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# RELAP5 and TRACE Constitutive Relations Comparison 

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## Publication Information

**Manuscript Completed:** February 2020**Date Published:** January 2021

**Prepared by:**S. Shin*, J. Lee*, A. Shin**, and M. Cho**

*Korea Advanced Institute of Science and Technology (KAIST)N7-1 4409 291 Daehak-ro. Yuseong-guDaejeon 34141 Republic of Korea

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

A system thermal hydraulic analysis code is generally used for nuclear power plant safety analysis, and it solves the fluid field equations to model two phase flow. System codes typically solve mass, momentum and energy equations of each phase numerically by approximating a one dimensional flow. However, the constitutive relation based on empiricism must be included for mathematical closure of the approximated governing equations.

The best available approach to evaluate the constitutive relation models is to compare the code results with the data from separate effect tests (SET). However, available experimental data cover only a small fraction of the thermal hydraulic conditions in SET. Thus, there is no available method to evaluate the constitutive relation models that include all regimes expected in applications of the system codes. This study proposes a new method for evaluating the constitutive relations by comparing between RELAP5 and TRACE to analyze the constitutive relation models of the two codes.

RELAP5 and TRACE are very different in the implemented constitutive relation models even though the field equations are almost identical. To compare the different constitutive relations of the two codes, models and correlations of each code are studied and compared. Then constitutive relation modules are extracted in a separate computational environment. The developed platform is based on MATLAB and REFPROP, and it is proven to reproduce the same values of RELAP5 MOD3.3 patch5 and TRACE 5.0 patch5, respectively.

In the wall heat transfer models, different heat transfer regimes are occasionally chosen in the two codes and they reveal substantial deviations. In the interfacial heat transfer models, while RELAP5 includes a void generation term, TRACE does not consider the term, which results in a difference in the calculation of interfacial heat transfer. In the wall friction models, almost all regimes show good agreement between TRACE and RELAP5 except for slug and transition flow regimes. In the interfacial friction models, the interfacial drag coefficient has a larger difference than the wall drag coefficient between TRACE and RELAP5.

This work was supported by the Nuclear Safety Research Program through the Korea Foundation Of Nuclear Safety(KoFONS), granted financial resource from the Nuclear Safety and Security Commission(NSSC), Republic of Korea. (No. 1903002).

Page Last Reviewed/Updated Monday, February 08, 2021