Nuclear fusion and fission | Difference between, Nuclear energy, Equations | aurayne

Nuclear Energy 

The curve of binding energy at (Eb) per nucleon, given in the figure 1.1, 

Figure 1.1

has a long flat median region between A = 30 and A = 170.  The binding energy per nucleon in this region is approximately constant (8.0 MeV).  For the light nucleus region, A <30, and for the heavy nucleus region, A> 170, the binding energy per nucleon is less than 8.0MeV.

Now, the higher the binding energy, the lower the total mass of a bonded system, such as a nucleus. Consequently, if nuclei with lower total binding energies are converted to nuclei with more binding energies, the net energy release will occur. 

This occurs when a heavy nucleus disintegrates into two or more intermediate mass fragments (Nuclear fission) or when the light nucleus fuses into a heavy nucleus (Nuclear fusion).  

Exothermic chemical reactions fall under conventional energy sources such as coal or petroleum. The energies involved here are in the range of electron volts.  

On the other hand, in a nuclear reaction, the emission of energy is of the order of MeV. Thus, for the same amount of matter, atomic sources produce a million times more energy than a chemical source.  

For example, fission of 1 kg of uranium produces 10^14 J of energy;  Compare this to the burning of 1 kg of coal which gives 10^7J of energy.

NUCLEAR FISSION  

New  emerge when we go beyond natural radioactive decay and study nuclear reactions by bombarding nuclei with other nuclear particles such as protons, neutrons, α-particles, and more.

One of the most important neutron-induced nuclear reaction is fragmentation.  An example of fission is when a uranium isotope U-235 breaks up into two intermediate-mass atomic fragments by bombardment with a neutron.

The same reaction can produce other pairs of intermediate mass fractions

The fragment products are radioactive nuclei;  They emit β particles sequentially to obtain stable end products.

The energy (Q value) released in the fission reaction of nuclei such as uranium is of the order of 200 MeV per fission nucleus. It is estimated as follows:

Let us take a nucleus with A = 240 which breaks into two pieces of A = 120.  Again

Binding energy (Eb) is about 7.6 MeV for A = 240 nuclei,

(Eb) is about 8.5 MeV for two A = 120 segment nuclei.

The gain in binding energy for the nucleon is about 0.9 MeV.

Therefore the total gain in binding energy is 240×0.9 or 216 MeV.

In fission events the dissolution energy first appears as the kinetic energy of fragments and neutrons. Eventually it gets transferred to the surrounding material appearing as heat.  

The source of energy in nuclear reactors, which produces electricity, is nuclear fission. The enormous energy emitted in the atomic bomb comes from uncontrolled nuclear fission. We will discuss some details in the next blog about how a nuclear reactor works.

NUCLEAR FUSION 

When two light nuclei form a larger nucleus, the energy is released, because the larger nucleus is more tightly bound, as seen from the binding energy curve in Fig. 1.1. Some examples of nuclear fusion reactions that release such energy are: 

  

In the first reaction (a), the two protons combine to form a deuteron and a positron with an energy release of 0.42 MeV.  

In reaction (b), the two deuterons combine to form the light isotopes of helium. 

In reaction (c), the two deuterons combine to form a triton and a proton. 

For fusion to occur, the two nuclei must come so close that the attractive short-range atomic force is able to affect them.  

However, since they are both positively charged particles, they experience coulomb repulsion. Therefore, they must have enough energy to overcome this coulomb barrier. The height of the barrier depends on the charges and radii of the two interacting nuclei.  

It can be shown, for example, that the barrier height for two protons is ~400 keV, and higher for nuclei with higher charges.  We can estimate the temperature at which two protons (on average) in a proton gas will have enough energy to overcome the coulomb barrier: 

(3/2) k T = K 400 keV, which gives T ~ 3 × 109 K. 

When fusion is achieved by increasing the temperature of the system so that the particles have sufficient kinetic energy to overcome the coupling repulsive behavior, it is called thermonuclear fusion.

Thermonuclear fusion is a source of energy production in the interior of wires. The temperature of the Sun's interior is 1.5×10^7 K, which is much lower than the estimated temperature required for the fusion of particles of average energy.  Clearly, fusion in the Sun involves protons whose energies are much higher than the average energy.

The fusion reaction in the Sun is a multi-step process in which hydrogen is burned into helium. Thus, fuel in the Sun is hydrogen at its core. The proton–proton (p, p) cycle by which this occurs is represented by the following sets of reactions:

For the fourth reaction to occur, the first three reactions must occur twice, in which case two light helium nuclei unite to form normal helium nuclei. If we consider the combination 2 (i) + 2 (ii) + 2 (iii) + (iv), the net effect is

Thus, four hydrogen atoms combine to form a 42He atom with 26.7MeV of energy.

Helium is not the only element that can be synthesized in the interior of a star. As the hydrogen in the core is depleted and helium is formed, the core begins to cool.  The star collapses under its own gravity causing the core temperature to rise.  

If this temperature rises to about 10^8K, fusion occurs again, this time with the helium nucleus in carbon. Such a process can arise through the fusion of higher and higher mass number elements. But more massive elements than near the peak of the binding energy curve in cannot be produced.

The age of the Sun is about 5×10^9year and it is estimated that there is enough hydrogen in the Sun that will keep it running for another 5 billion years.  

After that, the burning of hydrogen will stop and the sun will begin to cool and collapse under gravity, raising the core temperature. The outer shell of the Sun will expand, turning it into the so-called red giant.

Difference between nuclear fusion and nuclear fission:

Nuclear fission

• Fission is the division of a large atom into two or more smaller atoms.

• Fission produces highly radioactive particles.

• The fission reaction does not normally occur in nature.

• Requires critical mass and high-speed neutrons of matter.

• It takes less energy to split two atoms in a fission reaction.

• The energy released by fission is one million times greater than the energy released in chemical reactions, but less than the energy released by nuclear fusion.

• Atomic bomb works on the principle of nuclear fission.

Nuclear Fusion

• Fusion is the fusion of two or more light atoms into a larger atom.

• Some radioactive particles originate from the fusion reaction, but if a fission "trigger" is used, radioactive particles will result in it.

• Fusion occurs in stars like the Sun.

• High density, high temperature environment is required.

• Extremely high energy is required to bring two or more protons so close that atomic forces can overcome their electrostatic repulsion.

• The free energy by fusion is three to four times higher than the free energy by Nuclear fission.

• Hydrogen bomb works on the principle of nuclear fusion bomb.

Controlled thermonuclear fusion

The natural thermonuclear fusion process in a star is repeated in a thermonuclear fusion device. In controlled fusion reactors, nuclear fuel is to be heated to temperatures in the range of 10^8K to generate stable power. 

At these temperatures, the fuel is a mixture of positive ions and electrons (plasma). The challenge is to confine this plasma, as no container can tolerate such a high temperature. 

Many countries around the world, including India, are developing technology in this regard. If successful, fusion reactors would hope to supply nearly unlimited power to humanity.

Nuclear Fission | Nuclear fusion | Difference between nuclear fusion and nuclear fission | Nuclear Energy | Controlled Thermonuclear fusion|

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