In this system, 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.

The company is using computer models to anticipate how currently available materials would change over time, and is developing reactor designs that anticipate these changes. For example, if it's known that a material would swell in the conditions inside the reactor, the spaces between the fuel rods would be designed to accommodate this swelling, says Doug Adkisson, director of operations at Terrapower.

Terrapower has also developed designs for a passive cooling system. Like many other advanced reactor designs, Terrapower's uses molten sodium metal as the coolant. Sodium takes much longer to boil than water, which gives plant operators more time to respond to accidents. It would also be possible to use natural convection and air cooling in the event of a power outage—coolant wouldn't have to be continuously pumped into the reactor, as was the case at Fukushima. One danger of using sodium, however, is that it reacts violently when it's exposed to air or water.

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.