A ceramic seal based on a eutectic mixture of Na2O and Al2O3 for use in Na/NiCl2 (ZEBRA) cells was developed. The seal was tested in galvanic cells. The reproducibility of the emf data shows that the seal is fully impervious and can sustain high alkali pressure up to 1000 degrees C without cracking or degradation. The cross-sectional microstructure revealed liquid phase formation during sealing and reaction bonding with the ceramic components of the cell.