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Assessments of 3D Components in System Codes Against Separate Effect Tests (NUREG/IA-0551)

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Publication Information

Manuscript Completed: August 2024
Date Published: June 2025

Prepared by:
Yunseok Lee*, Taewan Kim*, Jae Soon Kim**, Dong Gu Kang**

*Department of Safety Engineering,
Incheon National University,
119 Academy-ro, Yeonsu-gu,
Incheon 22012, Republic of Korea

**Korea Institute of Nuclear Safety (KINS)
62 Gwahak-ro, Yuseong-gu,
Daejeon 34142, Republic of Korea

A. Hsieh, 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

Availability Notice

Abstract

As research focus intensifies on employing multi-rod representation for loss-of-coolant accidents (LOCA) analysis, the use of three-dimensional (3D) components of system codes has become essential for modeling the subchannel-scale multi-rod scheme. However, utilizing system codes at this subchannel-scale level is a novel endeavor, necessitating a comprehensive assessment of their capability. This study assesses the 3D components within the TRACE V5 patch 7 and MARS-KS 2.0 system codes against two separate effect tests: GE 3X3 and PSBT bundle experiments. The assessment confirmed that the 3D components of both codes inadequately predicted the phasic distribution observed in the experiments. Notably, TRACE significantly overpredicted vapor in comparison to MARS-KS. The discrepancy in vapor estimation between TRACE and MARS-KS stemmed from differing approaches to interfacial drag calculations. TRACE’s application of a drift flux model for vertical and horizontal flows led to larger interfacial drag calculations, resulting in underestimation of crossflows. In contrast, MARS-KS employed a drag coefficient model for horizontal flow, yielding smaller interfacial drag and increased crossflows, dispersing vapor throughout the entire cross-section of the bundle. These findings underscored the critical role of crossflow in subchannel-scale analysis, highlighting the imperative need to enhance crossflow models in both codes for accurate prediction of phasic distribution in the bundle. As a proposed improvement, this study suggests adopting the subchannel-mixing model—a turbulence model commonly used in state-of-the-art subchannel analysis codes. Notably, enhancing crossflows significantly bolstered the general code predictability, particularly when implementing secondary transport of two-phase mixtures using the subchannel mixing model.

Page Last Reviewed/Updated Monday, June 16, 2025