"The truth of the matter is, if people really want to solve the issue of greenhouse gases, civilian nuclear power, powering our energy grids by nuclear power is the best alternative available," Bush said last week.

The United Nations-backed Intergovernmental Panel on Climate Change is expected to recommend using nuclear power to curb greenhouse gases in a report to be released Friday.

Some environmentalists have said that global warming is such a serious threat that nuclear power should be reconsidered, in part because it can help phase out coal, which is the dirtiest fossil fuel.

The environmental advocacy group Greenpeace, though, said on Wednesday that building new US nuclear power plants is too costly and would take too long to replace fossil fuels, even with billions of dollars' worth of financial subsidies laced through a 2005 energy bill.

"Nuclear power is an expensive and dangerous distraction from real climate solutions," said Jim Riccio, a nuclear policy analyst for Greenpeace.

The report pointed to current construction of a new generator in Finland that is 18 months behind schedule and US$1 billion over budget. It also found that construction of a new nuclear plant takes about seven years on average.

Similarly, the Council on Foreign Relations, a Washington think tank, says it is almost impossible to build enough nuclear power plants to arrest the rise in earth temperatures. It would be hard for the nuclear industry to procure large amounts of reactor-grade construction materials and hire enough trained workers, the report said.

"Given the current US energy sources and patterns of use, nuclear energy alone does not provide a solution for at least the next few decades for significantly reducing the US contribution to global warming," wrote Charles Ferguson, the council's science and technology fellow.

To hold global carbon dioxide emissions at year 2000's levels, the world would need between 1,900 and 3,300 gigawatts of nuclear capacity by 2050, Ferguson wrote in the report, which was also sponsored by Washington and Lee University. The typical nuclear reactor produces about one gigawatt, or 1,000 megawatts, of power output. About one new reactor would have to come on line each week over the next four decades, he wrote.

Some 103 reactors currently generate about 20 percent of US electricity. The Energy Department said that in 2004 the electricity sector kept 142 million metric tons of carbon dioxide from entering the air by using nuclear power.

The Nuclear Energy Institute, which lobbies for big nuclear operators, said nuclear power plants are vital for the future.