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Now More Than Ever
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NGM.COM
JUNE 2011
The
BIRTH
Religion of
The World’s First Temple
Africa’s Super Park 60 The Secret World of Child Brides 78 Crazy Creatures in Tide Pools 100 Can China Go Green? 116 Why Rare Earths Are in Almost Everything 136
Göbekli Tepe, Turkey circa 9600 B.C.
TV BAND TO COME NGM.COM
NEXT Water-cooled nuclear power plants aren’t the only option.
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CAN WE FLY?
At the center of each poppy seedsize fuel particle is a uranium kernel. Layers of carbon and silicon carbide contain the radioactive material.
MELTDOWN-PROOF NUKES
The next generation of nuclear plants may be cooled by gas— helium gas. Such reactors
SEPTEMBER 2011
Pyrolytic carbon Silicon carbide Uranium Porous carbon
were proposed in the 1960s but rarely pursued; only a few have ever been built. But that
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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
Reactor core
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.
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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 engineers 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
national geo graphic
• o c tober
What’s Inside a Pebble? Each “pebble” in a pebblebed gas reactor is a graphite 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.
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How It Works A pebble-bed reactor is made up of about 400,000 pebbles. Heat from the fuel spheres is picked up by helium and can then be used to generate electricity—or to drive industrial processes such as oil refining and desalination.
Blowers move helium gas through the reactor and over the pebbles, where nuclear fission releases large amounts of energy, heating up the helium.
About 5,000 spheres move through the reactor each day, like gum balls through a vending machine. The constant circulation means no refueling interruptions.
If a pebble taken from the bottom can produce more power, it goes back in the top. If it’s spent, it’s stored as waste, and a new pebble is added.
KAITLIN M. YARNALL, NGM STAFF ART: STEFAN FICHTEL. SOURCE: ANDREW KADAK, MIT
AWARDS & RECOGNITION
Saving Orphan Elephants Forever Wild in the Adirondacks Girl Power in Brazil Lords of the Sahara The Race to the South Pole
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