CFD pre-test analysis of the fuel pin bundle simulator experiment in the NACIE-UP HLM facility

In the context of the studies on GEN. IV/ADS nuclear systems, the correct evaluations of the temperature distribution in the fuel pin bundle is of central interest. In particular, the use of lead or lead-bismuth eutectic (LBE) as coolant for the new generation fast reactors is one of the most promising choices. At ENEA-Brasimone R.C., large experimental facilities exist to study HLM free, forced and mixed convection in loops and pools: e.g. NACIE-UP is a large scale LBE loop for mixed convection experiments. In the context of the SEARCH FP7 project, an experiment has to be performed in the NACIE-UP facility to assess the coolability of a 19-pin wire-wrapped electrical bundle (Fuel Pin Simulator, FPS), with heat flux up to 1 MW/m2. The bundle is representative of the one adopted in the MYRRHA concept. A CFD analysis of fluid flow and heat transfer was carried out in the heavy liquid metal (LBE) cooled bundle test section of the NACIE-UP facility. The model includes the details of the wire-spacers as well as the entry region of the test section. A turbulence closure approach is adopted for all the simulations with = 3.5-107 nodes and a resolution of y+ = 1 - 4 at the wall in the range of interest. Results are compared with the up-to-date correlations on pressure loss and heat transfer and the experimental range is completely explored by CFD. The thermal structures of the test section are modelled and the role of conjugate heat transfer was assessed. Several highlights emerged from the numerical study for the experimental campaign. In particular, the accuracy in the measurement of heat transfer between rods and fluid was evidenced as weak point of the experimental test matrix. As a consequence the test matrix was modified.