Nuclear Fusion:
Nuclear fusion is the proces
Nuclear Fusion Reaction http://www.lancs.ac.uk/ug/hussainw/fusion.jpg
Nuclear Fusion Reaction http://www.lancs.ac.uk/ug/hussainw/fusion.jpg
s by which two light atomic nuclei combine to form one heavier atomic nucleus. This occurs when pressures are high enough to overcome the strong repulsive forces of the protons in the nuclei. When the nuclei fuses, they form a new element, and release excess energy in the form of a fast moving neutrino and a positron. The energy is ‘extra’ because the mass of the newly formed nuclei is less than the sum of the masses of the original two nuclei; the extra mass is converted into energy. The main fusion reaction involved occurs between the nuclei of the two hydrogen isotopes, Deuterium (D) and Tritium (T).

Basic Problem: Maintaining and Confining High Temperature and High Density Plasma
Although fusion power sounds extremely attractive right now, it still has not been developed into a viable energy source. The reason is because it isn't as simple as it sounds. Tritium and deuterium atoms don't randomly collide and give off energy. They must be heated up to extremely high temperatures (around 100 million degrees) in order for fusion to take place. So why can't we just heat them up? Well, when matter is present at the high temperatures necessary for fusion, it changes into another form. We're all familiar with the 3 main states of matter: solid, liquid, and gas. Each succeeding state is more energetic than the last. The atoms of a substance in the gaseous state move about much more than those of a substance in the solid state. Plasma, what has been called the "fourth state of matter," consists of a cloud of charged particles and is the most energetic of the four states of matter. Plasma is the most common form of matter in the universe. Because plasma exists only at such high temperatures, the electrons in the atoms of a substance in the plasmid state have enough potential energy to "break free" of the nucleus. Thus, the plasma is said to be made up of a cloud of positively and negatively charged particles. The positive ions are the atoms minus the electrons (and thus have a net positive charge). The negative particles are of course the electrons themselves that have been stripped from the atoms. One characteristic of matter in the plasma state is how difficult it is to confine. Naturally, anything present at such high temperatures is very energetic and therefore moves around a great deal, and anything that moves around a lot is hard to confine. The containment problem has become one of the biggest setbacks to the use of fusion power on earth.

Conditions for Fusion:
Even given a high enough temperature to overcome the coulomb barrier to nuclear fusion, a critical density of ions must be maintained to make the probability of collision high enough to achieve a net yield of energy from the reaction. The density required for a net energy yield is correlated with the confinement time for the hot plasma, so the minimum condition for a productive fusion reaction is typically stated in terms of the product of the ion density and confinement time, called Lawson’s Criterion. The calculated values are:
nt ≥ 1014 s/cm3 – deuterium tritium fusion
nt ≥ 1016 s/cm3 – deuterium deuterium fusion

Confinement Time for Fusion:
Confinement time in nuclear fusion devices is defined as the time the plasma is maintained at a temperature above the critical ignition temperature. To yield more energy from the fusion than has been invested to heat the plasma, the plasma must be held up to this temperature for some minimum length of time. Calculations of that minimum time are:
t = 2 x 1014 seconds (deuterium tritium fusion)
n
t = 5 x 1015 seconds (deuterium deuterium fusion)
n

Ion Density for Fusion:
Even given a high enough temperature to overcome the coulomb barrier to nuclear fusion, a critical density of ions must be maintained to make the probability of collison high enough to achieve a net yield of energy from the reaction. The density required for a net energy yield is associated with the confinement time for the hot plasma, so the minimum condition for a productive fusion reaction is typically stated in terms of the product of the ion density and confinement time, called Lawson’s Criterion. The calculated values are:
nt = 2 x 1014 sec/cm3 (deuterium tritium fusion)
nt = 5 x 1015 sec/cm3 (deuterium deuterium fusion)

