During the loss-of-flow-driven transient overpower experiments at the CABRI experiments, molten steel may contact the test channel wall for ∼20 s. Afterward, the test channel is again cooled down. The test channel wall is made from niobium, which was chosen because of its high melting point and low thermal neutron absorption cross section. However, liquid steel dissolves niobium. Tests revealed a solubility of niobium in steel and the dynamics of the solution process, which requires protection against the attack of steel. Surface oxidation of the niobium tube can be excluded. Before forming an oxide, niobium takes up oxygen and embrittles. Therefore, carbides and nitrides of refractories were examined. Solubility of TiC in steel is limited but still too high for a thin coating. The solubility of TiN is negligible within the considered temperature region. However, TiN grows with a basaltic structure on niobium and the crevices between the columnar crystals provide channels through which the liquid steel penetrates and reaches the substratum. Furthermore, TiN adheres poorly on niobium. Consequently, a multilayer coating was suggested, with a NbC basic layer for a good adhesion on niobium and two TiN layers that are interrupted by an intermediate TiC layer. Melt tests with liquid steel on coated specimens demonstrated the protective function of such multicoatings. Mandatory specifications require a pore-free precipitation of the coating material, no surface fissures of the substratum, and a surface roughness of the substratum well below the coating thickness. The sublayer has to reach a thickness of at least 1 (μm except for the top TiN layer, which has to be a minimum of 2 μm in order to cover the TiC dentrides. A niobium wire was installed coaxially in the niobium tube during the coating procedure. The coprecipitated coating on the wire proved to image the coating on the tube, providing an appropriate, nondestructive quality and thickness control for the coating on the tubes. Test coatings revealed that coatings can be completed or amended in a second step, even if the tubes are removed intermediately from the coating furnace. During the CABRI experiments, the coatings are subjected to sodium. Appropriate tests show that sodium does not deteriorate the protective function of the suggested multicoating, provided that the oxygen concentration of the sodium is limited. The protection of a multilayer coating against a steel attack can be extended if Al2O3 is applied as a top layer.