The computer code VESTA has been written to calculate the interactions of the characteristics of light water, advanced converter, and breeder reactors with respect to consumption of uranium, production of plutonium, and national electric power capacity, to about year 2020. Economic factors such as plutonium price, fuel fabrication cost, fuel-cycle minimization decisions, and plutonium-inventory time lags will be as important as technological factors such as thermal reactor type, breeder development rate, plutonium recycle techniques, and thermal-reactor specific power. For a wide range of conditions and levels of development effort, the introduction of low-gain breeders by 1975 and of high-gain breeders by 1990 will result in cumulative uranium usage by 2020 below the level at which very high-priced uranium ore will be required.