with the original RELAP5/MOD3.2 version.

The second case was run to check the impact of modelling a quasi-three-dimensional downcomer, modelled with three 1-D components joined with cross-flow junctions. For this case, the base nodalization used in the first case was modified by adding the new downcomer model, and then compared with the previous results. This case was run on the ALPHA SERVER 4100 with version RELAP5/MOD3.2.

Finally, for the third case analysed, two additional input decks were prepared. Both of these included the three-dimensional downcomer nodalization and the gap conductance model. The first calculation was done with RELAP5/MOD3.2 with its standard heat transfer package. The second calculation was done with the reflood model activated, and using the developmental version RELAP5/MOD3.2 fg.

The first case analysed showed minimal differences between the results obtained on the two platforms used. The second case analysed showed the impact of the downcomer nodalization and the three-dimensional effects are shown to be non-negligible. The third case analysed clearly shows the need for a specific reflood model for this kind of transient, instead of the standard heat transfer package. The standard heat transfer package produces a very oscillatory behavior under reflooding conditions.

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