Validation of the coupled calculation between RELAP5 STH code and Ansys FLUENT CFD code

This report, carried out at the DICI (Dipartimento di Ingegneria Civile e Industriale) of the University of, illustrates the activities performed in the framework of the AdP2013 and dealing with the application of an in-house developed coupling methodology between a modified version of RELAP5/Mod3.3 and Fluent commercial CFD code, to the NACIE (Natural Circulation Experiment) LBE experimental loop (built and located at the ENEA Brasimone research centre). In particular, the first part of this work deals with the application of the developed two way explicit coupling tool to a simulation representative of a natural circulation test with a heating power of about 21.5 kW. The second part of the document deals with the development of an implicit coupling scheme in order to increase the numerical stability of the coupled simulations and its application to simulate an experimental test representative of an isothermal gas enhanced circulation test. Obtained results show a good agreement with the experimental data; moreover, the robustness of the developed implicit scheme allows the use of larger time steps reducing significantly the computational time. Finally, in order to further reduce the computational efforts, the User Defined Function realized for the Fluent code to receive b.c. data from RELAP5 and to send b.c data to RELAP5 for each CFD time step was parallelized allowing to run the CFD code using multiple processor and thus reducing the computational time especially for those simulations involving a 3D-CFD geometrical domain.