The problem of unit commitment in an electric utility system is analyzed. The types of generating units considered include nuclear-steam, fossil-steam, thermalpeaking, conventional and pumped-storage hydro. The large problem of generating unit commitment, due to the inclusion of the nuclear unit, is decomposed into two stages. In the first stage, the relatively stable nuclear generation is optimized with respect to the generation from the large fossil-steam unit. Hourly generation levels for all the units in the system are determined in the second stage. AS a result of considering the startup-shutdown cost, the objective function has a fixed charge component in addition to first and second degree polynomials. Representation of the minimum turndown level and minimum Shutdown duration of the generator necessitates the use of O-1 variables along with continuous variables. The solution methodology presented here, applying an extension of the Lambda-Separable Programming, can handle these requirements efficiently. Application of the algorithm results in a minimum-cost generation schedule for all units in the system. Optimum generation levels of energy limited units are determined without using a preselected unit commitment order. The effect of startup-shutdown costs on the number of hours a Plant should operate is established. The cost and benefit of spent nuclear fuel reprocessing is analyzed and the effect of uranium prices on it (reprocessing) is shown. The model is tested using a sample system of six generating units. Hourly generation schedules, includig purchase and sale, are determined for two one-week periods while the nuclear generation is optimized for an entire year. / Ph. D.