RELAP5 Extended Station Blackout Analyses

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Publication Information

Manuscript Completed: January 2013
Date Published: March 2013

Prepared by:
A. Prošek, B. 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)

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

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Abstract

Following the accident at the nuclear power plant Fukushima in Japan the stress test were performed in European countries. Considering the stress tests specifications it was necessary to evaluate the consequences of loss of safety functions from any initiating event (earthquake or flooding) for loss of electrical power, including Station Blackout (SBO), loss of the ultimate heat sink or combination of both. In this report long term station blackout sequences for Krško twoloop pressurized water reactor with loss of normal or all secondary side heat sinks were performed. For calculations the latest RELAP5/MOD3.3 Patch 04 computer code was used. The verified standard RELAP5/MOD3.3 input model delivered by Krško nuclear power plant was used.

SBO scenario involves a loss of offsite power, failure of the redundant emergency diesel generators, failure of alternate current power restoration and the eventual degradation of the reactor coolant pump seals resulting in a long term loss of coolant. In the study different reactor coolant pump seal leaks were studied due to SBO. Besides, scenarios were performed for different primary side depressurizations performed by operator through the secondary side power operated relief valves, providing that turbine driven auxiliary feedwater pump is available. Finally, the effect of having some injection into the reactor coolant system was also evaluated. It can be concluded that calculated results obtained by RELAP5 give good indication about time available before core degradation started. The results suggest that RELAP5 can be used for extended SBO studies until core damage started. It is especially useful in studying maintaining core cooling function and time available before core uncovers as part of severe accident management. The benefit of using RELAP5 is in the fact that best estimate system codes are more accurate than severe accident codes in phases before core degradation started.