When the Atlantic Navigator docked in Baltimore harbor earlier this month, the freighter carried the last remnants of some of the nuclear weapons that the Soviet Union had brandished in the cold war. During the past 20 years more than 19,000 Russian warheads have been dismantled and processed to make fuel for U.S. nuclear reactors. In fact, during that period more than half the uranium fuel that powered the more than 100 reactors in the U.S. came from such reprocessed nuclear weapons.

In addition to reducing the risk of nuclear war, U.S. reactors have also been staving off another global challenge: climate change. The low-carbon electricity produced by such reactors provides 20 percent of the nation's power and, by the estimates of climate scientist James Hansen of Columbia University, avoided 64 billion metric tons of greenhouse gas pollution. They also avoided spewing soot and other air pollution like coal-fired power plants do and thus have saved some 1.8 million lives.

And that's why Hansen, among others, such as former Secretary of Energy Steven Chu, thinks that nuclear power is a key energy technology to fend off catastrophic climate change. "We can't burn all these fossil fuels," Hansen told a group of reporters on December 3, noting that as long as fossil fuels are the cheapest energy source they will continue to be burned. "Coal is almost half the [global] emissions. If you replace these power plants with modern, safe nuclear reactors you could do a lot of [pollution reduction] quickly."

Indeed, he has evidence: the speediest drop in greenhouse gas pollution on record occurred in France in the 1970s and ‘80s, when that country transitioned from burning fossil fuels to nuclear fission for electricity, lowering its greenhouse emissions by roughly 2 percent per year. The world needs to drop its global warming pollution by 6 percent annually to avoid "dangerous" climate change in the estimation of Hansen and his co-authors in a recent paper in PLoS One. "On a global scale, it's hard to see how we could conceivably accomplish this without nuclear," added economist and co-author Jeffrey Sachs, director of the Earth Institute at Columbia University, where Hansen works.

The only problem: the world is not building so many nuclear reactors.

Nuclear future
China leads the world in new nuclear reactors, with 29 currently under construction and another 59 proposed, according to the World Nuclear Association. And China has not confined itself solely to the typical reactors that employ water and uranium fuel rods; it has built everything from heavy-water reactors originally designed in Canada to a small test fast-reactor.

Yet, even if every planned reactor in China was to be built, the country would still rely on burning coal for more than 50 percent of its electric power—and the Chinese nuclear reactors would provide at best roughly the same amount of energy to the developing nation as does the existing U.S. fleet. Plus, nuclear requires emissions of greenhouse gases for construction, including steel and cement as well as the enrichment of uranium ore required to make nuclear fuel (or the downblending of uranium from nuclear weapons as in the case of the "Megatons to Megawatts" program). Over the full lifetime of a nuclear power plant, that means greenhouse gas emissions of roughly 12 grams of CO2-equivalent per kilowatt-hour of electricity produced, the same as wind turbines (which also require steel, plastics, rare earths and the like in their construction) and less than photovoltaic panels, according to the U.S. Department of Energy’s National Renewable Energy Laboratory.

In other parts of the world nuclear has begun to dwindle. Japan may never restart its nuclear plants in the wake of the multiple meltdowns at Fukushima Daiichi following the earthquake and tsunami in 2011, which also soured public opinion in many parts of the world. Germany still plans to eliminate nuclear power and even France has announced plans to reduce its reliance on reactors. In the U.S. the five new nuclear reactors under construction will replace the four aging reactors that closed in 2013, but as older reactors like Oyster Creek in New Jersey and Vermont Yankee continue to shut down, the number of reactors in the U.S. may be doomed to dwindle as well.

A big problem is cost. The construction of large nuclear power plants requires a lot of money to ensure safety and reliability. For example, for the U.S. to derive one quarter of its total energy supply from nuclear would require building roughly 1,000 new reactors (both to replace old ones and expand the fleet). At today's prices for the two AP-1000 reactors being built in Georgia, such an investment would cost $7 trillion, although that total bill might shrink with an order of that magnitude.

One other idea to cut cost is to begin building smaller reactors of so-called modular design. The Tennessee Valley Authority hopes to catalyze development of such reactors by installing one at its Clinch River site in Tennessee, former home of the U.S.’s failed attempt to build its own commercial fast reactor.