8.4.11 Solve Problems on the Production of Nuclear Power
Nuclear Fission
  1. Limited Supplies of Fissionable Uranium
A breeder reactor is a nuclear reactor that generates new fissionable material or in other words, it breeds new fuel.
Neutrons produced in the fission of 235-Uranium are absorbed by 238- Uranium and 239-Plutonium is produced via a set of reactions. Since 239-Plutonium is fissionable with slow neutrons, it can be utilized as fuel in a nuclear reactor after separation. Thus, a breeder reactor breeds new fuel from otherwise useless 238-Plutonium.
fast_breeder_reactor.gif
Breeder Reactor http://www.coolschool.ca/lor/PH11/unit9/U09L04/fast_breeder_reactor.gif

Nuclear Fusion
  1. Magnetic Confinement of Hot Plasma
    Inside the Tokamak http://www.plasma.inpe.br/LAP_Portal/LAP_Site/Figures/Tokamak_Schematic.gif
    Inside the Tokamak http://www.plasma.inpe.br/LAP_Portal/LAP_Site/Figures/Tokamak_Schematic.gif
Generates fusion energy that uses magnetic fields to confine the fusion fuel in the form of a plasma. The best and most reliable machine known to man till present is called the Tokamak. A tokamak is toroid shaped and utilizes a combination of two magnetic fields: one directed along the axis of the toroid produced by current-carrying conductors and a second produced by a current that passes through the plasma. This not only helps confinement but heats the plasma up as well. In 1994, TFTR, a type of tokamak reactor, produced a world-record 10.7 million watts of controlled fusion power, enough to meet the needs of more than 3,000 homes.

Inside the Fusion Reactor Tokamak - http://www.youtube.com/watch?v=tdQmPUI23mA



2. Intertial Confinement Fusion (Plasma)
It is a process where nuclear fission reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium.
nuclea11.gif
Operating System of the ICF http://eo.ucar.edu/staff/dward/sao/fit/images/nuclea11.gif
It is then struck simultaneously from several directions by intense laser beams. The intense amount of energy in the device heats and ionizes the pellet into a plasma. As the outer layers evaporate, the collisions they make with the ions in the core of the pellet drives it inward. This collapse or violent compression raises the density to about 1,000 times normal density and further heats the core totemperatures at which fusion normally occurs. The energy released by these reactions will then heat the surrounding fuel, which may also begin to undergo fusion. The aim of ICF is to produce a condition known as "ignition", where this heating process causes a chain reaction that burns a significant portion of the fuel. Typical fuel pellets are about the size of a pinhead and contain around 10 milligrams of fuel: in practice, only a small proportion of this fuel will undergo fusion, but if all this fuel were consumed it would release the energy equivalent to burning a barrel of oil. The confinement time is very short, of about 10-11 to 10-9 seconds, during which time the ions don’t move appreciably because of their own inertia.

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"Breeder reactor -." Wikipedia, the free encyclopedia. 01 Apr. 2009 <http://en.wikipedia.org/wiki/Breeder_reactor>.

"Energy Matters: Methods of Containing Plasma." Oracle ThinkQuest Library. 01 Apr. 2009 <http://library.thinkquest.org/20331/types/fusion/methods.html>.

Giancoli, Douglas C. Physics Principles With Applications. 5th ed. Upper Saddle River: Prentice Hall, 2002.

"Lawson Criteria for Nuclear Fusion." Test Page for Apache Installation. 01 Apr. 2009 <http://hyperphysics.phy-astr.gsu.edu/Hbase/NucEne/lawson.html#c3>.

"Nuclear fission -." Wikipedia, the free encyclopedia. 30 Mar. 2009 <http://en.wikipedia.org/wiki/Nuclear_fission>.

"Nuclear Fusion." NukeWorker.com - Nuclear Jobs & Resumes - Nuclear Career Resource. 31 Mar. 2009 <http://www.nukeworker.com/study
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"Nuclear Fusion." NukeWorker.com - Nuclear Jobs & Resumes - Nuclear Career Resource. 02 Apr. 2009 <http://www.nukeworker.com/study/nuclear_energy/ne5-fusion.shtml>.

"Nuclear fusion power -." Encyclopedia of Earth. 02 Apr. 2009 <http://www.eoearth.org/article/Nuclear_fusion_power>.

"YouTube - Inside the Fusion Reactor DIII D tokamak." YouTube - Broadcast Yourself. 01 Apr. 2009 <http://www.youtube.com/watch?v=tdQmPUI23mA>.