uranium reserves. Even if you look at reserves which are more expensive now, by this time, the nuclear power would be getting considerably more expensive. You might have 30 or 40 years at 10 terrawatts. And what if we need 20 or 30 or 40 terrawatts for 100 years? Now you're done. NARRATOR: Conventional reactors cannot solve the problem. But there is one possibility. Breeder reactors, which turn a more abundant form of uranium into plutonium, can potentially extend nuclear's lifespan as a fuel source for hundreds of years. But breeders are a very complex technology, and because they produce plutonium, they raise serious issues about nuclear proliferation. So today virtually no research is being done. [www.pbs.org: More on the nuclear alternative] HENRY JACOBY, Mass. Institute of Technology: A high priority, and one that's now not being followed in the United States, is to find a socially acceptable version of nuclear power. I think one of the great tragedies of the current R&D- it's not a tragedy in that necessarily we are not building more nuclear power because there is a lot of social reasons to worry about nuclear power. The problem is we're not doing the research that would be needed to search for a nuclear power option that would be socially acceptable. NARRATOR: Today there is no known energy technology capable of delivering the amounts of carbon-free energy to stabilize greenhouse gases in the atmosphere. The most commonly cited cures for global warming - efficiency improvements, renewable energy