An Advanced Radioisotope Power System (ARPS) in the 150 watt class is currently under development for space applications. The ARPS is a potential power source for potential NASA missions to Europa in 2003 and Pluto/Kuiper in 2004. The ARPS system, which has been described in recent literature (Hendricks, 1997), will utilize a multi-tube Alkali Metal Thermal to Electric Converter (AMTEC) cell design. Designated EPX-1, the cell is designed to survive ground handling and launch stresses and provide reliable power for the duration of the missions. The EPX-1 AMTEC cell is based on the PX series of demonstration cells (Borkowski, 1997, and Sievers, 1998). The planned operating temperatures and life requirements for these missions have made an all refractory metal design a requirement. Several other design enhancements have been implemented to meet specific mission requirements and improve predicted cell reliability. The EPX-1 cell will use a porous molybdenum capillary system for sodium transport and a Nb-1Zr cylindrical housing as containment. The number of different materials in the cell is being kept to a minimum. The beta”-alumina solid electrolyte (BASE) tubes will utilize both ceramic to metal seals and mechanical restraints to provide a high reliability connection to the cell structure with the metal to metal joints in the Nb-1Zr structure electron beam welded. The final design EPX-1 cell will be tested and the results analyzed to assure that the cells will meet the mission requirements.