2. Nuclear fission

The importance of nuclear fission for the production of energy is obvious. In fission reactions, a heavy nucleus is split into two lighter fragments and two or three neutrons. About 180 MeV of energy is produced in the fission of an actinide to one of its most probable daughter pairs. This means that 1 kg of uranium (235U) is capable of producing enough energy to keep a 100-Watt light bulb running for about 25,000 years [2].

In the 1930s, scientists, particularly Hans Bethe, discovered that it is fusion that has been powering the Sun and stars since their formation [4]. A "fusion reactor" buried deep in the Sun's interior produces in one heartbeat the energy of 100 billion nuclear bombs. Beginning in the 1940s, researchers began to look for ways to initiate and control fusion reactions to produce useful energy on Earth. We now have a very good understanding of how and under what conditions two nuclei can fuse together.

The fusion of hydrogen into helium in the Sun and other stars occurs in three stages. First, two ordinary hydrogen nuclei (1H), which are actually just single protons, fuse to form an isotope of hydrogen called deuterium (2H), which contains one proton and one neutron. A positron (eþ) and a neutrino (ν) are also produced. The positron is very quickly annihilated in the collision with an electron, and the

Once created, the deuterium fuses with yet another hydrogen nucleus to produce 3He—an isotope of 4He. At the same time, a high-energy photon, or γ ray,

The final step in the reaction chain, which is called the proton-proton cycle, takes place when a second 3He nucleus, created in the same way as the first, collides

The net result of the proton-proton cycle is that four hydrogen nuclei combine to create one helium nucleus. The mass of the end product is 0:<sup>0475</sup> � <sup>10</sup>�<sup>27</sup> kg less than the combined mass of the 3He nuclei. This mass difference, known as mass defect in the parlance of nuclear physics, is converted into 26.7 MeV of energy as

The proton-proton cycle is particularly slow—only one collision in about 1026 for the cycle to start. As the cycle proceeds, the Sun's temperature rises, and eventually three 4He nuclei combine to produce 12C. Despite the slowness of the proton-proton cycle, it is the main source of energy for the Sun and for stars less massive than the Sun. The amount of energy released is enough to keep the Sun shining for billions of years. Besides the proton-proton cycle, there is another important set of hydrogenburning reactions called the carbon-nitrogen-oxygen (CNO) cycle that occurs at higher temperatures. Although CNO cycle contributes only a small amount to the Sun's luminosity, it dominates in stars that are more massive than a few times the Sun's mass. A star like Sirius with somewhat more than twice the mass of the

An obstacle called the Coulomb barrier caused by the strongly repulsive electro-

static forces between the positively charged nuclei prevents them from fusing

and fuses with another 3He, forming 4He and two protons. In symbols,

1H <sup>þ</sup> 1H ! 2H <sup>þ</sup> <sup>e</sup><sup>þ</sup> <sup>þ</sup> <sup>ν</sup>: (1)

2H <sup>þ</sup> 1H ! 3He <sup>þ</sup> <sup>γ</sup>: (2)

3He <sup>þ</sup> 3He ! 4He <sup>þ</sup> 21H: (3)

3.1 Fusion in the Sun

Nuclear Fusion: Holy Grail of Energy

DOI: http://dx.doi.org/10.5772/intechopen.82335

neutrino travels right out of the Sun:

known from Einstein's equation <sup>E</sup> <sup>¼</sup> mc2.

4. Coulomb barrier

5

Sun derives almost all of its energy from the CNO cycle.

is produced. The reaction is

#### 2.1 Fission reactors

All nuclear power plants in operation today rely on controlled fission of the isotopes of uranium and plutonium [3]. The reactor functions primarily as an exotic heat source to turn water into pressurized steam. Only the source of heat energy differs—nuclear power plants use fissile radioactive nucleus, while nonnuclear power plants use fossil fuel. The rest of the power train is the same. The steam turns the turbine blades, the blades generate mechanical energy, the energy runs the generator, and the generator produces electricity. The major improvement is the elimination of the combustion products of fossil fuels—the greenhouse gases, which have destroyed our environment beyond repair.

