United States Nuclear Regulatory Commission - Protecting People and the Environment

Assessment of RELAP5/MOD2 Cycle 36.04, Against the Loviisa–2 Stuck-Open Turbine By-Pass Valve Transient on September 1, 1981 (NUREG/IA–0047)

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Publication Information

Date Published: March 1992

Prepared by:
Vesa Yrjola

Technical Research Centre of Finland
Nuclear Engineering Laboratory
P.O. Box 169
SF–00181 Helsinki, Finland

Prepared as part of:
The Agreement on Research Participation and Technical Exchange
under the International Thermal-Hydraulic Code Assessment
and Application Program (ICAP)

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

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Abstract

An overcooling type transient that took place in the LOVIISA Unit 2 has been analyzed using the RELAP5/MOD2 code. The code version was cycle 36.04.

The Loviisa Power Plant consists of two Soviet VVER-440 type pressurized water reactors having a net electric output of 445 MWe each. In VVER-440 reactors, the primary circuit comprises six parallel loops, each with a horizontal steam generator, a main circulation pump and main loop isolation gate valves. The reactor has significant differences compared to a typical western PWR, such as loop seals both in the hot and cold leg and horizontal steam generators.

The transient that occurred on September 1, 1981 was initiated from full power by a reactor trip. Incorrect operation of the level gauges in four steam generators caused the trip signal. An associated stuck-open failure of one turbine by-pass valve caused a fast cool-down. The high pressure safety injection started to operate, but was quickly turned off by the operator. The downcomer temperature decreased from 265 °C to 215 °C in fifteen minutes. The cooling down ceased when the operator closed the shut-off valve of the open by-pass line.

Although the plant data are not gathered as comprehensively as those from the extensively instrumented test facilities, the real plant transients are important in order to verify the scaling capability of the current one-dimensional codes to large three-dimensional power plants. The transient data together with the start-up commissioning tests also form a good data base when the applicability of the nodalization model for accident analysis is tested.

This work was performed at the Technical Research Centre of Finland (VTT) in co-operation with the utility Imatran Voima Oy (IVO), which owns and operates these plants. Many people in both organizations have contributed to the work and their support and assistance is acknowledged.

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