United States Nuclear Regulatory Commission - Protecting People and the Environment

Assessment of RELAP5/MOD3.3 against Single Main Steam Isolation Valve Closure Events at the Krško Nuclear Power Plant (NUREG/IA-0223)

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Publication Information

Manuscript Completed: November 2003
Date Published: April 2010

Prepared by:
Iztok Parzer, Borut Mavko

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

A. Calvo, NRC Project Manager

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

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Abstract

Recently, the last frozen version of RELAP5/MOD3.3 was released, before the code will be merged with another best estimate thermal-hydraulic system code (TRAC) into an integrated thermal-hydraulic code. Therefore, it is of utmost importance to assess models built using the RELAP5/MOD3.3 code against real plant transients before the code merger.

The paper presents the RELAP5/MOD3.3 analysis of two abnormal events that occurred at the Krško Nuclear Power Plant in September 1995 and January 1997, before replacement of the steam generators (SGs) in 2000. The events resulted from the sudden closure of a main steam isolation valve (MSIV). In the September 1995 event, a malfunction in the MSIV in SG-1 caused inadvertent valve closure. In the January 1997 event, the valve stem in SG-2 was broken, also causing sudden valve closure. Researchers obtained valuable plant data from these real plant transients and performed the RELAP5/MOD3.3 code assessment.

The analyses used a full two-loop plant model, developed at the Jožef Stefan Institute in Ljubljana, Slovenia. The model assumed old Westinghouse D4-type SGs with preheaters. It assumed that 18-percent of the U-tubes were plugged in both SGs.

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