That never-completed breeder reactor is part of a legacy of failed U.S. research and development of new types of reactors, such as the Experimental Breeder Reactor that ran successfully in Idaho for nearly 30 years. "It's a shame that the U.S. essentially stopped R&D on advanced nuclear power a few decades ago," Hansen noted. "By now we should be in a position where a country like China would have some options other than coal."

New dawn?
That said, nuclear reactors are beginning to get the kind of scientific attention not seen since at least the end of the cold war. Novel designs with alternative cooling fluids other than water, such as Transatomic Power's molten salt–cooled reactor or the liquid lead–bismuth design from Hyperion Power, are in development. Alternative concepts have attracted funding from billionaires like Bill Gates. Transatomic Power even won the top prize from energy investors at the 2013 summit of the Advanced Research Projects Agency–Energy, or ARPA–E, in 2013. "The intellectual power of what's been done in the nuclear space should allow for radical designs that meet tough requirements," Gates told ARPA–E's 2012 summit, noting that the modeling power of today's supercomputers should allow even more innovation. "When you have fission, you have a million times more energy than when you burn hydrocarbons. That's a nice advantage to have."

ARPA–E itself, however, has no program to develop alternative reactors because of the expense of proving out novel designs and the long timescales required to develop any of them. "We searched a lot in nuclear," ARPA–E's former director Arun Majumdar, now at Google, said in a interview with Scientific American earlier this year. "We realized that in the nuclear business, investing $30 [million] to $40 million, I'm not sure it would have moved the needle. … That is something that I wish I had had the budget to try."

With more money for development of novel designs and public financial support for construction—perhaps as part of a clean energy portfolio standard that lumps in all low-carbon energy sources, not just renewables or a carbon tax—nuclear could be one of the pillars of a three-pronged approach to cutting greenhouse gas emissions: using less energy to do more (or energy efficiency), low-carbon power, and electric cars (as long as they are charged with electricity from clean sources, not coal burning). "The options for large-scale clean electricity are few in number," Sachs noted, including geothermal, hydropower, nuclear, solar and wind. "Each part of the world will have different choices about how to get on a trajectory with most of the energy coming from that list rather than coal."

As long as countries like China or the U.S. employ big grids to deliver electricity, there will be a need for generation from nuclear, coal or gas, the kinds of electricity generation that can be available at all times. A rush to phase out nuclear power privileges natural gas—as is planned under Germany's innovative effort, dubbed the Energiewende (energy transition), to increase solar, wind and other renewable power while also eliminating the country's 17 reactors. In fact, Germany hopes to develop technology to store excess electricity from renewable resources as gas to be burned later, a scheme known as “power to gas,” according to economist and former German politician Rainer Baake, now director of an energy transition think tank Agora Energiewende. Even worse, a nuclear stall can lead to the construction of more coal-fired power plants, as happened in the U.S. after the end of the nuclear power plant construction era in the 1980s.

Hansen, for one, argues that abundant, clean energy is necessary to lift people out of poverty and begin to reduce greenhouse gas emissions from a swelling human population. Nuclear is one of the technologies available today—and with room for significant improvement and innovation. In that sense, natural gas is a bridge fuel to disaster, even with some form of CO2 capture and storage, and the world must immediately transition to renewables and nuclear.

But significant hurdles remain, not least the decades required for design, licensing and construction of even existing nuclear technologies, let alone novel ideas. That may mean advanced nuclear power cannot contribute much to efforts to combat climate change in the near term, which leaves current reactor technology as the only short-term nuclear option—and one that is infrequently employed at the global scale at present.

In the same way, U.S. nuclear power plants have not eliminated the threat of nuclear weapons despite 20 years of megatons to megawatts. Russia retains an estimated 8,500 nuclear warheads—and the U.S. some 7,700—despite the best efforts to fission the problem away. The problem of fission and climate change is equally stuck at present. But, as Hansen wrote in an additional assessment of his new analysis, "Environmentalists need to recognize that attempts to force all-renewable policies on all of the world will only assure that fossil fuels continue to reign for base-load electric power, making it unlikely that abundant affordable power will exist and implausible that fossil fuels will be phased out."