Correction to This Article
This article incorrectly listed Portsmouth, N.H., as one of three cities where the United States is storing depleted uranium. It is Portsmouth, Ohio, along with Paducah, Ky., and Oak Ridge, Tenn.
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Miniature nuclear reactors might be a safe, efficient source of power

Smaller reactors might also be cheaper than their bigger siblings, according to Ingersoll. It costs a lot of money to build a gigawatt reactor (i.e., one that produces 1,000 megawatts), in part because it has to be put together on-site, one piece at a time, by expert welding crews. In today's larger reactors, all of which are being built outside the United States, the reactor vessel -- that's the cylindrical vault in which the fission takes place -- is about 90 feet in diameter, much too large to be shipped overland in one piece. Reactor vessels for mini plants would be as small as about 10 feet across and could be loaded onto a train or maybe even onto the bed of a trailer. Once the pieces get to the site, they can be bolted together with far less manpower and equipment.

Small reactors can also be daisy-chained together as a city grows. Consider this: Today's typical 1,000-megawatt reactor can power a city of about a million homes. What happens when the population grows by 20 percent? A second big reactor would be expensive and unnecessary. Smaller increments are more manageable.

A big 'but'

Not convinced? You're not alone. Opponents point out that small reactors don't begin to solve the most worrisome aspects of nuclear energy: dangerous waste and the potential that the fuel could be used to build a weapon.

When you remove used-up fuel from a reactor, it's incredibly hot and very radioactive. The spent rods are moved into a concrete-encased pool, where they spend years cooling off. Eventually, they should be moved to a long-term storage facility, but one doesn't exist yet. (No one wants nuclear waste stored in their state.) As a result, there are more than 50,000 tons of uranium soaking in pools next to U.S. reactors, waiting for a permanent home.

As for the risks of nuclear proliferation, it's hard to separate generating nuclear power from building nuclear weapons, because the first few steps of the process are nearly identical.

Uranium can take several forms, which differ based on the number of neutrons in the nucleus. When miners pull uranium from a mine, more than 99 percent of what they get is U-238, which isn't all that useful for either power or weapons.

Today's nuclear reactors want the slightly lighter U-235. So the uranium ore is hauled off to enrichment plants, which keep pulling U-238 out of the sample until 3 to 5 percent of what's left is U-235. At that point, the sample can power a reactor.

"But you can use the same enrichment plant and just keep putting the stuff through over and over again until you get into the 90 percent range," says nuclear scientist John Gilleland, who runs TerraPower. "At that level, you can pipe all that stuff together and make a bomb."

Since you use the same ingredients, equipment and processes to create reactor and bomb fuel, countries such as Iran, which is suspected of having nuclear-weapons ambitions, can plausibly tell the world they're enriching for power-generation purposes for a long time before their true intentions are undeniable.

Many also fear that a nuclear cooling tower might look like a bull's-eye to a terrorist. The towers on small reactors would be narrower and shorter than those at big plants, since most of the reactor can be buried. While there has never been an attack on a nuclear plant, it remains a concern.

If it works . . .

TerraPower, the venture that has received Gates's backing, is working to develop a system that will solve some of the problems with today's nuclear reactors. Its "traveling wave reactor" would be powered almost completely by fuel rods containing depleted uranium, the concentrated U-238 that enrichment plants throw away after extracting U-235. (TerraPower plans to start with a 500-megawatt reactor, but it could be scaled smaller or larger as needed.)

Here's how it would work: A tiny amount of U-235 is introduced and split open, releasing a whole bunch of neutrons. The depleted uranium in the fuel rod absorbs one of the neutrons. The resulting form of uranium soon decays into plutonium, a very potent fissile material. When more neutrons strike the plutonium, huge amounts of energy are released, as well as some more neutrons to keep the reaction going.

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