A distinction can be made between an “asymptotic” nuclear symbiosis and a “transitional” nuclear symbiosis. With the currently projected nuclear energy growth, there appears to be no urgency for introducing nuclear systems required for the asymptotic era before, say, another 50 to 75 years. Consequently, more attention should be directed toward desirable systems for the transitional era, i.e., the next 30 to 50 years, that might ultimately ease the commercialization road to the asymptotic system. In the classical asymptotic system, an optimal system would consist of a relatively small number of fast breeder reactor plants operating largely as “fuel factories” and a larger number of thermal spectrum near breeder reactor plants serving as “energy factories.” The breeding ratio of U-238 Pu in the FBR plants would be chosen at unity simply to assure that the FBR primary fuel could sustain itself. Excess breeding capacity would be utilized to convert Th-232 and U-233 in the blankets. This excess fuel would then be used to supply makeup requirements for the NBR plants. With an FBR breeding ratio of 1.4 and an NBR conversion ratio of 0.9, approximately four NBR plants could be supported for each FBR plant. Perhaps more importantly, the sale of U-233 to the NBR could be used to offset a probable higher capital cost of the FBR plants. In this way, a capital cost penalty of some 30 to 40% relative to LWR plants might be allowable for FBR plants. In the next three to five decades the potential merits of a transitional system might be more relevant. In this period it is more likely that the much larger number of thermal spectrum reactors will dictate the preferred types of fuels for commerce. Under these circumstances the neutronic value of U-233 will tend to be high relative to that of Pu-239, particularly if the thorium fuel cycle is well established by then. Then transmuter reactors burning plutonium and producing U-233 can realize a very substantial profit from the fuel credits. In this case the sale of U-233 to NBR plants could allow an even larger capital cost penalty for FBR plants of around 50 to 70% relative to LWR plants. The importance of this is that the normally expected higher capital costs associated with the commercialization of fast spectrum reactors might be largely compensated by this strategy during the transitional period. The effectiveness of this approach depends on the prior introduction of the thorium cycle, and reactor plants that can use the thorium cycle effectively. Under these conditions it would seem that U-233 and plutonium fuels would seek the appropriate fuel values to catalyze the introduction of the fast spectrum transmuter reactors. However, there might also be considerable merit in establishing a “Bred Fuel Bank” institution that might help in establishing and maintaining policies on bred fuel values to expedite the transitional symbiotic strategy.