The results of performance analyses of a refractory Nb-1Zr/C-103 vapor anode multi-tube alkali-metal thermal-to-electric conversion (AMTEC) cell are presented and discussed. This cell could be used with a radioisotope heater unit To provide electric power from tens to a few hundreds of watts. In the tens of kilowatts electric range, the AMTEC cells could be used with a parabolic solar concentrator or a nuclear reactor heat source. The present cell measures 41.27 mm in diameter and is 125.3 mm high and has eight sodium beta"-alumina solid electrolyte (BASE) tubes, which are connected electrically in series to provide a load voltage in excess of 3 V. The hot structure of the cell, including the hot plate, the BASE tube support plate, the hot plenum wall and conduction stud, the evaporator standoff and porous wick and the side wall facing the BASE cubes, is made of Nb-1Zr. The cell's colder structure, which includes the condenser structure, the interior thermal radiation shield, the casing and wick of the liquid sodium return artery and the side wall above the BASE tubes, is made of C-103. This niobium alloy is stronger and has a lower thermal conductivity than Nb-1Zr, reducing the parasitic heat conduction losses in the cell wall, hence enhancing the cell's performance. The base cell weighs 163.4 g and delivers 7 W-e at 17% conversion efficiency and load voltage of 3.3 V (cell specific mass of 23.4 g/W-e). These performance parameters were for TiN BASE electrodes characterized by B = 75 AK(1/2)/m(2) Pa and G = 50, assuming a BASE/electrode contact resistance of 0.06 Omega cm(2) and a BASE braze structure leakage resistance of 3 Omega. Also, the inner surfaces of the thermal radiation shield and the cell wall above the BASE tubes were covered with low emissivity rhodium. The temperatures of the BASE brazes and the evaporator were below the recommended design limits (1123 and 1033 K, respectively), and the temperature margin was greater than or equal to +20 K to avoid sodium condensation inside the BASE tube. shorting the cell. When high performance electrodes. characterized by B = 120 AK(1/2)/m(2) Pa and G = 10, were used, the cell's electric power increased to 8.38 W, at 3.5 V, and the efficiency increased to 18.8%, decreasing the specific mass of the cell to 19.7 g/W-e without exceeding any of the design temperature limits. (C) 2000 Elsevier Science Ltd. All rights reserved.