Blockage fuel pin simulator experiments and simulation

摘要

This work reports the experimental results and post-test analysis carried out in the heavy liquid metal-operated NACIE-UP facility in the framework of the HORIZON2020 SESAME project. NACIE-UP is a rectangular loop, where the two vertical pipes, which work as riser and downcomer, are 8 m long and the two horizontal pipes are 2.4 m long. A prototypical Blockage Fuel Pin Simulator test section is installed in the bottom part of the riser, whereas a shell and tubes heat exchanger is placed in the upper part of the downcomer. Several degrees of internal blockage were tested in the facility. The degree of blockage is fixed by moving rod mechanism in the bottom part of the test sections and by blocking it with a cam asymmetric tool. Experimental data showed a maximum temperature closer to the blockage at 30 mm from the obstacle. The peak value is around 45 degrees C in the experimental conditions. Although the typical phenomenology is clear and it is repeated in each condition, lots of data were produced in different configurations by varying blockage degree and mass flow rate. This data base can be used to validate CFD codes and numerical methods applied to internal blockage in grid-spaced fuel assembly. A CFD numerical post-test validation activity is carried out on a limited number of cases. The CFD numerical model reproduces the geometry of the test section in a detailed way. The model is described in detail both in terms of geometry and of meshing technique. Different numerical models were tested with RANS and URANS simulations. For the single sector blockage numerical and experimental results are compared in detail. Results show a maximum in the temperature field just behind the blockage and this feature is also evidenced by experimental tests, although the quantitative comparison is not always fully satisfactory. Both numerical and experimental results show two separated main effects of the blockage: a local effect with a maximum in temperature field behind the blockage and an overall effect with a local maximum at the end of the active region in the blocked subchannels. The comparison of experimental and numerical data shows that an unsteady RANS simulation provides a lower temperature peak in the recirculation region downstream the blockage in better accordance with the experimental data; anyway, the width of the temperature peak is almost similar to the steady state RANS. The better agreement of the unsteady RANS simulation clearly indicates that a more accurate approach like a LES simulation could reduce the gap with the experimental results.