Alternate thorium fuel cycles were evaluated on a consistent basis in a Combustion Engineering Standard System 80 plant to provide a base-line for comparison of resource utilization with the conventional uranium cycle and with more advanced concepts. The economic motivation and the technical feasibility of employing thorium-based fuels in present PWRs were also determined. In order to achieve a significant increase in generating capacity for a given uranium ore resource, thorium utilization must be widespread. This necessitates the use of thorium cycles employing highly-enriched uranium; increases in the energy generated per mined ton of ore on the order of 18 to 34 percent can be realized in the long term with these fuel cycles. Increases in energy generation for the highly-enriched uranium/thorium fuel cycles are obtained, however, at the expense of higher separative work requirements (about 25 percent) and higher uranium ore requirements during the early cycles. As a consequence, greater 30-year levelized fuel cycle costs are obtained for these fuel cycles. Cycles employing plutonium enrichment have a more limited long term impact on uranium ore utilization because of the large fraction of the nuclear generating capacity which must be operated on the conventional uranium cycle to provide start-up andmore » makeup plutonium inventories. However, about 20 percent more energy can be generated per kilogram of plutonium consumed in the thorium cycle than in the uranium cycle, and hence the thorium-based cycle represents a superior way of utilizing existing plutonium stockpiles. Comparison of the characteristics of uranium- and thorium-based cores indicates that thorium fueling is feasible, and modifications to PWRs designed to accommodate plutonium recycle do not appear to be required.« less