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July 27, 2011

Terrapower traveling wave reactor design changed to be more buildable

A recent design for a nuclear reactor known as a traveling wave reactor looks similar to some conventional nuclear designs, but the way it operates is very different. Credit: Terrapower

Technology Review - Terrapower, a startup funded in part by Nathan Myhrvold and Bill Gates, is moving closer to building a new type of nuclear reactor called a traveling wave reactor that runs on an abundant form of uranium. The Terrapower reactor would burn up all of the actinides (uranium, plutonium). Most of the uranium that it uses would not need to be enriched.

The company has changed its original design to make the reactor look more like a conventional one. The changes would make the reactor easier to engineer and build. The company has also calculated precise dimensions and performance parameters for the reactor. Terrapower expects to begin construction of a 100-megawatt demonstration plant in 2016 and start it up in 2020. It's working with a consortium of national labs, universities, and corporations to overcome the primary technical challenge of the new reactor: developing new materials that can withstand use in the reactor core for decades at a time. It has yet to secure a site for an experimental plant—or the funding to build it.

In the new design, the heat is always generated in about the same area within the reactor core—near the center. As a result, it's easier to engineer the systems to extract and use the heat to generate electricity.

One challenge with this design is ensuring that the steel cladding that contains the fuel in the fuel rods can survive exposure to decades of radiation. Current materials aren't good enough: for one thing, they start to swell, which would close off the spaces between the fuel rods through which coolant is supposed to flow. To last 40 years, the materials would need to be made two to three times more durable, Terrapower says.

Terrapower's next steps include finalizing the design and finding partners to build the plants. It's been in talks with organizations in China, Russia, and India. Gilleland says the company expects to have an announcement about partners within the next few months



In the original Terrapower design, the reactor core was filled with a large collection of uranium 238. The process of converting it starts at one end, producing plutonium that's immediately split to generate heat and convert more uranium to plutonium. The reaction moves from one end to the other—in a "traveling wave"—until no more reactions can occur.

In the new design, the reactions all take place near the reactor's center instead of starting at one end and moving to the other. To start, uranium 235 fuel rods are arranged in the center of the reactor. Surrounding these rods are ones made up of uranium 238. As the nuclear reactions proceed, the uranium 238 rods closest to the core are the first to be converted into plutonium, which is then used up in fission reactions that produce yet more plutonium in nearby fuel rods. As the innermost fuel rods are used up, they're taken out of the center using a remote-controlled mechanical device and moved to the periphery of the reactor. The remaining uranium 238 rods—including those that were close enough to the center that some of the uranium has been converted to plutonium—are then shuffled toward the center to take the place of the spent fuel.

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