Physics of Fusion
From Thermal-FluidsPedia
The thermonuclear fusion in the sun involves a series of reactions starting with the fusion of two atoms of hydrogen nuclei (protons) to produce deuterium and a positron (positive electron). The deuterium subsequently interacts with a proton to produce a helium-3 isotope. Finally, two He-3 atoms are fused together to produce a He-4 nucleus (alpha particle). The reactions involved in the solar thermonuclear process are far too slow to be useful for producing energy here on earth. A better approach would be to use deuterium or a mixture of deuterium and tritium. Two specific reactions of particular interest are:
D + D ->100 million degrees 3He + 1n + 3.3 MeV
D + T ->45 million degrees 4He + 1n + 17.6 MeV
Unfortunately, these isotopes will not fuse to each other at ordinary temperatures and pressures because they are all positively charged. The attractive forces exist only over very short distances; the repulsive forces between the positive nuclei and negative electrons are too strong to allow these nuclei to bring them close enough unless their average speeds (temperature) are raised. The D-D reaction has an ignition temperature of 100 million degrees, much higher than the ignition temperature of 45 million degrees needed for a D-T reaction. If normal hydrogen is used as the fuel for fusion, the required temperature would be even greater.
The D-T reaction is currently the reaction of choice (Figure 1). Deuterium is naturally present in seawater, but at a very low concentration–one atom for every 7,000 atoms of hydrogen. Tritium is also very rare and must be manufactured. One possible source of tritium is lithium, which is plentiful both in the earth’s crust and in seawater. Indeed, one gallon of sea water contains enough hydrogen isotopes for fusion to equal the energy which would be released by burning 300 gallons of gasoline. Large amounts of tritium can be obtained if an isotope of lithium (Li-6) is bombarded by neutrons in a reactor.
1n + 6Li 4He + 3T
neutron + lithium -> helium + tritium
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Fusion Dilemma
The basic challenges facing the commercial development of fusion reactors are (a) to achieve ignition with a net gain in the energy, beyond the break-even point; and (b) to find a method that supports fusion chain reaction in a sustained manner. The break-even point occurs when the amount of energy released by the nuclear reaction exceeds the amount of energy required to raise the temperature (and pressure) of the reactants to the level needed to initiate fusion (namely, Q of 1). These requirements can be reduced to three tasks of acquiring:
1. Very high temperatures (In excess of 100 million degrees). This is needed to overcome the repulsive forces of the positively charged nuclei.
2. Very high compressions. This increases the collision frequency and the rate of reaction.
3. Long residence times. This is needed to keep the reactants in proximity long enough to sustain the reaction; i.e. to increase energy production at a rate greater than that needed to initiate the reaction.
In 1991, the Princeton Tokomak was the first fusion reactor to reach this critical break-even point for about two seconds, opening the door for further research toward development of commercial nuclear fusion reactors. The current world record for a sustained high-temperature, high-pressure plasma is held by JT-60, at 24 seconds.
To make the reaction sustainable, we need to find a way to allow this reaction to proceed at a much larger scale and in a continuous way. Obviously, no vessel exists that can tolerate the extreme temperatures and pressures required for the fusion reaction to take place.
Three approaches have been investigated: a) gravitational confinement, b) inertial confinement, and c) magnetic confinement. Gravitational confinement is what happens in the sun, where gases are compressed and heated by gravitational forces. Gravitational confinement cannot be duplicated on earth. Inertial confinement mimics a short-lived micro-miniature star; a spherical capsule containing a deuterium-tritium (D-T) mixture is heated and compressed by an array of powerful lasers directed toward the pellet for a few nanoseconds. In the third approach, magnetic confinement, hot ionized gases are confined in a toroidal path that is held away from the reactor walls by strong magnetic fields.
Inertial confinement was first used in the design of the Shiva (a) reactor at Lawrence Livermore National Laboratory (LLNL) in California. The original lasers were later replaced by NOVA lasers which were ten times more powerful (Figure 2). Very strong laser pulses strike and uniformly heat the deuterium-tritium pellet (made of substances such as lithium deuteride, the active ingredient of a hydrogen bomb) placed inside a vacuum sphere. The laser beams blast and incinerate the pellet and compress the gas to very high temperatures and pressures that are required to initiate fusion (1). The next generation high powered lasers are being constructed to deliver at least 60 times more energy than any previous laser system. The National Ignition Facility (NIF) -- scheduled to be completed in 2010 – plans to deliver two million joules (2 MJ) of ultraviolet laser energy by focusing its 192 giant lasers to about 4 billionths of a second (producing 500 trillion watts of power) on a tiny 0.5-millimeter-diameter capsule in the center of its target chamber, creating conditions similar to those that exist only in the cores of stars and giant planets and inside nuclear weapons (2).
