Thermal-hydraulic test and analysis of the ENEA TF conductor sample for the EU DEMO fusion reactor

The ENEA conductor for the EU DEMO Nb3Sn Toroidal Field (TF) magnets, cooled by supercritical He, features a rectangular cross section with two small pressure relief channels (“holes”), separated from the cable bundle by means of a flat spiral, twisted together with the last cabling stage. A well instrumented short sample of the ENEA TF conductor has been tested in SULTAN at SPC, Villigen (Switzerland) in 2016, aimed at its thermal-hydraulic characterization, and the test results are presented here. A correlation for the friction factor in the small holes is derived, best fitting the results of a set of computational- fluid dynamics (CFD) simulations. The new correlation (combined with existing correlations for the He friction factor in the bundle region) is shown to allow a proper reproduction of the measured hydraulic characteristic of the conductor. The heat slug propagation tests are used to calibrate the hole-to-bundle heat transfer coefficient in the 4C thermal-hydraulic code and to estimate the characteristic length for the homogenization of the He temperature on the conductor cross section, following a localized thermal perturbation.