Coupling between System and CFD codes for the analysis of thermal-hydraulic phenomena relevant for LMFR

The focus of the present research activity was the experimental and numerical investigations, aiming to analyse some thermal-hydraulic phenomena peculiar of the heavy liquid metal-cooled systems like the Lead Fast Reactors (LFR). The two activities, experimental and numerical, were carried out in parallel during the three years of research and performed at the Department of Civil and Industrial Engineering (DICI) of the University of Pisa and at the Experimental Thermo-fluid-dynamic Laboratory (FSN-ING-TESP) of ENEA Brasimone R.C., in the framework of a collaboration between the two institutes. The main thermal-hydraulic aspects, treated during this work, were the core coolability in normal and accidental conditions, as well as the transition from forced to natural circulation and the consequent forced and natural convection heat transfer in Heavy Liquid Metal (HLM) systems. These topics are relevant for the safety objectives of the Generation-IV reactors, being related to the assessment of the decay heat removal capabilities by means of passive safety systems. The experimental activity focused on the evaluation of the heat transfer coefficient in a wire-spaced fuel bundle cooled by HLM. For this purpose, a 19 wire-spaced pins bundle was installed in the loop-type facility NACIE-UP located at ENEA Brasimone R.C., which employs lead-bismuth eutectic (LBE) as main coolant. Several experimental tests were performed in steady state and transient conditions, investigating natural and mixed circulation flow regimes. The 67 thermocouples in the test section allowed to measure wall and LBE temperatures and to calculate the heat transfer coefficient in five different sub-channels. Local and section-averaged Nusselt numbers were computed and graphically reported as a function of the Péclet number. These latter data were compared with correlations existing in literature, developed for rod bundles configurations cooled by liquid metals. The obtained results enrich the set of data available in the literature, which are essential for the characterization of this phenomenon and for the assessment of empirical correlations to describe it. The numerical studies concerned the development and the use of multi-scale approach, i.e. coupled calculation between a modified version of the system thermal-hydraulic code RELAP5/Mod3.3 and the CFD code ANSYS Fluent. Indeed, the use of multi-scale simulations have been emerging during the last years, due to their potentiality to model different levels of detail in a unique simulation and to account for the mutual feedbacks among the different sections of the overall system. In this context, a coupling methodology have been developed at the University of Pisa since 2012, characterized by “non-overlapping” domain decomposition, “on-line” and “two-ways” exchange of data and a semi-implicit numerical scheme. The MATLAB® software was used to manage the information transfer between the two codes. In this work, the semi-implicit numerical scheme was optimized, and the methodology was extended to heat transfer problems by implementing the coupling at the thermal boundaries with no hydraulic interaction. The developed tool was tested with simple test cases in loop-type and pool-type applications and was assessed through the comparison with the available experimental data. It was used to analyze the fuel pin simulator in the NACIE-UP facility, proving to be useful to calculate the total pressure losses, to investigate the flow distribution and the heat transfer in a wire-spaced core configuration. In the application to the CIRCE facility, the coupling methodology allowed to calculate the main thermal-hydraulic phenomena during an accidental scenario in a pool reactor, such as the transition from forced to natural circulation flow, the removal of residual heat thanks to dedicated passive systems and thermal stratification. The overall comparison of the numerical results with data obtained from NACIE-UP and CIRCE experiments was rather satisfactory, qualifying the use of the set-up coupling technique for the simulation of loop-type and pool-type systems. The developed multi-scale approach proved to bring remarkable advantages in the analysis of entire large/medium-sized facilities but with a focus on the 2D/3D flow characteristics in a limited portion of the domain.