Benchmark calculation of the MEGAPIE target (M1)

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Maciocco, Luca
Moreau, Vincent
Sorrentino, Luca
Buono, Stefano
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The benchmark calculations performed by CRS4 with Star-CD on a reference geometry of the MEGAPIE target are presented in this report (benchmark M1). Scope of the benchmark is a comparison of the results obtained by the various partners involved in the MEGAPIE project using different codes and turbulence modelling approaches. The considered target geometry is the one with the final part of the guide tube slanted at an angle of about 9 degrees. The Pb-Bi flow in the last 2150 mm of the target have been simulated, including the calculation of the thermal field in all the solid structures (window, hull and flow guide). Due to geometrical symmetry, only half of the real domain was considered. Turbulence was simulated using a Chen k-ε model, combined with a Two-layer model in the most critical near-wall regions (window and flow guide in the spallation region) and with Wall Functions along the riser and the down-comer. Modified wall functions for low Prandtl number fluids were implemented. Results are presented for both cases with the beam footprint major axis parallel (benchmark M1.0) and normal (benchmark M1.1) to the guide-tube slant. In order to estimate the effect of the variation of the turbulent Prandtl number on the heat exchange, two calculation have been performed, one with Prt = 0.9 and one using a relationship Prt = f(Ret, Pr), yielding a locally variable turbulent Prandtl number. Results show a very complex flow pattern in the spallation region, with 3D vortex structures being generated in the reversing region and dragged along the rising duct. In case M1.0 with Prt = 0.9, results show maximum window temperatures of 521 °C and 487 °C in the external and internal side respectively, with a maximum Pb-Bi temperature of 486 °C located nearby the window centre. The maximum flow velocity is 1.35 m/s. A significant heat exchange takes place across the 1.5 mm thick flow guide, causing a mean temperature increase along the down-comer of about 34 °C. Due to the high Reynolds number of the flow, the effect of using a variable Prt is limited to near wall regions, where the heat exchange is slightly reduced. The combination of a lower heat exchange across the flow guide (resulting in a lower temperature increase of the Pb-Bi along the down-comer) and a worse window cooling yielded a maximum window temperature of 524°C, namely 3 °C more than in the case with Prt = 0.9. In case M1.1, maximum window temperatures of 447 °C and 414 oC were found using Prt = 0.9 with a maximum Pb-Bi temperature of 423 °C located in the central part of the spallation region. Using a variable Prt, window temperatures increased of about 2 °C while a 1 °C lower maximum Pb-Bi temperature was found.
Megapie project , turbulence model , spallation target , CFD