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A material system that resists lead-bismuth attack and retains its strength at very high temperatures has been developed that enables increased outlet temperature and the promise of allowing increased coolant velocity and efficiency of lead- and lead-bismuth-cooled reactors if the behavior reported here is confirmed by long-term tests. The development of this system represents an enabling technology for lead-bismuth-cooled reactors. The system is a functionally graded composite (FGC), with separate layers engineered to perform corrosion resistance and structural functions. Alloy F91 was chosen as the structural layer of the composite because of its strength and radiation resistance. An Fe-12Cr-2Si alloy was developed based on previous work in the Fe-Cr-Si system, and was used as the corrosion-resistant cladding layer because of its chemical similarity to F91 and its superior corrosion resistance in lead and lead-bismuth in both oxidizing and reducing environments. The availability of the FGC will have significant impacts on lead-bismuth reactor design. The-allowable increases in outlet temperature and coolant velocity lead to a large increase in power density--either to a smaller core for the same power rating or to more power output for the same-size core. In this paper, we report on the overall design of the FGC. We also discuss the general implications for lead-bismuth-cooled reactor design. In a future paper, we will discuss the fabrication and the initial evaluation of the actual product produced using commercial processing methods. [ABSTRACT FROM AUTHOR]