Nuclear Fusion (English)

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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

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

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

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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