Core Exit Temperature Response during an SBLOCA Event in the Ascó NPP (NUREG/IA-0498)

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Publication Information

Manuscript Completed: January 2018
Date Published: December 2018

Prepared by:
J. Freixa, V. Martínez-Quiroga, F. Reventós

Department of Physics
Universitat Politècnica de Catalunya
ETSEIB, Av. Diagonal 647, Pav. C
08028 Barcelona
Spain

Kirk 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:
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

Core exit temperature (CET) measurements play an important role in the sequence of actions during accidental conditions in pressurized water reactors (PWR). Given the difficulties in placing measurements in the core region, CET readings are used as criterion for the initiation of procedures because they can indicate a core heat up scenario. However, the CET response have some limitation in detecting inadequate core cooling, this is simply because the measurement is not taken in the position where the cladding excursion occurs and the superheated steam is generated. The Group of Thermal Hydraulics of the Technical University of Catalonia has conducted analytical studies to assess the performance of RELAP5 and the nodalization approaches for CET predictions through post-test analyses of the ROSA-2 Test 3 experiment. These studies have led to deriving a different nodalization approach for the core region and UP with a 3-dimensional representation.

The information learned with post-test analyses has been transferred to the NPP model through Kv scaling calculations. The scalability between the LSTF and the Ascó NPP has been analyzed in order to select the best scaling Kv factor for the specific scenario. The necessary changes in the nodalization in order to correctly reproduce the CET response, as indicated by the post-test calculations, have been added to the Ascó NPP model. The final step of the work presented here was to adapt the boundary conditions to a more realistic situation in the NPP in order to evaluate the relation between the CET and the PCT.

Finally, when the CET signal was activated in the Ascó NPP, the PCT measured was in the range of [777, 906] K depending on which CET measurement was considered as a reference. Due to the high temperatures at the time the set point is triggered, the effectiveness of the AM actions are at stake and therefore future studies should be focused on the analysis of the evolution of the scenarios after the CET signal is reached and the assessment of the CET set-point value.