Energy that is released as a result of fission of heavy atom nuclei (e.g., uranium) in atomic reactors has been utilized already for a good many years. Another possible method of nuclear energy production is by the fusion of the nuclei of the lightest chemical elements. Fusion of this kind called nuclear (or thermonuclear) fusion may occur, for example, when in a mixture of the two gases deuterium and tritium (which are heavy hydrogen isotopes indicated by the symbols 2H and 3H respectively) the atom nuclei collide with one another with sufficiently high energy (i.e., with sufficiently high relative velocity) In such circumstances the electrostatic repulsion operating between the nuclei (which are positively charged) can be overcome, so that the colliding nuclei “fuse” together. This phenomenon is accompanied by the release of individual nuclear components with high kinetic energy.
The principal processes that may occur in a mixture of deuterium and tritium are:
D+D = 3He+n+3.25 MeV 3He+D = 4He+p+ 18.3 MeV
D+D = T+p+4MeV T+D = 4He+n+17.6MeV
D denotes a deuterium nucleus (deuteron), T a tritium nucleus (triton), p a proton and n a neutron, while 3He and He denote helium nuclei with mass numbers equal to 3 and 4, respectively. The energy that is released is expressed in mega-electron volts (1 MeV = 4.45 > 10-20 kWh).
There are many other possible ways of achieving the fusion of light atom nuclei. In the sun and other stars there occur, besides the fusion of hydrogen nuclei, complex fusion processes involving the participation of heavy nuclei. It is these processes that produce the vast amounts of energy that arc constantly radiated into space by the stars. The devastating effect of a hydrogen-bomb explosion is also due to the energy released by fusion processes of this type: with an atomic bomb as a detonator, an explosive or uncontrolled chain reaction of nuclear fusion processes is initiated. “Taming the hydrogen bomb” in the sense of bringing these reactions under control in a so-called fusion reactor would place a tremendous new source of energy at man’s disposal. Using pure deuterium as “fuel,” it would be possible to produce about 1025 kWh of energy from the quantity of heavy water (deuterium oxide) estimated to be present in the natural waters of the earth.
As already stated, fusion processes take place only when nuclei of atoms collide at high velocities. To achieve these, the gas serving as fuel must be heated to such extremely high temperatures T that the average energy kT of the particles (k denotes the Boltzmann constant) is of the order of magnitude of the potential wall U0. Since the potential wall is to a certain extent “permeable” to the particles whose energy is less than Uo (quantum-mechanical tunnel effect), nuclear reactions already take place at kinetic-energy values of 10 to 100 keV. Hence, sufficiently large numbers of nuclei can react with one another in “thermal collisions,’ if kT= 100 keV i.e., T =100 million degrees centigrade. Nuclear reactions caused by thermal collisions are generally called thermonuclear reactions.
Since each fusion results in the release of several MeV of energy, it is possible in principle to achieve a positive energy balance even when only a small proportion of the nuclei react with one another. If the energy that is released can be kept together for some length of time, spontaneous heating of the gas may be initiated. At these extremely high temperatures the gas employed is completely ionized: i.e., all the atoms have been split up into freely moving electrons and “naked” nuclei. The gas has thus become completely ionized plasma. Because of this, it can be compressed into a relatively small space and be contained there by the action of a sufficiently strong magnetic field (approx. 100 kilogauss); (It is, of course, not possible to contain the plasma in any sort of material receptacle, as this would be vaporized at these high temperatures.) The escape of neutrons and radiation (more particularly the so-called “bremsstrahlung,”) inevitably causes losses of energy. A positive energy balance and therefore a continuation of the fusion processes can be achieved only if the energy of the electrically charged reaction products that are formed is able to make up for these losses. This is the case with a mixture comprising 50% deuterium and 50% tritium at temperatures of more than 50 million degrees centigrade (for pure deuterium it would require a temperature of 400 million degrees).
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