Reinventing Nuclear Power

nuclear fusion

nuclear fusion

A fusion-fission hybrid reactor could produce clean electricity and remove dangerous nuclear waste from the planet. If it ever works.

The light of 192 lasers at Lawrence Livermore National Laboratory’s National Ignition Facility travels more than a half-mile through a stadium-size building toward its target. Along the way, the beams are amplified, shaped and focused into the world’s most powerful laser, capable of delivering power in a pulse that lasts 20 billionths of a second and peaks at 500 million megawatts.

The target of all of this fury is tiny–a gold capsule the size of an extra-strength Advil. The goal is to mash the contents of the capsule, a BB-size pellet of hydrogen frozen to nearly absolute zero, until the hydrogen atoms fuse into helium and release a gush of energy. This fusion is the same reaction that takes place in the center of the sun and stars, and at the business end of a nuclear bomb.

“NIF is by far the biggest hammer in the world right now,” states Peter J. Wisoff. A former Space Shuttle astronaut who now runs the laser’s operations, 50 miles east of San Francisco, Wisoff retains a no-nonsense demeanor from his days making sure people returned to Earth safely. His current job may be harder. Wisoff will try to use NIF’s $3.5 billion hammer, which took 12 years to build and was just completed in March, to ignite for the first time on Earth a controlled fusion reaction.

If NIF succeeds, it will be a transcendent moment for science. But this is more than just a race for a Nobel Prize: Fusion’s powerful pull is that it has the potential to turn a modest amount of seawater into a large supply of clean energy. NIF’s plan to use fusion for energy is especially dramatic. NIF scientists have proposed building a reactor that uses both fusion and fission to deliver clean energy and nearly eliminate nuclear waste from the planet.

Generations of scientists have been frustrated by fusion’s complexities. There are elaborate experimental fusion reactors all over the globe, and they have made steady but achingly slow progress toward a controlled, self-sustaining burn. Eight nations, including the U.S., are cooperating to build a $15 billion reactor in France scheduled to be completed in 2016. Its own chief scientist says reaching the goal will require a miracle.

[For a different fusion-fission approach, see “Texas Smoosh ‘Em.”]

NIF’s director, the outspoken Edward Moses, is undaunted. He dismisses all previous attempts at fusion as lighting the edge of a pile of wet leaves. “Poof, and then it’s out,” he says. “We’re going to burn the pile. We are on the edge of burn.”

The National Ignition Facility was conceived in response to a nuclear-weapon test ban signed by George H.W. Bush in 1992. The Department of Energy’s national laboratories were charged with trying to understand bomb physics in such exquisite detail that weapon performance could be modeled accurately without blowing anything up.

This is still the laser’s primary mission. Astrophysicists will also use it to try to re-create the conditions at the center of supernovas, to understand how the elements that make up our solar system–and our bodies–are created.

But energy could be NIF’s greatest legacy. Fusion is the process of forcing the nuclei of atoms so close together that they fuse into a nucleus of a new element. (Fission, of course, is the opposite: energy produced by splitting nuclei.) The new, fused nucleus weighs less than the original two. With a bow to Einstein’s famous law, the lost mass is transformed into energy, mostly in the form of a torrent of energetic neutrons (NIF’s tiny reaction produces 10 quintillion–10 with 18 more zeros–in 10 trillionths of a second).

Nuclei repel one another powerfully. For fusion to occur, they have to be submitted to high temperatures and pressures. That’s where NIF’s lasers come in. They compress a pellet of a type of hydrogen found in seawater by a factor of about 40,000, like squashing a basketball to the size of a pea. The pressure and heat produced, greater than 100 million degrees and 100 billion times the pressure of the Earth’s atmosphere, cause fusion.

The precision that is required seems far-fetched. There are 60,000 control points that help guide the light from the lasers to the target. That fleeting pulse, a 20-nanosecond flash, can’t arrive at once or the tiny round target will warp, making ignition impossible. Imagine trying to collapse that basketball while keeping it round and not letting any air out.

The laser pulse, then, has to be shaped. The first light to hit must deliver only about 1% of full power. The power then oscillates, decreasing slightly before increasing in steps that last between 10 and 100 trillionths of a second. And all 192 lasers must produce the same shaped pulse in the same trillionths of a second. “Our conditions are ten times the temperature and ten times the density of the sun, and we’re getting there in billionths of a second,” says John Lindl, NIF’s chief scientist. “It’s not just making a flamethrower–it’s making a precise flamethrower.”

If NIF’s aim is true, the fusion it sparks will produce enough energy to sustain the reaction and fuse all of the fuel–the sought-after “burn.”

Then comes the hard part: Using this technique to make energy. NIF has proposed building a prototype energy reactor that would use both fusion and nuclear fission. A fusion chamber would be surrounded by a blanket of fissionable material, like nuclear waste, that would serve as an additional fuel source.

This helps address the drawbacks of both fusion and fission. Even if NIF’s lasers achieve burn, they will be extraordinarily inefficient, producing only 1% of the energy needed to fire the lasers. NIF scientists think they can improve efficiency dramatically but not enough to make fusion energy alone feasible. Fission, meanwhile, produces nasty waste. Moses says that together with emerging laser amplification technology that is less power hungry, and the huge energy gain from the fission reaction, a combo reactor would deliver 200 times the energy it consumes. It would work by using the neutrons from fusion to help turn nuclear waste into nuclear fuel and then burn it until almost none is left. Today’s nuclear reactors are fueled by uranium that is partially “enriched.” Still, only 3% of the fuel’s energy is used. Left behind is the unusable uranium and other radioactive by-products that the nation has yet to figure out what to do with.

With the blended reactor, the neutrons from fusion reactions would wedge themselves into the nuclei of this waste, making them unstable enough to split and produce heat. NIF scientists estimate the leftover waste would require only 5% of the space of the proposed Yucca Mountain repository. A heat exchanger running through the whole thing would collect heat from the fusion reaction and the fission reaction and run a turbine, capable of producing one to two gigawatts of power, about the same as today’s nuclear power plants. Moses imagines having a demonstration plant running by 2020 and commercial technology by 2030.

First, he has a long line of hurdles to clear, beyond just making the laser more efficient. NIF can fire at full strength only once a week, or else the laser light will fry the optics. For the reactor to work, it would need to fire ten times a second. This would require much better and stronger optics, the likes of which have yet to be invented. Also, it would require several hundred million new targets a year. Now held in gold capsules, they’d have to be made more cheaply. And a process has to be invented that can insert these targets of hydrogen, which must be kept frozen to a few degrees above absolute zero, ten times a second.

Says NIF chief scientist Lindl: “If we’ve learned anything, we’ve learned nature isn’t going to give this up easily.”

Sourced and published by Henry Sapiecha 116th APRIL 2009


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