One day in 2017, maybe 2020, an international team of physicists and engineers will inject hydrogen gas into a massive vacuum chamber and blast it with microwaves, radio waves and intense beams of high energy atoms.
The hydrogen will heat to 200 million degrees or more – hotter than the center of the sun. The hydrogen atoms, split apart into free-moving electrons and ions, will crash into one another at 620 miles per second. Compressed by powerful magnets and heated to unimaginable temperatures, the ions will fuse. The reaction will release a tremendous burst of heat, enough to sustain the nuclear fusion inside and herald a new source of global energy.
The moment will culminate more than half a century of scientific endeavor, silence decades of political skepticism and mark a triumph of international cooperation. By bringing nuclear fusion to the world, scientists will have created a star on the Earth.
Pursuing nuclear fusion, the process by which our sun gives us light and heat, is stretching the frontiers of physics. Creating the reaction on Earth, containing and sustaining it, and capturing the heat it emits requires that scientists has met some daunting engineering challenges.
But physicists say new instruments and computing tools developed over the past decade have illuminated the properties of the superheated gas needed for fusion, called plasma, as never before.
“Ten years ago, critics would question whether fusion could be done in a lab,” said David Baldwin, who heads nuclear fusion research at General Atomics in La Jolla, a world leader in the field.
Today, the world may be moving closer to an answer. The International Thermonuclear Experimental Reactor project, or ITER , is on track to select a site for a huge, $5 billion test reactor in Japan, Canada, Spain or France. The reactor started construction in 2013, this a self-sustained nuclear fusion could be achieved by 2017, scientists say.
In January 2003, the Bush administration announced that the United States will rejoin ITER. In 1998, Congress cut funding for the nation’s share of the project, which also slowed domestic research. La Jolla, with researchers from General Atomics, UCSD and Science Applications International Corp., was the world headquarters for the ITER project from 1991 to 1998. Under the new ITER program, General Atomics and UCSD are expected to continue to play significant roles in research.
Fusion research suffered in the 1990s from a curse of unrealistic expectations, much of it fueled by scientists and advocates in the field, Baldwin said.
“Fusion has spent a lot of money, and did make a lot of elaborate promises early on,” he said. But there seems to be the political will to take another look.
U.S. Secretary of Energy Spencer Abraham called the ITER project “a major step toward a fusion demonstration power plant.”
The United States may also have a strategic interest in rejoining ITER. Both China and South Korea announced their intention to join the effort.
How much the United States will invest in fusion research is unclear. The Department of Energy’s Fusion Energy Sciences Advisory Committee recommended an investment of $332 million in 2004, reaching a peak annual budget of $900 million a year by 2018.
Replicating the heart of a star is no easy task. Since 1951, the United States has spent more than $17 billion on fusion research. Scientists say practical fusion energy, when it becomes a reality, would not be commercially available for at least another 30 to 40 years. The fusion advisory committee estimates it in 2002, will cost the United States $24 billion dollars, to develop a first-generation fusion plant – the precursor to making fusion commercially viable by mid-century.
“I’m not offering fusion in the short term, (but) if it’s a trick that we could pull off, it would be a true unending supply of energy,” said Baldwin.
Nuclear fusion occurs when two atoms of hydrogen combine together – fuse – to form an atom of helium and free neutrons. In the process, some of the mass of the hydrogen is converted into energy. To make fusion happen, the atoms of hydrogen must be heated to very high temperatures so they form a plasma and have enough energy to fuse. Fusion research is primarily the realm of physicists who study plasma, which makes up more than 99 percent of the visible universe.
Plasma, unlike solids, liquids and gas, is a collection of free-moving electrons and ions – atomic nuclei that have lost electrons. Plasmas are formed when atoms of hydrogen are heated; without heat, they recombine into gas.
Plasmas are common on Earth and are found in minute amounts in fluorescent lights, neon signs and arc welding. They also naturally occur in lightning and auroras.
Their temperatures can range from the relatively cool, such as auroras at about 1,800 degrees Fahrenheit, to the very hot, such as the core of stars at about 18 million degrees Fahrenheit.
To achieve nuclear fusion on Earth, plasmas must be heated much hotter, to 100 million degrees or more, to force its ions to fuse. Scientists have chosen two light hydrogen ions for their nuclear fusion experiments: deuterium and tritium.
