This article highlights that fusion holds out hope for an abundant supply of clean energy, fueled by forms of hydrogen drawn from seawater, with no emissions into the atmosphere of CO2, no oxides of nitrogen or sulfur, and no mercury or cadmium. Talk of a future hydrogen economy has raised the profile of fusion research, and the field has gotten a couple of boosts during the past few months. The arsenal is aging, and the major powers have agreed not to set off any nuclear weapons, even for safety testing. By studying the physics of inertial confinement fusion and the response of weapon materials placed near a fusion fuel capsule, the caretakers of the bombs and missiles can better understand the aging process. Fusion energy has been a long time coming and still has a long way to go. There are skeptics who question whether it can happen at all.
Researchers have hunted for the keys to fusion energy for 50 years, and they may need 50 more.
The lure that keeps them on the chase is summed up in five words by Richard Hawryluk, deputy director of the Princeton Plasma Physics Laboratory: "There is no Yucca Mountain."
Yucca Mountain, in Nevada, has been nominated as the site where the United States will store waste from current nuclear fission reactors until radioactivity tails off, perhaps thousands of centuries in the future.
Fusion holds out hope for an abundant supply of clean energy, fueled by forms of hydrogen drawn from seawater, with no emissions into the atmosphere of CO2, no oxides of nitrogen or sulfur, no mercury or cadmium.
Barring a leak, fission reactors avoid harmful emissions, too. So what's the difference?
Besides the abundance of hydrogen fuel, fusion promises to yield a waste that, while not benign, is mild by comparison with the leftovers of nuclear fission. It was at the lab where Hawryluk works that the world's first large-scale disposal of fusion waste was completed by a crew wearing coveralls and masks.
Talk of a future hydrogen economy has raised the profile of fusion research, and the field has gotten a couple of boosts during the past few months.
The international fusion research program, the ITER project, said it reached "an historic milestone" in February, when representatives from both the United States and China attended a meeting of the group in St. Petersburg, Russia.
Then Sandia National Laboratories reported confirmation in March that its Z pinch machine was producing thermonuclear neutrons by fusion, encouragement that the system could be made some day to produce high-yield fusion energy.
Fusion experiments with commercial aims use hydrogen as a fuel. For the highest yields, the fuel is a combination of two isotopes of hydrogen. Both isotopes have one proton and one electron like common hydrogen atoms, but one of them, deuterium, has picked up a neutron in its nucleus. Tritium, the other isotope, has two neutrons and is radioactive. Deuterium is found with low frequency in nature. There is one atom of deuterium for every 6,700 ordinary hydrogen atoms. Tritium, very rare as a natural occurrence, can be produced by nuclear fission and in a fusion reactor from lithium.
A fusion reactor has to heat the fuel until it reaches the plasma state, when it is so hot that electrons and nuclei separate to form a swirling cloud.
Odd Neutron Out
If the free nuclei collide with sufficient force, they will fuse and their parts will create a new substance. In the case of the hydrogen isotopes, the new substance is helium. The helium nucleus has two protons and two neutrons. After deuterium and tritium fuse, there's a neutron left over. The mass of the reaction products is less than the mass of the initial nuclei, and so energy is released.
Fusion is the process that fuels the hydrogen bomb and the sun.
Fusion energy is a long way off, and it isn't cheap today. According to its own published documents, ITER is expected to cost more than $5 billion, including the $1 billion or so spent on R&D since 1992.
Sandia has 400 people, including scientists and engineers, working on its fusion project, according to Alton Romig, the lab's chief technology officer. About 150 are full-time employees. The fusion energy output of the Z machine comes to about 4 millijoules.
Experiments at the Princeton lab and the plans of the ITER organization use a torus, a doughnut-shaped vacuum vessel in which the plasma reaches a temperature several times the heat of the sun.
The Princeton Plasma Physics Laboratory, operated by Princeton University in Plainsboro, NJ, is the U.S. Department of Energy's primary magnetic fusion laboratory. Decommissioning the lab's Tokamak Fusion Test Reactor was the world's first experience in disposing of waste products from a controlled fusion machine using deuterium and tritium.
The reactor, which had operated for about 15 years, was shut down in 1997. Disposal was completed last fall.
