Experimental and theoretical MHD performance of a round pipe with an NaK-compatible Al2O3 coating

Abstract

Abstract A key feasibility issue for the international thermonuclear experimental reactor (ITER) vanadium/lithium breeding blanket is the question of insulator coatings. Design calculations show that an electrically insulating layer is necessary to maintain an acceptably low magnetohydrodynamic (MHD) pressure drop. To begin experimental investigations of the MHD performance of candidate insulator materials and the technology for putting them in place, a new test section was prepared. Aluminum oxide was chosen as the first candidate insulating material because it may be used in combination with NaK in the ITER vacuum vessel and/or the divertor and MHD performance tests could begin early in Argonne's liquid-metal experiment (ALEX) because NaK was already the working fluid in use. Details on the methods used to produce the aluminum oxide layer, as well as the microstructures of the coating and the aluminide sublayer, are presented and discussed. The overall MHD pressure drop, local MHD pressure gradient, local transverse MHD pressure difference and surface voltage distributions in both the circumferential and axial directions are reported and discussed. The overall MHD pressure drop, measured at 30 and 85°C, was higher than the perfectly insulating case, but many times lower than the bare-wall case. It was demonstrated that the increase in MHD pressure drop above the theoretical values is largely due to the presence of instrumentation penetrations in the test section walls, which provide current paths from the fluid to the walls of the pipe, resulting in local areas of near-bare-wall MHD pressure drop.