Reactor Trip Analysis at Krško Nuclear Power Plant (NUREG/IA-0221)

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Publication Information

Manuscript Completed: July 2007
Date Published: January 2010

Prepared by:
Andrej Prošek, Borut Mavko

Jožef Stefan Institute
Jamova cesta 39
SI-1000 Ljubljana, Slovenia

A. Calvo, NRC Project Manager

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

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

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Abstract

The reactor trip, which occurred at the Krško Nuclear Power Plant (NPP), on April 10, 2005, has been analyzed with the RELAP5/MOD3.3 Patch 03 computer code, the most current version released. The analysis was performed by the Jožef Stefan Institute, Reactor Engineering Division. The RELAP5 input model delivered by the Krško NPP was used. The purpose of the analysis was to evaluate the RELAP5 computer code against plant-measured data and validate the RELAP5 input model for the Krško NPP, which is a two-loop Westinghouse pressurized-water reactor. The event analyzed was a malfunction, which occurred during a power reduction sequence when regular, periodic testing of the turbine valves was performed. The malfunction led to a plant trip. All of the plant's safety systems responded according to the design specification, so the event caused no hazard to the environment or the plant staff and did not challenge the plant's safety. The RELAP5/MOD3.3 Patch 03 calculations agree very well with the plant-measured data, when operator actions are modeled properly. The analysis found that the long-term transient evolution is very sensitive to the steamflow after reactor trip. However, the calculation showed that the measured steamflow was larger than the calculated steam generated by available decay heat and therefore measurement of steamflow after reactor trip was not reliable. Therefore, the steamflow was tuned to obtain as good a match to the measured SG pressure as possible. The value of steamflow was physically reasonable but it is not known if this value reflects the reality of the transient.