Among the growing field of startups attempting to bring advanced nuclear reactors to market, TerraPower stands out—partly because it’s well-funded, and partly because of who’s doing the funding. TerraPower is a spin-off of Intellectual Ventures, the technology investment firm founded by Microsoft cofounder Nathan Myhrvold. Bill Gates is both a primary investor in and a vocal champion of the company (see “Q&A: Bill Gates”). The company has also drawn attention because it’s pursuing a novel reactor technology, known as a traveling wave reactor, that is elegant in conception and powerful in potential (see “Advanced Reactor Gets Closer to Reality” and “Nathan Myhrvold: The Wealthy Should Fund Innovation”).

But six years after it was founded, TerraPower has not yet produced a working prototype. Last week, at Oak Ridge National Laboratory in Tennessee, the company revealed that it is now pursuing a different advanced reactor concept: a molten chloride reactor that uses a liquid as both the coolant, which transfers energy in the form of heat, and the fuel in which the nuclear reactions occur.

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