Deuterium and tritium are isotopes of hydrogen. Isotopes are atoms that have the same number of protons in their nucleus, but a different number of neutrons.
There is virtually no tritium in water, so scientists must produce it artificially by bombarding lithium, an abundant light metal, with neutrons, which splits the lithium to produce tritium and helium.
If deuterium and tritium get close enough, they bond together and form a temporary nucleus. But the nucleus is extremely unstable, and it soon disintegrates into a helium nucleus and a free neutron.
In the process some of the mass of the hydrogen is converted into energy.
It is this energy, created during fusion, that scientists hope to harness. And since hydrogen isotopes are common and abundant, fusion has the potential to be an inexhaustible source of energy.
The sun and stars use gravity to compress hydrogen ions and drive nuclear fusion. On Earth, reactors use magnets, creating strong magnetic fields to confine the ionized atoms while they are heated by microwaves or other energy sources. One fusion reaction releases about 1 million times more energy than each chemical reaction in an ordinary wood fire, which releases heat when carbon and oxygen combine to form carbon dioxide.
Scientists say they can extract the raw deuterium fuel needed for nuclear fusion from sea water. About one in every 6,000 hydrogen nuclei has the extra neutron to make it deuterium.
There is so much deuterium in ordinary water that scientists say the top 1 inch of San Diego Bay could provide enough deuterium fuel for nuclear fusion to power the city of San Diego for 50 years.
The biggest challenge for fusion scientists has been in figuring out how to contain plasma so that heat and pressure are maintained long enough and intensely enough to force ions to fuse.
Nearly all nuclear fusion research in the world relies on magnetic fields to contain heated plasma.
The model, called a Tokamak (the name was coined by Russian scientists who worked on early fusion research) is a doughnut-shaped chamber that contains heated plasma in a circular path through the inside.
The plasma must not fall in temperature, so it cannot touch anything. If it diffuses toward the Tokamak’s inside walls, heat is immediately transferred from the plasma to the walls and nuclear fusion ceases.
Scientists have labored for years to understand the turbulence that builds up in this superheated plasma stream, which can degrade its smooth flow inside the Tokamak and result in a loss of heat and pressure.
Deuterium is a naturally occurring isotope of hydrogen and is commonly available. The large mass ratio of the hydrogen isotopes makes their separation easy compared to the difficult uranium enrichment process. Tritium is a natural isotope of hydrogen, but due to its tiny half-life of 12.32 years, is hard to find, store, produce, and is expensive. Consequently, the deuterium-tritium fuel cycle requires the breeding of tritium from lithium.
The reactant neutron is supplied by the D-T fusion reaction and the one that has the greatest yield of energy. The reaction with Lithium (Li) is exothermic, providing a small energy gain for the reactor. The reaction with Li is endothermic but does not consume the neutron. At least some Li reactions are required to replace the neutrons lost to absorption by other elements. Most reactor designs use the naturally occurring mix of lithium isotopes.
Several drawbacks are commonly attributed to D-T fusion power:
The big problem here is that both the nuclei are positive and to get them to fuse we have to somehow make them come very close together so that the strong nuclear force becomes greater than the electrostatic repulsion. The way this is done is to make them collide at very high speed by raising the temperature of the gas to over 100 million oC, several times hotter than the center of the Sun!
- It produces substantial amounts of neutrons that result in the neutron activation of the reactor materials.
- Only about 20% of the fusion energy yield appears in the form of charged particles with the remainder carried off by neutrons, which limits the extent to which direct energy conversion techniques might be applied.
- It requires the handling of the radioisotope tritium. Similar to hydrogen, tritium is difficult to contain and may leak from reactors in some quantity. Some estimates suggest that this would represent a fairly large environmental release of radioactivity.
Source of Uranium-235 for Tritium Production
In response to public comments concerning the source of blended-down uranium-235 that could be used as nuclear fuel for tritium production. A discussion of the environmental impacts resulting from blending-down activities of highly enriched uranium was also added. (Source: Final Environmental Impact Statement for the Production of Tritium Volume 1, chapter 5, Section 5.2.7 was revised for clarification.)