Because of its abundance in nature, most nuclear reactors use uranium as fuel. Natural uranium contains 0.7% of the fissile 235U; the rest is non-fissile 238U. When 235U is bombarded with a slow neutron, it captures the neutron to form 236U, which undergoes fission producing two lighter fragments and releases energy together with two or three neutrons. The neutrons produced in the reaction cause more fission resulting in a self-sustaining chain reaction. A reactor is considered safe when a self-sustained chain reaction is maintained with exactly one neutron from each fission inducing yet another fission reaction.

#### 2.2 Problems and concerns with fission reactors

Although fission-based nuclear reactors generate huge amounts of electricity with zero greenhouse gas emissions, and thus was hailed as a solution not only to global warming but also to global energy needs, nuclear energy is now seen by many, and with good reasons, as the misbegotten stepchild of nuclear weapons programs. Besides, it is by no means certain that the safety systems designed to shut down the reactor in the event of a runaway reaction are 100% foolproof and will work as designed. Another area of great concern is the hazards associated with the disposal of highly radioactive waste products.

What has raised our fear in regard to nuclear power more than anything else are the accidents at Chernobyl in 1986 and Fukushima in 2011. They were a sobering reminder of what we can expect from an accident due to catastrophic reactor failure or human errors. The Fukushima disaster in particular has shattered the zero risk myth of power reactors and heightened our concern about the invisibility of the added lethal component, nuclear radiation. Consequently, they have spurred our interest in the other source of nuclear energy—fusion.

### 3. Nuclear fusion

Nuclear fusion is the process in which two lighter nuclei, typically isotopes of hydrogen, combine together under conditions of extreme pressure and temperature to form a heavier nucleus, resulting in the release of enormous amount of energy. The fusion of four protons to form the helium nucleus 4He, two positrons, and two neutrinos, for example, generates about 27 MeV of energy.

### Nuclear Fusion: Holy Grail of Energy DOI: http://dx.doi.org/10.5772/intechopen.82335

In the 1930s, scientists, particularly Hans Bethe, discovered that it is fusion that has been powering the Sun and stars since their formation [4]. A "fusion reactor" buried deep in the Sun's interior produces in one heartbeat the energy of 100 billion nuclear bombs. Beginning in the 1940s, researchers began to look for ways to initiate and control fusion reactions to produce useful energy on Earth. We now have a very good understanding of how and under what conditions two nuclei can fuse together.

### 3.1 Fusion in the Sun

neutrons. About 180 MeV of energy is produced in the fission of an actinide to one of its most probable daughter pairs. This means that 1 kg of uranium (235U) is capable of producing enough energy to keep a 100-Watt light bulb running for

Nuclear Fusion - One Noble Goal and a Variety of Scientific and Technological Challenges

All nuclear power plants in operation today rely on controlled fission of the isotopes of uranium and plutonium [3]. The reactor functions primarily as an exotic heat source to turn water into pressurized steam. Only the source of heat energy differs—nuclear power plants use fissile radioactive nucleus, while nonnuclear power plants use fossil fuel. The rest of the power train is the same. The steam turns the turbine blades, the blades generate mechanical energy, the energy runs the generator, and the generator produces electricity. The major improvement is the elimination of the combustion products of fossil fuels—the greenhouse gases, which

Because of its abundance in nature, most nuclear reactors use uranium as fuel. Natural uranium contains 0.7% of the fissile 235U; the rest is non-fissile 238U. When 235U is bombarded with a slow neutron, it captures the neutron to form 236U, which undergoes fission producing two lighter fragments and releases energy together with two or three neutrons. The neutrons produced in the reaction cause more fission resulting in a self-sustaining chain reaction. A reactor is considered safe when a self-sustained chain reaction is maintained with exactly one neutron from

