A benchmark study based on a representative design of the megapie spallation source target

Several groups are performing Computational Fluid Dynamics (CFD) simulations to assess the feasibility of cooling the liquid-metal spallation source target currently being designed for installation in the SINQ facility at the Paul Scherrer Institute in Switzerland. Since it is not possible to fully verify the cooling principle by means of mock-up experiments, due to the difficulty in reproducing a volumetric heating effect equivalent to a proton beam (for SINQ, up to 1 GW/m3), the calculations must be validated against data from special effects tests and by means of Benchmarking exercises. The paper describes the results of the first Benchmark, which concentrates on the lower target region where the spallation reactions take place, and on the critical structure - the window - through which the beam enters the target enclosure. Four organisations have participated in the Benchmark, using two commercial CFD packages, but working independently. The exercise has demonstrated that, because of low-Prandtl-number effects, the use of CFD tools to calculate local heat transfer data in liquid-metal systems is not as reliable as with traditional coolants. In particular, adequate mesh resolution in thermal boundary layers is required, and the use of low-order differencing schemes is not recommended. These, and other points, are discussed in detail in the paper