United States Nuclear Regulatory Commission - Protecting People and the Environment

Analysis of the THETIS Boildown Experiments Using RELAP5/MOD2 (NUREG/IA-0014)

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Publication Information

Date Published: July 1989

Prepared by:
M.G. Croxford, P.C. Hall

Central Electricity Generating Board
Barnwood, Gloucester
GL4 7RS
United Kingdom

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

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

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Abstract

To test the ability of RELAI5iM-OD2 to model two phase mixture level and fuel rod heat transfer when the core has become partially uncovered, post test calculations have been carried out of a series of boildown tests in the AEEW THETIS out-of-pile test facility. This report describes the comparison between the code calculations and the test data.

Excellent agreement is obtained with mixture level boildown rates in tests at pressures of 40 bar and 20 bar. However at pressures below 10 bars the boildown rate is considerably overpredicted. A general tendency for RELAP5/IOD2 to overpredict void fraction below the two-phase mixture level is observed, which is traced to defects in the interphase drag models within the code. The heat up of exposed rods above the two-phase mixture level is satisfactorily calculated by the code.

The results support the use of RELAP5/MOD2 for analysis of high pressure core boildown events in PWRs.

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