This work describes the coupling methodology between a modified version of RELAP5/Mod3.3 and ANSYS Fluent CFD code developed at the University of Pisa. The described coupling procedure can be classified as "two-way," nonoverlapping, "online" coupling. In this work, a semi-implicit numerical scheme has been implemented, giving greater stability to the simulations. A MATLAB script manages both the codes, oversees the reading and writing of the boundary conditions at the interfaces, and handles the exchange of data. A new tool was used to control the Fluent session, allowing a reduction of the time required for the exchange of data. The coupling tool was used to simulate a loop system (NACIE facility) and a pool system (CIRCE facility), both working with Lead Bismuth Eutectic and located at ENEA Brasimone Research Centre. Some modifications in the coupling procedure turned out to be necessary to apply the methodology in the pool system. In this paper, the comparison between the obtained coupled numerical results and the experimental data is presented. The good agreement between experiments and calculations evinces the capability of the coupled calculation to model correctly the involved phenomena.

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In the frame of the SESAME project, a benchmarking activity was proposed to validate the existing system thermal-hydraulics codes for Heavy Liquid Metal reactors. More specifically, blind simulations on three well-defined experiments were carried out on the NACIE-UP facility, using CATHARE by ENEA, ATHLET by GRS, RELAP5-3D by University of Roma and RELAP5/Mod3.3 by University of Pisa. The numerical models were calibrated in terms of system thermal losses and gas enhanced circulation by means of the outcomes from specific experimental preliminary tests. The present discussion expose, compare and analyze the numerical results of some representative parameters (primary lead-bismuth eutectic (LBE) mass flow rate, temperatures and pressure) charaterizing the system behaviour in transiet scenarios in a “pre-test” blind numerical assessment.

This paper is related to fluid-structure interaction analysis of sodium cooled fast reactors core (Na-FBR). Sudden liquid evacuation between assemblies could lead to overall core movements (flowering and compaction) causing variations of core reactivity. The comprehension of the structure behavior during the evacuation could improve the knowledge about some SCRAMs for negative reactivity occurred in PHÉNIX reactor and could contribute on the study of the dynamic behavior of a FBR core. An experimental facility (PISE-2c) is designed composed by a Poly-methyl methacrylate hexagonal rods (2D-plan similitude with PHÉNIX assembly) with a very thin gap between assemblies. Another experimental device (PISE-1a) is designed and composed by a single hexagonal rod for testing the dynamic characteristics. Different experiments are envisaged: free vibrations and oscillations during water injection. A phenomenological analysis is reported showing the flow behavior in the gap and the structure response. Also computational simulations are presented in this paper. An efficient numerical method is used to solve Navier-Stokes equations coupled with structure dynamic equation. The numerical method is verified by the comparison of analytic models and experiments.

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.