The inside wall of the torus was covered with a blanket of tiles. Their surfaces were contaminated with tritium compounds from the fuel.
As Charles Gentile, head of the lab 's tritium group, explained, there are no gamma rays or X-rays from tritium, so it is not an external hazard. It releases beta radiation. Tritium is toxic when it is ingested or inhaled.
Tritium has a half-life of about a dozen years, and so could be considered safe within a hundred years or so. That's in sharp contrast to plutonium created in a pressurized water reactor, for instance, which has to be isolated from all contact with the biosphere for 500,000 years, a longer period of time than the record of our species.
Plutonium, along with spent fuel and the byproducts of a fission reactor, must be handled by robots. Work teams in the room where the torus was disassembled wore protective clothing and air filtration masks for some of the tasks.
The torus had to be cut into segments, loaded into containers, and trucked away to disposal sites. The decommissioning project was overseen by the lab 's construction manager, Erik Perry, an ASME member.
The disassembly team, under lead engineer Bob Parsells, ran tests on mockups and settled on diamond wire cutting as the best method to use on the torus. The method allows for remote control so people step out of the immediate area, and it could be adapted to control dust and fumes. It is an established technology for cutting stone and concrete.
As Perry described it, the wire had beads every inch or so embedded with industrial-grade diamond chips. He and others were concerned that, as the wire sawed through the stainless steel of the torus, metal would cling to the diamond chips and interfere with the cutting. That's where concrete came into the picture.
The disposal team needed to seal radioactive waste, including tritiated carbon dust, inside the vacuum vessel. They decided to fill the space inside the torus with a lightweight concrete to contain the tritium-tainted materials.
According to Perry, the concrete has a foaming agent that traps air so, when the mixture sets, it weighs about 35 pounds per cubic foot, less than a quarter the weight of conventional concrete. It can float on water, Perry said. Torus segments filled with the stuff would not be too heavy for their shipping containers.
Besides trapping radioactive materials, the concrete had another use. Its abrasiveness cleaned the stainless steel residue from the cutting wire.
After engineers tested the system on mockups and before they started on the torus itself, workers in full bubble suits removed tiles from the parts of the interior wall where the cuts would fall. These tiles were left in bags inside the torus to be engulfed by the concrete filler. The protective suits were disposed of with other radwaste.
Meanwhile, other tests found an alternative to the water normally used to cool the cutting wire.
As an isotope of hydrogen, tritium readily mixes with water to form heavy molecules, which can enter the body easily. Linnea Wahl, an environmental health physicist at Lawrence Berkeley National Laboratory, said, "Any time you have water around tritium you can wind up with tritiated water."
The usual estimate is that half the tritiated water ingested will be lost from the body in 10 days. One remedy for swallowing tritiated water is beer drinking, which will flush the system, she said.
The Princeton team used a spray of biphase carbon dioxide as the coolant. Perry said that, as the gas cooled the wire, pellets of solid CO2 acted like sand blasting to keep the diamond chips free of debris.
The wire cutting apparatus worked entirely enclosed in a housing. Vents ran dust through a series of HEPA filters that caught 99.999 percent of contaminants, Gentile of the tritium group said.
According to Perry, exterior parts of the reactor were made of steel containing cobalt. The cobalt had become mildly activated over the years, and the reactor parts had to be treated as radiation waste, too. Workers handling these parts wore cotton overalls, which were laundered and reused.
The waste has been sent to two disposal sites. Some of it went to the DOE's Hanford site in Washington, and most of it is at the Nevada Test Site.
The Way Back
The ITER group plans to build the world's largest tokamak reactor where it will conduct a 20-year experiment. The U.S. withdrew from the project in 1999 for financial reasons, and is now talking about a return. The contribution the U.S. intends to make and the amount of money it is willing to spend remain open questions at this point.
China is moving to join the group for the first time. Core members at present are Canada, Japan, the Russian Federation, and the European Union.
ITER derives from "International Thermonuclear Experimental Reactor," a name no longer in use, and the acronym is a Latin word that can be translated as "way." The torus will contain plasma heated to 100 million C and confined in magnetic fields generated by superconducting coils. Nuclei in the plasma will collide with such force that they fuse. In theory, when enough nuclei fuse, the reaction will release 10 times the power supplied to the plasma during the fusion process.
