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At a global conference sponsored by the International Atomic Energy Agency last week in Paris, Russia and India described large demonstration plants that will start operating next year and further deployments that are still in the design phase. China, meanwhile, described a broad R&D effort to make fast reactors comprise at least one-fifth of its nuclear capacity by 2030.

By breaking down the longest-lasting and hottest components of spent fuel from light-water reactors, fast reactors would need only 2 percent of the space required by a conventional reactor to store spent fuel. Fast reactors would also reduce the time that the waste must remain in storage from roughly 300,000 years to just 300. “Are they going to eliminate the need for geological repositories? No. But it will reduce the burden,” says Thierry Dujardin, acting deputy director general for the Organization for Economic Cooperation and Development’s Paris-based Nuclear Energy Agency.

Despite that enticing promise, however, the inherent hazards of today’s state-of-the-art fast reactors also loomed large at the Paris confab, which concluded a few days before Monday’s two-year anniversary of the Fukushima accident in Japan. At the conference, Dujardin said that fuel safety and prevention of severe accidents need to be “high priorities” for fast reactor research.

The problem with most fast reactors in construction or development is the molten sodium that cools their cores. Molten sodium is highly corrosive and explodes on contact with water and oxygen. Most dangerous, however, is that the sodium-cooled fast reactor, or SFR, exhibits what physicists call positive reactivity. Unlike conventional reactors, which experience their fastest possible chain reaction when operating at full power, the SFR’s chain reaction is capable of further acceleration than its equipment is designed to handle. This puts such reactors at greater risk of a runaway reaction that could cause a core meltdown or breach its steel containment vessel.

Many technical presentations at last week’s meeting focused on improved materials and designs intended to protect SFRs from the most extreme accidents imaginable. But alternative core designs were also well represented, and some countries are hedging their bets by testing the alternatives. A U.S. company, Transatomic Power, recently revealed designs for a new kind of molten salt reactor, which has different safety characteristics than a reactor cooled by molten sodium metal and should be compact and cheap to manufacture (see “Safer Nuclear Power at Half the Price”).

This dual approach is visible within the fast reactor program of Rosatom, Russia’s state nuclear corporation. Valery Rachkov, scientific director of the Leipunski Institute of Physics and Power Engineering within Rosatom, says Russia needs fast reactors to sustain its nuclear power program. Light-water reactors under construction in Russia will give the country an additional 10 gigawatts of nuclear power capacity by 2020—­a 42 percent jump—but further growth will become difficult unless Russia can manage its spent fuel, Rachkov says.

Hence Rosatom’s 2.5-billion-euro ($3.25-billion) investment directed not only at fast reactor technology but also facilities to recycle spent fuel into fuel for fast reactors. Rosatom has operated its BN-600, a 600-megawatt fast reactor, since 1980 at the Beloyarsk nuclear power plant. Rosatom expects to start operating an upgraded 880-megawatt version at Beloyarsk next year. That would be close to the 1,000-megawatt size of some commercial nuclear reactors.

Ivanovitch Zagorulko, a fast reactor specialist at Rosatom’s Leipunski Institute, says the BN-600 experienced serious sodium leaks only during its first four years of operation. And he says a 1987 incident—in which particle contaminants building up in the sodium coolant caused an acceleration of its chain reaction—was solved with an improved purification system and tighter airflow control during maintenance to keep contaminants out. He adds that the BN-800 provides further safety enhancements.

But Zagorulko says there is still a “big gap” between the BN-800’s design and the international safety criteria that Rosatom intends to meet with a 1,200-megawatt commercial-scale fast reactor, the BN-1200, now in the design phase. Sergey Shepelev, a representative of Afrikantov OKBM, a subsidiary of Rosatom, refused to discuss the BN-600’s 1987 incident during an open-panel session. When questioned after the session, Shepelev said there were “many versions” of the incident and that it was not known “which is right,” but that he was certain the BN-1200 was “absolutely a safe” design.

Rosatom is also developing another fast reactor cooled with molten lead. Lead coolant is less corrosive than sodium and chemically inert to water and air. It has never been used in a power plant, but the reactors in Russia’s nuclear submarines have long been cooled with a lead alloy. Rosatom’s plan calls for a 300-megawatt lead-cooled demonstration plant to be operating at Beloyarsk by 2020.

Some countries are more devoted to existing fast reactor technology. Indian researchers argued vehemently for the safety of sodium-cooled reactors at the Paris meeting. India’s 500-megawatt SFR demonstration plant is nearing completion at Kalpakkam, and the state-owned Indian Nuclear Power Corporation has a green light to build two more 500-megawatt SFRs at the site.

Redundant passive safety systems are one answer, according to Narayanasamy Mahendran, an engineer with Indian Nuclear Power. Backup cooling loops, for example, use convection alone to draw heat from the reactor and dump it into the air above the reactor building. Their plant has four such loops of two distinct designs. Any two should be capable of keeping a reactor cool in the event of a station blackout like the one that upended Fukushima. Similarly, he says, the core control rods are suspended by electromagnets and can thus passively drop by gravity to instantaneously scram the reactor during a station blackout.

European, Japanese, and U.S. researchers at Paris had research advances to note but no funding to support large demonstration projects. For the U.S., the focus is on finding repositories for interim and long-term waste storage. “The U.S. will be focused on geologic disposal for at least a few decades,” says Peter Lyons, the U.S. assistant secretary of energy for nuclear energy.

Absent funding in Japan and Europe is largely due to the corrosive impact of Fukushima. France is going it alone on Europe’s only well-funded fast-reactor program: a 650-million-euro design called Astrid that incorporates some bold next-generation components. For example, solid-state electromagnetic pumps move sodium coolant. They are expected to be more efficient and reliable than pumps with moving parts.

However, Astrid’s future hangs on a French energy policy review that got underway last month that could yet see the country turn away from nuclear power (see “Will France Give Up Its Role as a Nuclear Powerhouse?”). Pierre Le Coz, the project’s manager at France’s Atomic Energy Commission, says that if France has begun pulling away from nuclear energy in five years, when Astrid’s design is mature, they probably won’t get the green light to build.

Japan’s fast reactor program once led the world, but it’s now frozen—along with all but two of Japan’s nuclear reactors. Successive Japanese prime ministers seek to redefine Japan’s energy policy in the wake of the Fukushima accident. Each of the Japanese speakers last week began their talks with a reminder of the over 100,000 people who are still displaced from their homes—some of whom will never return—and of the fisheries and large forests that are still contaminated.

They were just as mindful of the accident’s impact on their colleagues’ efforts to advance nuclear energy. As Shunsuke Kondo, chairman of the Japanese Atomic Energy Commission, put it in his address: “The fact that this accident has raised concerns around the world about the safety of nuclear power generation is something Japan takes with great seriousness.”