This work describes the activity performed at the University of Pisa concerning the application of an in-house developed coupling methodology between a modified version of RELAP5/Mod.3.3 and the ANSYS Fluent commercial CFD code to a pool system. Mono-dimensional codes, like RELAP5, are commonly used for thermal-hydraulic analysis of entire complex systems. Nevertheless, their one-dimensional feature represents a limit in the analysis of such problems where significant 3D phenomena are involved. On the other hand, CFD codes standalone are usually employed to simulate relatively small domains. The use of System Thermal-Hydraulic + CFD coupled calculations can overcome these issues, allowing the simulation of a complete system, but with a part of the domain reproduced with the CFD code.
In this work, the coupled calculation technique was used to simulate a PLOHS + LOF transient in the HLM experimental facility CIRCE (CIRCulation Experiment), located at the ENEA Brasimone research centre. The paper initially calls up the coupling procedure adopted, consisting in a “two-way” coupling. MATLAB software, used as external interface, manages the exchange of data between the system and the CFD code. The numerical method adopted for the coupling is the implicit scheme. Then, the main features of the CIRCE facility are briefly described, so are the two computational domains employed in this study. In particular, the CFD code was used to model the CIRCE pool (8 m high) and the Decay Heat Removal (DHR) heat exchanger. Due to the long duration of the transient simulated, a 2D axial-symmetric domain was chosen in order to reduce the computational time. The test section, placed inside the pool and consisting in a heat source and a heat sink, and the secondary side of the heat exchanger, were modeled with RELAP5. The use of the coupling tool allowed to set realistic boundary conditions in the calculation, more representative of the experimental ones. The main numerical results obtained from the PLOHS + LOF coupled calculation were compared with experimental data. Calculated LBE mass flow rates in the test section and in the DHR showed good agreement with experimental data. Some discrepancies with respect to the experimental trends were noticed for LBE temperatures; these should be related to some simplifications introduced in the model. Nevertheless, obtained outcomes represent a preliminary guideline for the improvement of the modeling for future works.