Thursday, March 05, 2009

The Traveling-Wave Reactor

Traveling Wave Nuclear ReactorIntellectual Ventures, an invention company, believes they've developed a "new reactor design [that] could make nuclear power safer and cheaper." Published by MIT's Technology Review magazine:

a traveling-wave reactor requires very little enriched uranium, reducing the risk of weapons proliferation. (Click here for a larger diagram). The reactor uses depleted-uranium fuel packed inside hundreds of hexagonal pillars (shown in black and green). In a “wave” that moves through the core at only a centimeter per year, this fuel is transformed (or bred) into plutonium, which then undergoes fission. The reaction requires a small amount of enriched uranium (not shown) to get started and could run for decades without refueling. The reactor uses liquid sodium as a coolant; core temperatures are extremely hot--about 550 ºC, versus the 330 ºC typical of conventional reactors.


As it runs, the core in a traveling-­wave reactor gradually converts nonfissile material into the fuel it needs. Nuclear reactors based on such designs "theoretically could run for a couple of hundred years" without refueling, says John G­illeland, manager of nuclear programs at Intellectual Ventures.

Gilleland's aim is to run a nuclear reactor on what is now waste. ­Conventional reactors use uranium-235, which splits easily to carry on a chain reaction but is scarce and expensive; it must be separated from the more common, nonfissile uranium-238 in special enrichment plants. Every 18 to 24 months, the reactor must be opened, hundreds of fuel bundles removed, hundreds added, and the remainder reshuffled to supply all the fissile uranium needed for the next run. This raises proliferation concerns, since an enrichment plant designed to make low-enriched uranium for a power reactor differs trivially from one that makes highly enriched material for a bomb.

But the traveling-wave reactor needs only a thin layer of enriched U-235. Most of the core is U-238, millions of pounds of which are stockpiled around the world as leftovers from natural uranium after the U-235 has been scavenged. The design provides "the simplest possible fuel cycle," says Charles W. Forsberg, executive director of the Nuclear Fuel Cycle Project at MIT, "and it requires only one uranium enrichment plant per planet."

The trick is that the reactor itself will convert the uranium-238 into a usable fuel, plutonium-239. Conventional reactors also produce P-239, but using it requires removing the spent fuel, chopping it up, and chemically extracting the plutonium--a dirty, expensive process that is also a major step toward building an atomic bomb. The traveling-wave reactor produces plutonium and uses it at once, eliminating the possibility of its being diverted for weapons. An active region less than a meter thick moves along the reactor core, breeding new plutonium in front of it.

The traveling-wave idea dates to the early 1990s. However, Gilleland's team is the first to develop a practical design. Intellectual Ventures has patented the technology; the company says it is in licensing discussions with reactor manufacturers but won't name them. Although there are still some basic design issues to be worked out--for instance, precise models of how the reactor would behave under accident conditions--Gilleland thinks a commercial unit could be running by the early 2020s.

While Intellectual Ventures has caught the attention of academics, the commercial industry--hoping to stimulate interest in an energy source that doesn't contribute to global warming--is focused on selling its first reactors in the U.S. in 30 years. The designs it's proposing, however, are essentially updates on the models operating today. Intellectual Ventures thinks that the traveling-wave design will have more appeal a bit further down the road, when a nuclear renaissance is fully under way and fuel supplies look tight.

"We need a little excitement in the nuclear field," says Forsber­g. "We have too many people working on 1/10th of 1 percent change."
Sounds very promising! Be sure to watch the animated video as well.

Hat tip to FuturePundit.


Anonymous said...

Sounds a lot like a candle-type reactor. I just hope the fuel containment won't fail after a few decades. These reactor types always push mechanical fatigue to their limits on paper...

skyler said...

give it time. hopefully with a few more innovations in storage/containment ability this will become a viable solution in the days to come.

Patrick said...

I see a serious problem with this design, which is the neutron balance. Once the wave has traveled a bit through the bloc, something like half of the neutrons will be directed into the burned-up part and not contribute to the chain reaction, nor contribute to the breeding, and I don't see how you can keep this going. After all, a breeder needs 2 useful neutrons per fission (one for the chain, and one for the breeding), and with plutonium, you get at most 2.9 or so, so you can permit to loose overall at most 0.9 neutrons. But if you loose half of them (by geometry) you've already lost 1.4 or so.

Gunnar said...

Maybe the core innovation is a kind of Maxwell's demon who is able to direct the neutron's deflector shields effective axis towards the wave front.

Anonymous said...

make a sphere of fuel and start fission in the centre. what's the problem?