Fission Or Fusion Essay Research Paper Fission

Fission Or Fusion Essay, Research Paper

Fission or Fusion

I think that right now, fission is the only way that we can get more

energy out of a nuclear reaction than we put in. First, the energy per fission

is very large. In practical units, the fission of 1 kg (2.2 lb) of uranium-235

releases 18.7 million kilowatt-hours as heat. Second, the fission process

initiated by the absorption of one neutron in uranium-235 releases about 2.5

neutrons, on the average, from the split nuclei. The neutrons released in this

manner quickly cause the fission of two more atoms, thereby releasing four or

more additional neutrons and initiating a self-sustaining series of nuclear

fissions, or a chain reaction, which results in continuous release of nuclear

energy. Naturally occurring uranium contains only 0.71 percent uranium-235; the

remainder is the non-fissile isotope uranium-238. A mass of natural uranium by

itself, no matter how large, cannot sustain a chain reaction because only the

uranium-235 is easily fissionable. The probability that a fission neutron with

an initial energy of about 1 MeV will induce fission is rather low, but can be

increased by a factor of hundreds when the neutron is slowed down through a

series of elastic collisions with light nuclei such as hydrogen, deuterium, or

carbon. This fact is the basis for the design of practical energy-producing

fission reactors.

In December 1942 at the University of Chicago, the Italian physicist

Enrico Fermi succeeded in producing the first nuclear chain reaction. This was

done with an arrangement of natural uranium lumps distributed within a large

stack of pure graphite, a form of carbon. In Fermi’s “pile,” or nuclear reactor,

the graphite moderator served to slow the neutrons.

Nuclear fusion was first achieved on earth in the early 1930s by

bombarding a target containing deuterium, the mass-2 isotope of hydrogen, with

high-energy deuterons in a cyclotron. To accelerate the deuteron beam a great

deal of energy is required, most of which appeared as heat in the target. As a

result, no net useful energy was produced. In the 1950s the first large-scale

but uncontrolled release of fusion energy was demonstrated in the tests of

thermonuclear weapons by the United States, the USSR, Great Britain, and France.

This was such a brief and uncontrolled release that it could not be used for the

production of electric power.

In the fission reactions I discussed earlier, the neutron, which has no

electric charge, can easily approach and react with a fissionable nucleus ,for

example, uranium-235. In the typical fusion reaction, however, the reacting

nuclei both have a positive electric charge, and the natural repulsion between

them, called Coulomb repulsion, must be overcome before they can join. This

occurs when the temperature of the reacting gas is sufficiently high, 50 to 100

million ? C (90 to 180 million ? F). In a gas of the heavy hydrogen isotopes

deuterium and tritium at such temperature, the fusion reaction occurs, releasing

about 17.6 MeV per fusion event. The energy appears first as kinetic energy of

the helium-4 nucleus and the neutron, but is soon transformed into heat in the

gas and surrounding materials.

If the density of the gas is sufficient?and at these temperatures the

density need be only 10-5 atm, or almost a vacuum?the energetic helium-4 nucleus

can transfer its energy to the surrounding hydrogen gas, thereby maintaining the

high temperature and allowing subsequent fusion reactions, or a fusion chain

reaction, to take place. Under these conditions, “nuclear ignition” is said to

have occurred. The basic problems in attaining useful nuclear fusion conditions

are to heat the gas to these very high temperatures, and to confine a

sufficient quantity of the reacting nuclei for a long enough time to permit the

release of more energy than is needed to heat and confine the gas. A subsequent

major problem is the capture of this energy and its conversion to electricity.

At temperatures of even 100,000? C (180,000? F), all the hydrogen atoms

are fully ionized. The gas consists of an electrically neutral assemblage of

positively charged nuclei and negatively charged free electrons. This state of

matter is called a plasma. A plasma hot enough for fusion cannot be contained by

ordinary materials. The plasma would cool very rapidly, and the vessel walls

would be destroyed by the temperatures present. However, since the plasma

consists of charged nuclei and electrons, which move in tight spirals around

strong magnetic field lines, the plasma can be contained in a properly shaped

magnetic field region without reacting with material walls.

In any useful fusion device, the energy output must exceed the energy

required to confine and heat the plasma. This condition can be met when the

product of confinement time t and p

Внимание, отключите Adblock

Вы посетили наш сайт со включенным блокировщиком рекламы!
Ссылка для скачивания станет доступной сразу после отключения Adblock!

Скачать