simulation, coupling, interface, testing, CFD, CFX, RELAP5.


The modern direction in the study of the safety of nuclear power plants is to ensure the maximum level of detail in process modeling with a satisfactory level of computational resources. One of the approaches of such task solving is the coupled use of special software with the required levels of detail, for example, systemic thermohydraulic codes with computational hydrodynamics codes.

This article describes the developed coupling module between the system thermohydraulic code RELAP5/Mod. 3.2 and the CFX computational hydrodynamics code, using an external explicit interfacing scheme without overlap, in which CFX acts as the main code and RELAP5 as the secondary one. The development of this module is part of the work on integrated coupled modeling of severe accidents and makes it possible to model the defining processes and phenomena that occur during a severe accident within the reactor vessel using the CFD model, and the elements of the reactor unit using the RELAP5 model with an acceptable cost of resources and with a high level of detail. To check the coupling technology, the module was tested on simplified models containing serially connected RELAP5- and CFD-models of horizontal pipes, as well as on a closed loop. In view of the further use of the interface module on a full-fledged model of the VVER-1000, testing is performed for three main interface types: "input flow", "output flow" and their combination "input/output flow" on a closed loop. After completion of the testing, the accuracy and credibility of the results of the coupled calculations was evaluated by comparing them with the results of autonomous RELAP5 calculations. Results of the tests calculations support the possibility of using the RELAP5-CFX coupling module on more complex and large-scale RELAP5 and CFD models.


Nuclear Safety Analysis Division 2001, RELAP5/Mod3.3 code manual Volume I: Code Structure, System Models, and Solution Methods., vol. 1.

ATHLET Mod 3.0 Cycle A. User’s Manual, (2012). GRS-P-1/ Vol. 1 Rev. 6 – GRS, Garching bei München, Germany.

TRACE V5. 0, 2007.Theory Manual - Field Equations, Solution Methods, and Physical Models, USNRC, Washington DC.

Lavialle G 2006, CATHARE 2 V2.5_1: User’s Manual, SSTH/LDAS/EM/2005-035.

D. Bestion, System Code Models and Capabilities, Thicket-2008, Italy, 2008, p. 26.

Y. Onyshchuk, S. Klevtsov. Problems of modeling in-vessel severe accident phase. Scientific journal “POWER ENGINEERING: economics, technique, ecology”. 2021. №3. pp. 111 – 121.

ANSYS CFX-Solver Manager User's Guide. ANSYS, Inc. Release 18.2. ANSYS Drive Canonsburg, ANSYS, Inc., 2017, p. 200.

CFX Command Language. CFX-5.5.1. p 22. URL:

Miguel A. Fernandez: Coupling schemes for incompressible fluid structure interaction: implicit, semi-implicit and explicit, (2011).