National Geographic : 2011 Oct
• NEXT Pyrolytic carbon Silicon carbide Uranium The next generation of nuclear plants may be cooled by gas--- helium gas. Such reactors were proposed in the 1960s but rarely pursued; only a few have ever been built. But that could change. Though issues such as fuel storage are a potential concern, gas reactors---which can't melt down---may be a way forward in the wake of Japan's nuclear disaster. Using helium as a coolant has at least a couple of advantages. For one thing, it's inert, so it can't become radioactive like the water in water-cooled plants. For another, gas reactors are more efficient at generating electricity than water-cooled ones, because they run much hotter. That heat, produced carbon free, has gotten the attention of chemical, fertilizer, and oil companies. Though it's still in the developmental stage, "this technology could be a real game changer," says Fred Moore of Dow Chemical. At the core of the reactor's safety is a clever fuel design. Instead of the uranium fuel rods used in water reactors, gas-reactor fuel takes the form of uranium bits scattered among graphite "pebbles." Graphite is a great moderator, slowing down the neutrons and keeping their reaction in the proper temperature range. Andrew Kadak of MIT, who visited a small prototype pebble-bed reactor in China four years ago, watched engi- neers turn off the cooling system. "It naturally shut down," he says. "It was incredible. Especially in light of Fukushima, this is a reactor that doesn't melt down." ---Juli Berwald MELTDOWN PROOF NUKES At the center of each poppy seed- size fuel particle is a uranium kernel. Layers of carbon and silicon carbide contain the radioactive material. What's Inside a Pebble? Each "pebble" in a pebble- bed gas reactor is a graph- ite fuel sphere the size of a tennis ball. Nine grams of uranium are dispersed among some 15,000 tiny particles within the graphite. During a recent three-year test at Idaho National Lab, 300,000 fuel particles were heated to 2300°F and bombarded with neutrons. Not a single particle leaked radioactive material---strong evidence of the fuel's safety. Porous carbon Water-cooled nuclear power plants aren't the only option.