The preliminary results seem to indicate that this approach will probably not be as successful as the magnetic confinement approach used in a number of other facilities throughout the world (JT-60 in Japan, Tokomak Fusion Test Reactor (TFTR) in Princeton (Figure 3), and EU’s Joint European Torus (JET) located in England).
Magnetic confinement is a process where hot hydrogen plasma is circulated inside a huge coil of wire (magnetic bottle) that is cooled until it becomes a superconductors. If a current flows through this coil, it creates a magnetic field that confines the plasma within the torus shaped cavity. The plasma is continuously heated by the magnetic field as it circulates through the coil. Once it reaches the fusion temperature, deuterium and tritium fuse to form helium and neutron. The excess energy is carried by neutrons and deposited in a lithium jacket and produce electricity.
The 500 MW International Thermonuclear Experimental Reactor (ITER, “the way” in Latin) being based on Tokomak design is a joint, €10 B ($16 B) cooperative effort between the European Union, Japan, the US, Russia, China, India, and South Korea. ITER will be built in Cadarache, France, for operation in or near 2016 (Figure 4).
The initial goal is to sustain a deuterium-tritium chain reaction and generate ten times as much energy as was put into it, to a Q of 10. Furthermore, research will be conducted to test critical issues of fusion safety (such as finding materials that can withstand the high-energy neutrons created by the ITER, coolants, etc.) that must be resolved before the commercial reactors can be built.
Cold Fusion
When people talk about fusion, they are mostly referring to “hot fusion”. There has been much talk about the possibility of “cold fusion” in the last couple of decades. In 1989, Stanley Pons of the University of Utah, and Martin Fleischmann from the University of Southampton in the United Kingdom, claimed that they had achieved nuclear fusion in a simple photochemical cell (3). Cold fusion takes advantage of an elementary particle called a muon. Muons have negative charges, and when they collide the nucleus they cancel positive charges of protons, thus eliminating the need for extremely hot temperatures to overcome the repulsive forces of hydrogen nuclei (4).
These experiments generated enormous publicity around the world, but were later denounced by leading experts who could not verify and reproduce the results. Although there are still some researchers who believe that cold fusion is possible, no credible experimental proof has yet to be presented!
References
(1) Maniscalco, J. A., “Inertial Confinement Fusion,” Ann. Rev. Energy, 5; 33-60, 1980.
(2) Yamanaka, C., “Inertial confinement fusion: The quest for ignition and energy gain using indirect drive,” Nucl. Fusion 39 825 (1999).
(3) Editorial (April, 2004c): “DOE Warms to Cold Fusion”. Physics today.
(4) Rafelski, H. E., et al., “ Cold fusion: muon-catalysed fusion,” J. Phys. 9: At. Mol. Opt. Phys. 24 (1991) 1469-1516.
(5) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Additional Comments
(a) Shiva often called the Destroyer or The Lord of the Dance is the Hindu God of many hands and is attributed with great power and physical prowess.
Further Reading
Bodansky, Nuclear Energy Principles, Practices, and Prospects, Second Ed., Springer, 2004.
Seaborg, G., T., Peaceful Uses of Nuclear Energy, University Press of the Pacific, 2005.
International Journal of Nuclear Engineering and Design, Direct Science Elsevier Publishing Company, devoted to the Thermal, Mechanical, Material and Structural Aspects of Nuclear Fission.
Journal of Fusion Energy, Springer Netherlands. It features articles pertinent to development of thermonuclear fusion.
External Links
Federation of American Scientists (http://www.fas.org/nuke/intro/nuke/index.html).
International Atomic Energy Agency (http://www.iaea.org).
DoE Office of Nuclear Energy, Science & Technology (http://www.ne.doe.gov).
American Nuclear Society, (http://www.ans.org).
World Association of Nuclear Operator (WANO) (http://www.wano.org.uk).