The impact of the nuclear option in the national energy outlook on the environment and the U.S. economy is analyzed with the MARKAL-MACRO energy systems computer code. The base case projection by the U.S. Energy Information Administration is the starting point for this study. The possibility of license renewal of the current fleet of U.S. nuclear power plants is considered as well as the introduction of cost-competitive advanced light water reactors. Electricity energy sector projections for fossil fuel plants, renewable energy sources, and nuclear power plants are analyzed on a least cost basis. The impact of constraints on the emissions of greenhouse gases is included in the analysis. It is found that it would be economically favorable to introduce as many as 300 additional nuclear power plants in the United States by the year 2025 to meet emission constraints of limiting emission to the 1990 level in the years beyond 2010.