The acid test for ITER is a long way off. According to Federico Casci, a spokesman for ITER at the Max Planck Institut for Plasmaphysik in Munich, Canada, Spain, France, and Japan have offered to host the reactor and the sites have checked out equally well on technical grounds. There is no deadline yet for a final decision.
He said it will take eight to 10 years to construct ITER. Operations will start with two and a half years of experiments with hydrogen, then one year with deuterium alone. Only after that will tests mix deuterium and tritium.
It takes a lot of electric power to create the conditions for nuclei to fuse and release energy. Experiments produce less energy than they use. Closest to break-even so far has been a machine in the United Kingdom called the Joint European Torus, or JET, which has released fusion energy equal to 65 percent of the electricity that was poured into its plasma. JET's plasma volume is about 90 cubic meters.
The ITER torus will have a plasma volume of 837 cubic meters, more than nine times the volume of JET. Human-engineered fusion has never been tried on this scale before. The reactor is expected to generate 500 to 700 megawatts of fusion power.
The Squeeze Is On
Sandia's Z machine fuses the nuclei in its fuel not by collision in a racetrack, but by squeezing them together harder than they repel each other. (The machine is called "z" because the pinch is made vertically, or along the z axis.)
The Z machine is an example of an approach to fusion called " inertial confinement." According to David Hammer, a professor of electrical and computer engineering at Cornell University, the primary reason for investing in inertial confinement fusion devices is to develop more detailed knowledge of thermonuclear detonations.
The arsenal is aging, and the major powers have agreed not to set off any nuclear weapons, even for safety testing. By studying the physics of inertial confinement fusion and the response of weapon materials placed near a fusion fuel capsule, the caretakers of the bombs and missiles can better understand the aging process.
Developing commercial sources of fusion energy is a desirable goal, Hamn1er said, "and the Z machine, or science generated on the Z machine, may contribute to that."
Researchers at Sandia had observed neutrons coming from the Z machine before, but were unsure whether they were being produced by fusion of fuel-in this case, deuterium alone-or were coming from another source during the high-energy process.
The system sends 20 million amps of current in a burst lasting a few hundred-billionths of a second. The electromotive force is approximately 3 million volts. The surge passes through 360 or so tungsten wires, each 7 micrometers in diameter, strung like harp strings around the foam container that holds a capsule of fuel.
According to Neal Singer, a Sandia spokesman, the tungsten wires on all sides vaporize and the magnetic field generated by that burst of electricity collapses the matter. When the imploding mass collides with the foam, kinetic energy is transformed into X-rays. The bombardment by X-rays and vaporized tungsten particles heats and compresses the fuel capsule. Its diameter will collapse from 2 millimeters to 160 micrometers in 7 nanoseconds.
According to Hammer, the temperature in the capsule at maximum compression reaches 11.6 million Kelvin, at which point fusion reactions can occur.
In a series of experiments completed in late March, researchers inserted xenon gas in the capsule. The gas prevented the capsule from getting hot during compression. Thus, the neutron yield dropped sharply, as predicted.
The yield during normal operation is about 10 billion neutrons, Singer said.
Another inertial confinement experiment, the National Ignition Facility, will use lasers to achieve fusion. The facility, at Lawrence Livermore National Laboratory in California, has been conducting initial tests with four laser beams. At full strength, possibly in 2008, it will have 192 beams capable of producing 1.8 megajoules of energy.
It will be used to simulate thermonuclear reactions of weapons and is expected to contribute to the development of harnessed fusion energy, according to a Livermore spokesman, Bob Hirschfeld. He added that, although it is not designed specifically to do so, the laser fusion machine could develop into a source for commercial energy. No one can say until the facility is up to speed.
Fusion energy has been a long time coming and still has a way to go. There are skeptics who question whether it can happen at all. One of them, a fission specialist, said: "Fusion has been 50 years away since I was a boy."
But human inventiveness has taught everyone that the world can be full of surprises. After all, exactly 100 years ago this month, men couldn't fly.
ITER may yield 10 times the energy poured into its plasma.