An economic model for the back end of the nuclear fuel cycle was developed for a once-through cycle, a standard reprocessing cycle, and a reprocessing cycle with fractionation of cesium and strontium. The development of the model was performed under the expected political constraints and scenario for the first nuclear waste repository. Technical issues concerning the repository design were analyzed, in particular the thermal design. A parametric thermal analysis was performed for waste emplaced in five different geologic formations: salt, granite, basalt, shallow tuff, and deep tuff. The results of the thermal analysis, in the form of maximum permissible loadings, were incorporated into the economic model. The economic analysis was performed for a variety of situations in order to compare the five possible repository host rocks, the three different back end cycles, and different locations of the Monitored Retrievable Storage Facility (MRS). A sensitivity analysis was also performed for evaluating the effect of variations in some parameters in the final cost of the system;The results of the thermal analysis indicated that the maximum permissible thermal loadings in granite and tuff were relatively high, whereas for basalt the thermal loadings are very restricted for both spent fuel and reprocessed waste and for salt the loadings are very restricted for spent fuel only. The results of the economic analysis showed that a repository in basalt always resulted in higher costs than in any other rock; the other repository media yield comparable costs, except in the case of spent fuel disposal in salt, where the costs are higher. Co-location of the MRS with the repository results in a lower system cost than locating it away from the repository. The regular reprocessing cycle presents the lower storage plus disposal costs among the three cycles studied, and disposal of spent fuel has the highest costs associated. The model proved to be very sensitive to variations of the discount rate, the storage facility capital cost, and the delay of repository backfilling after waste emplacement. Lower waste storage plus disposal costs can be obtained by delaying disposal in the fractionation waste cycle in any rock or disposal of any waste form in basalt. In the other cases, aging the waste before disposal does not reduce, in general, the total cost.