Pre-test analysis of thermal-hydraulic behaviour of the NACIE facility for the characterization of a fuel pin bundle

This report, carried out at the DIMNP (Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione) of the University of Pisa in collaboration with ENEA Brasimone Research Centre, illustrates the pre-test thermo-fluidynamic analysis of the NACIE (Natural Circulation Experiment) facility, built at ENEA, in its new configuration of the heat exchanger and of the heater system. In particular, the first part of the work regards the study performed by RELAP5/Mod3.3 system code, modified in order to take into account LBE fluid properties and the appropriate convective heat transfer correlations. The code was employed to support the design of SEARCH experimental campaign, devoted to characterize the performance of a wire spaced fuel bundle relevant for MYRRHA facility (i.e. heat exchange and pressure drop) in shutdown conditions and providing data for code validation. For this purpose, low heat power simulations on NACIE facility have been performed to investigate the established loop natural flow rate and related parameters for increasing values of loop hydraulic resistance. The second part of the work concerns the first application, to a simplified representation of NACIE facility, of the coupling between the RELAP5 thermal-hydraulic system code and the CFD Fluent commercial code. Preliminary comparative analysis among the simulations performed by RELAP5-Fluent coupled codes and by RELAP5 stand-alone code showed very good agreement among them, giving confidence to this innovative coupling strategy..