How It Works: Traveling-Wave Reactor

"We need to pursue every available path to achieve a really big breakthrough," Bill Gates told Jeff Goodell, a writer who focuses on energy and environmental issues, in the last issue of Rolling Stone. Gates, the Microsoft billionaire known for his work combating disease in the developing world and poverty -- as well as his ban on all Apple products in his home -- has taken on a new challenge: climate change and energy issues. "This is a global thing, and it's really hard for people to get their minds around the amount of reduction required," said Gates, who is described as a "radical consumerist" in the interview's introduction. He doesn't want to cut our energy use; that's unfeasible, he believes. Instead, he wants to focus on developing an unlimited supply of carbon-free energy.

"I certainly don't want the government to only pick a few paths, because our probability of success is much higher if we're pursuing many, many paths," said Gates. He's investing in a lot of options, but, over the course of the interview, he makes it clear that he believes most in the power of nuclear energy. Gates is the primary investor in TerraPower, a branch of Intellectual Ventures, that is currently working on developing a Traveling-Wave Reactor (TWR).

But what makes the TWR any different from a traditional nuclear reactor?

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Unlike traditional nuclear reactors that rely on enriched uranium to produce power, the Traveling-Wave Reactor (TWR) can function, for the most part, on waste uranium, a byproduct of the current reactor design. A relatively tiny amount of enriched uranium is required by the reactor to get started, but it then runs on the waste, making and consuming its own fuel. The benefit of this design is that the reactor doesn't require constant refueling and waste removal. It can run -- it is thought -- for decades without refueling. This, the companies currently working on a TWR design insist, makes nuclear power safer and cheaper.

With the traditional nuclear reactor design, spent uranium rods must be removed every 18 to 24 months, safely stored and replaced with hundreds of new rods.

First proposed in the 1950s, the TWR produces plutonium and uses it immediately, thereby eliminating the possibility that the fuel can be extracted from the machine and used to create nuclear weapons. Traditional reactors also produce plutonium -- P-239 -- but using it requires the removal of the spent fuel. The spent fuel must then by choped up so that the plutonium can be chemically extracted; this is an expensive process that is a critical step in the construction of an atomic bomb.

It wasn't until the 1990s, though, that potential designs for the TWR started to surface.

The traveling wave that the reactor's name references "moves" through the unit's core at a rate of only one centimeter per year. The wave doesn't actually move at all, but this is the easiest way to describe it. Instead, fuel is pushed into the burning region. This fuel is transformed into plutonium and then undergoes fission. The waste is stored behind the wave. As it runs, the wave converts nonfissile material into fuel.

As a coolant, the reactor uses liquid sodium. The temperature in the core far exceeds that of traditional reactors: about 550 degrees Celsius compared to 330 degrees Celsius.

The United states currently has about 85 tonnes of weapons-grade plutonium. That would be enough to power about 20 TWRs, according to Dr. George S. Stanford, a nuclear reactor physicist who was part of the Argonne National Laboratory team that developed the Integral Fast Reactor (IFR) on which the TWR is said to be based.

The material is there and the money is there, but no TWR has been successfully constructed yet. A handful of companies are working to make the TWR a reality, but dealing with the strong backlash against the dangers associated with nuclear power will be the biggest hurdle to overcome.

Image: TerraPower.