RELAP5/MOD3.3 analysis of steam generator tube rupture (SGTR) accident for NPP Krško (NUREG/IA-0442)

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Publication Information

Manuscript Completed: December 2013
Date Published: March 2014

Prepared by:
V. Benčik, D. Grgić

University of Zagreb, Faculty of Electrical Engineering and Computing
Unska 3
10000 Zagreb, Croatia

K. Tien, NRC Project Manager

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

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

Steam Generator Tube Rupture (SGTR) event leads to contamination of the secondary side due to leakage of the radioactive coolant from the Reactor Coolant System (RCS) through the broken Steam Generator (SG) tube(s). Unlike other loss of coolant accidents, an early operator action is necessary to prevent radiological release to environment. The authors have analyzed SGTR for NPP Krško (NEK) using RELAP5/MOD3.3 code for two basic scenarios; i.e. with and without offsite power available. The plant model has been updated taking into account the Resistance Temperature Detector Bypass Elimination (RTDBE) project realized during the 2013 outage. The actions from the standard Emergency Operating Procedures (EOPs) were modelled and the efficiency of operator actions to prevent radiological release to environment was evaluated. The time of the start of the operator action was selected as a critical parameter influencing occurrence of the release of the contaminated inventory (steam and liquid). In order to stop the steam release from the ruptured SG for the scenario with offsite power available the operator action has to be taken 15 minutes after transient begin, whereas the liquid solid condition and the liquid discharge can be prevented for operator action performed not later than 45 minutes after transient begin. For the scenario with offsite power not available the operator action has to be performed not later than 20 minutes to prevent the broken SG liquid solid condition and the liquid discharge. For both cases the operator action successfully stops the primary to secondary leakage and the inventory release to the environment.