United States Nuclear Regulatory Commission - Protecting People and the Environment

Evaluation of TRACE Spacer Grid Model with FLECHT-SEASET Reflood Test (NUREG/IA-0481)

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Publication Information

Manuscript Completed: December 2016
Date Published: August 2017

Prepared by:
Byung-Gil HUH, Ae-Ju CHEONG, Kyung Won LEE

Korea Institute of Nuclear Safety
62 Gwahak-ro, Yuseong-gu
Daejeon, 34142, Korea

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

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Abstract

The effects of spacer grid model of TRACE V5.0 patch 4 were assessed for the Full-Length Emergency Core Heat Transfer Separate Effects and Systems Effects Tests (FLECHT-SEASET) that were the typical reflood heat transfer tests. The FLECHT-SEASET test section was modeled in the VESSEL component of TRACE and the 161 heated rods in 17x17 assemblies were modeled as a single HTSTR component. The injected flow rates and temperatures were provided as a function of time by a FILL component connecting to the bottom of the lower plenum. The BREAK component was used to set the pressure boundary at the top of the test section. The main parameters of the spacer grid were defined by the experimental design data and eight egg-crate grids were modeled in the VESSEL component of TRACE. The calculations for eight tests of FLECHT-SEASET revealed that when the spacer grid model was used, the rod temperatures decreased and the rods were quenched at an earlier time in most other tests. In addition, as the reflood rate increased, the lower peak rod temperature and the earlier quenching time were predicted. When the test pressure was lower, the higher rod temperature and the later rod quenching were predicted since the liquid approached a relatively lower saturation temperature faster. When the subcooling degree was higher, the reduced degree of quenching time due to the spacer grid was further decreased because the higher subcooling degree enhanced the heat transfer rate. Sensitivity studies were performed to identify the effect of the grid locations and the difference from the spacer grid model of RELAP5. In this study, the effect of the spacer grid model in TRACE is shown well to simulate the FLECHT-SEASET reflood heat transfer tests. However, since the droplet breakup and the grid rewetting models were not fully implemented yet, there were some limitations in quantitatively predicting their effects. The comparison with the RELAP5 revealed that the current RELAP5 version had some errors in implementing the spacer grid model, and the effect of the spacer grid of TRACE could have been over-estimated for the rod temperature behaviors as compared with RELAP5.

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