Abstract Most prior net-energy studies of nuclear-power systems accounted only for the direct consumption of fuels and the indirect consumption of energy embodied in physical materials when making such estimates. Most ignored the energy embodied in labor, government, and financial services. In this study, total economic cost is used as a surrogate to estimate the total input-energy cost of constructing, operating, financing, and disposal of nuclear-power systems. Although the cost and performance data used in this study are from light-water reactor systems experience, it is assumed that fast-neutron reactors may be substituted for light-water reactors when economic conditions dictate. We make the conservative assumption that the cost and performance characteristics of fast-neutron reactors will be similar to those of light-water reactors. We conclude that the operation of a large nuclear-power system, involving a continuing construction program of starting one new 1000-MW system each month for 100 yrs, would yield a relatively small amount of net energy, under optimistic assumptions. Under less-optimistic assumptions the net-energy yield is negligible to negative. The average net-energy yield increases, somewhat, when optimistic assumptions are added to account for the possibility of improved efficiency in an all-electric economy.