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3-4: Nuclear Fusion

On the top page of the present Part 3, 3-1:The Origin of the Nuclear Energy, we took an example of exothermic (exoergic) nuclear reaction in which two deuterons combine together to form helium-3 () and emit a neutron (n) and release an energy of 3.27 MeV. This fusion reaction process is written as

We interpreted the reason why such a big amount of energy as 3.27 MeV is released in this process as follows: After this fusion reaction, the binding energy of the produced helium-3 () is 7.719 MeV which is considerably larger than the sum of the binding energies of the two deuterons before the reaction, 4.449 MeV.
Like this, the binding energy as a function of the mass number A fluctuates much in the region of light nuclei. Utilizing this fluctuation, one can get large amounts of energy through nuclear fusion reaction.
Let us see the experimental data of the binding energies of the light nuclei in detail in the following figure.

[Nuclear Binding Energies]
This figure shows the experimental data of the binding energies per nucleon in the region of light nuclei. This is essentially the same as the last one at the bottom of the page: "2-3: Mass of Nuclei, Binding Energy", but the data for nuclei of smaller mass number than 20 can be seen in further detail.

Casting a glance at the above figure, we can see that, if very light nuclides, e.g. proton (p), deuteron (d), helium-3 ( ) or triton (t), combine together to form such a slightly heavier one as helium-4 ( ), a large amount of energy would be released.
(Needless to say, proton, deuteron, helium-3, triton, and helium-4 are the atomic nuclei of hydrogen(protium), deuterium, helium-3, tritium and helium-4 atoms, respectively.)
In addition to the above d-d reaction (Eq. (1)), the following reactions

are also assumed to be capable to be utilized for fusion power. Among these reactions, the easiest to utilize is the d-t reaction (Eq. (2)). (See the following figure.)

[The d-t Reaction]
If deuteron and triton collide at a high speed, a fusion reaction occurs and a large amount of energy (= 17.58 MeV) is released.

The device in which nuclear fusion reaction takes place continuously to extract energy is called a nuclear fusion reactor. Fusion is clearly a more "mass-energy efficient" process than fission, because the difference of the binding energies per nucleon before and after the reaction is larger in fusion than in fission. In this sense, fusion is more favorable than fission. While experiments for utilization of fusion reaction continue, no actual fusion reactor exist yet.
To develope a reactor for practical use, many difficult problems should be overcome. Among them, the most important is the confinement of plasma.

[Confinement of Plasma]
Let us take up the above d-t reaction as an example. d and t are the nuclei of deuterium and tritium elements, respectively. In these elements, the nuclei are surrounded by electrons and usually exist as electrically neutral. Namely, d and t are ions of these neutral elements. The gases of these ions are called plasma.
Therefore both d and t in the plasma have +e charges, and the Coulomb repulsive force works between them. In order for d and t to fuse together, they must get closer to each other and collide at a very high speed overcoming the Coulomb repulsive force. For this, one must confine the plasma at a state of extremely high temperature and pressure; e.g. higher than 0.1 GK (= giga Kelvin).
How can we confine such an enormously high temperature and pressure of plasma? This is none other than the problem of plasma confinement. To carry out a Research and Development of a fusion power reactor, the construction of ITER (International Thermonuclear Experimental Reactor) has internationally been negotiated.

[Resources for Fusion Power]
Deuterium is one of naturally occurring isotopes of hydrogen whose abundance is 0.015% (in number of atoms). It can be taken from heavy water, whoes resources are quite rich on the earth. It is easy to separate deuterium from hydrogen isotopes.
In contrast to this, tritium is not naturally occurring. It must be produced in an artificial way; for this purpose, the fusion reaction shown by the above Eq. (3) considered to be suitable. The resources of lithium (Li) appears rather rich on the earth.
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