Although fission-based nuclear reactors generate huge amounts of electricity with zero greenhouse gas emissions, and thus was hailed as a solution not only to global warming but also to global energy needs, nuclear energy is now seen by many, and with good reasons, as the misbegotten stepchild of nuclear weapons programs. Besides, it is by no means certain that the safety systems designed to shut down the reactor in the event of a runaway reaction are 100% foolproof and will work as designed. Another area of great concern is the hazards associated with the

What has raised our fear in regard to nuclear power more than anything else are the accidents at Chernobyl in 1986 and Fukushima in 2011. They were a sobering reminder of what we can expect from an accident due to catastrophic reactor failure or human errors. The Fukushima disaster in particular has shattered the zero risk myth of power reactors and heightened our concern about the invisibility of the added lethal component, nuclear radiation. Consequently, they have spurred our

Nuclear fusion is the process in which two lighter nuclei, typically isotopes of hydrogen, combine together under conditions of extreme pressure and temperature to form a heavier nucleus, resulting in the release of enormous amount of energy. The fusion of four protons to form the helium nucleus 4He, two positrons, and two

about 25,000 years [2].

have destroyed our environment beyond repair.

each fission inducing yet another fission reaction.

2.2 Problems and concerns with fission reactors

disposal of highly radioactive waste products.

3. Nuclear fusion

4

interest in the other source of nuclear energy—fusion.

neutrinos, for example, generates about 27 MeV of energy.

2.1 Fission reactors

The fusion of hydrogen into helium in the Sun and other stars occurs in three stages. First, two ordinary hydrogen nuclei (1H), which are actually just single protons, fuse to form an isotope of hydrogen called deuterium (2H), which contains one proton and one neutron. A positron (eþ) and a neutrino (ν) are also produced. The positron is very quickly annihilated in the collision with an electron, and the neutrino travels right out of the Sun:

$$\mathbf{^1H} + \mathbf{^1H} \rightarrow \mathbf{^2H} + \mathbf{e^+} + \boldsymbol{\nu}.\tag{1}$$

Once created, the deuterium fuses with yet another hydrogen nucleus to produce 3He—an isotope of 4He. At the same time, a high-energy photon, or γ ray, is produced. The reaction is

$$\text{H}^{2}\text{H} + \text{}^{1}\text{H} \rightarrow \text{}^{3}\text{He} + \text{y.} \tag{2}$$

The final step in the reaction chain, which is called the proton-proton cycle, takes place when a second 3He nucleus, created in the same way as the first, collides and fuses with another 3He, forming 4He and two protons. In symbols,

$$\text{\textquotedblleft He} + \,^3\text{He} \to \,^4\text{He} + 2\,^1\text{H}. \tag{3}$$

The net result of the proton-proton cycle is that four hydrogen nuclei combine to create one helium nucleus. The mass of the end product is 0:<sup>0475</sup> � <sup>10</sup>�<sup>27</sup> kg less than the combined mass of the 3He nuclei. This mass difference, known as mass defect in the parlance of nuclear physics, is converted into 26.7 MeV of energy as known from Einstein's equation <sup>E</sup> <sup>¼</sup> mc2.

The proton-proton cycle is particularly slow—only one collision in about 1026 for the cycle to start. As the cycle proceeds, the Sun's temperature rises, and eventually three 4He nuclei combine to produce 12C. Despite the slowness of the proton-proton cycle, it is the main source of energy for the Sun and for stars less massive than the Sun. The amount of energy released is enough to keep the Sun shining for billions of years.

Besides the proton-proton cycle, there is another important set of hydrogenburning reactions called the carbon-nitrogen-oxygen (CNO) cycle that occurs at higher temperatures. Although CNO cycle contributes only a small amount to the Sun's luminosity, it dominates in stars that are more massive than a few times the Sun's mass. A star like Sirius with somewhat more than twice the mass of the Sun derives almost all of its energy from the CNO cycle.
