NUCLEAR FUSION

When two light nuclei combine to form a heavier nucleus, the process is called nuclear fusion.

Because the mass of the final nucleus is less than the combined masses of the original nuclei,

there is a loss of mass accompanied by a release of energy.

1
1𝐻 + 11𝐻 → 21𝐻 + 𝑒 + + 𝑣

1
1𝐻 + 21𝐻 → 31𝐻𝑒 + 𝛾

These reactions occur in the core of a star and are responsible for the of energy from the star. The

second reaction is followed by either hydrogen–helium fusion or helium–helium fusion

1
1𝐻 + 32𝐻 → 42𝐻𝑒 + 𝑒 + + 𝑣

3
2𝐻𝑒 + 32𝐻𝑒 → 42𝐻𝑒 + 11𝐻 + 11𝐻

These fusion reactions are the basic reactions in the proton–proton cycle, believed to be one of

the basic cycles by which energy is generated in the Sun and other stars that contain an

abundance of hydrogen. Most of the energy production takes place in the Sun’s interior, where

the temperature is approximately 1.5 × 107 𝐾. Because

such high temperatures are required to drive these reactions, they are called thermonuclear

fusion reactions. All the reactions in the proton–proton cycle are exothermic. In the cycle four

protons combine to generate an alpha particle, positrons, gamma rays, and neutrinos.

Terrestrial Fusion Reactions

Controlled fusion is the ultimate energy source because of the availability of its fuel source:

water. For example, if deuterium were used as the fuel, 0.12 g of it could be extracted from 1 gal

of water at a cost of about four cents. This amount of deuterium would release approximately

1010 J if all nuclei underwent fusion. By comparison, 1 gal of gasoline releases approximately
108 J upon burning and costs far more than four cents.

An additional advantage of fusion reactors is that comparatively few radioactive by-products are

formed. For the proton–proton cycle, for instance, the end product is safe, nonradioactive helium.

Unfortunately, a thermonuclear reactor that can deliver a net power output spread over a

reasonable time interval is not yet a reality and many difficulties must be resolved before a

successful device is constructed.

The Sun’s energy is based in part on a set of reactions in which hydrogen is converted to helium.

The proton–proton interaction is not suitable for use in a fusion reactor because the event

requires very high temperatures and densities.

The process works in the Sun only because of the extremely high density of protons in the Sun’s

interior.

The reactions that appear most promising for a fusion power reactor involve deuterium ( 21𝐻 ) and

tritium ( 31𝐻 ):

2
1𝐻 + 21𝐻 → 32𝐻𝑒 + 10𝑛 Q = 3.27 MeV

2
1𝐻 + 21𝐻 → 31𝐻 + 11𝐻 Q = 4.03 MeV

2
1𝐻 + 31𝐻 → 42𝐻𝑒 + 10𝑛 Q = 17.59 MeV

Deuterium is available in large quantities lakes and oceans and is very cheap to extract. Tritium,

however, is radioactive (half-life of 12.3 years) and undergoes beta decay to 32𝐻𝑒. For this

reason, tritium does not occur naturally to any great extent and must be artificially produced.

One major problem in obtaining energy from nuclear fusion is that the Coulomb repulsive force

between two nuclei, which carry positive charges, must be overcome before they can fuse. The

main problem then is to give the two nuclei enough kinetic energy to overcome this repulsive
force. This can only be accomplished by raising the fuel to extremely high temperatures (

approximately 108 𝐾 ). At these high temperatures, the atoms are ionized and the system consists

of a collection of electrons and nuclei, commonly referred to as a plasma.

whether or not a thermonuclear reactor is successful are the ion density n and confinement time

t, which is the time interval during which energy injected into the plasma remains within the

plasma. Both the ion density and confinement time must be large enough to ensure that more

fusion energy is released than the amount required to raise the temperature of the plasma. For a

given value of n, the probability of fusion between two particles increases as t increases. For a

given value of t, the collision rate between nuclei increases as n increases.

Fusion Reactor Design

In the D–T fusion reaction 21𝐻 + 31𝐻 → 42𝐻𝑒 + 𝑛0𝑛 Q = 17.59 MeV the alpha particle carries

20% of the energy and the neutron carries 80%, or approximately 14 MeV. A diagram of the

deuterium–tritium fusion reaction is shown in the figure below.

Because the alpha particles are charged, they are primarily absorbed by the plasma, causing the

plasma’s temperature to increase. In contrast, the 14-MeV neutrons, being electrically neutral,

pass through the plasma and are absorbed by a surrounding blanket material, where their large

kinetic energy is extracted and used to generate electric power.

lithium in a closed heat-exchange loop, thereby producing steam and driving turbines as in a

conventional power plant. The figure below shows a diagram of such a reactor.

It is estimated that a blanket of lithium approximately 1 m thick will capture nearly 100% of the

neutrons from the fusion of a small D–T pellet.

The capture of neutrons by lithium is described by the reaction

1
0𝑛 + 63𝐿𝑖 → 31𝐻 + 42𝐻𝑒
where the kinetic energies of the charged tritium 31𝐻 and alpha particle are transformed to

internal energy in the molten lithium. An extra advantage of using lithium as the energy-transfer

medium is that the tritium produced can be separated from the lithium and returned as fuel to the

reactor.

Advantages and Problems of Fusion

If fusion power can ever be harnessed, it will offer several advantages over fission generated

power:

1) low cost and abundance of fuel (deuterium),

2) impossibility of runaway accidents,

3) decreased radiation hazard.

Some of the anticipated problems and disadvantages include:

1) scarcity of lithium,

2) limited supply of helium, which is needed for cooling the superconducting magnets used

to produce strong confining fields

3) structural damage and induced radioactivity caused by neutron bombardment.