JEE Main & Advanced Chemistry Surface & Nuclear Chemistry / भूतल और नाभिकीय रसायन Nuclear Fission And Nuclear Fusion

(1) Nuclear fission : The  splitting of a heavier atom like that of uranium – 235 into a number of fragments of much smaller mass, by suitable bombardment with sub-atomic particles with liberation of huge amount of energy is called Nuclear fission. Hahn and Startsman discovered that when uranium-235 is bombarded with neutrons, it splits up into two relatively lighter elements.

\[_{92}{{U}^{235}}{{+}_{0}}{{n}^{1}}\to {{\,}_{56}}B{{a}^{140}}{{+}_{36}}K{{r}^{93}}+3{{\,}_{0}}{{n}^{1}}\]+ Huge amount of energy

Spallation reactions are similar to nuclear fission. However, they differ by the fact that they are brought by high energy bombarding particles or photons.

Elements capable of undergoing nuclear fission and their fission products. Among elements capable of undergoing nuclear fission, uranium is the most common. The natural uranium consists of three isotopes, namely \[{{U}^{234}}(0.006%)\], \[{{U}^{235}}(0.7%)\] and \[{{U}^{238}}(99.3%)\]. Of the three isomers of uranium, nuclear fission of \[{{U}^{235}}\] and \[{{U}^{238}}\] are more important. Uranium-238 undergoes fission by fast moving neutrons while \[{{U}^{235}}\] undergoes fission by slow moving neutrons; of these two, \[{{U}^{235}}\] fission is of much significance. Other examples are \[P{{u}^{239}}\] and \[{{U}^{233}}\].

Uranium-238, the most abundant (99.3%) isotope of uranium, although itself does not undergo nuclear fission, is converted into plutonium-239.

\[_{92}{{U}^{238}}+{{\,}_{0}}{{n}^{1}}\to {{\,}_{92}}{{U}^{239}}\] ; \[_{92}{{U}^{239}}\to {{\,}_{92}}N{{P}^{239}}+{{\,}_{-1}}{{e}^{0}}\]

 \[_{93}N{{p}^{238}}\to {{\,}_{94}}P{{u}^{239}}+{{\,}_{-1}}{{e}^{0}}\]

Which when bombarded with neutrons, undergo fission to emit three neutrons per plutonium nucleus. Such material like U-238 which themselves are non-fissible but can be converted into fissible material (Pu-239) are known as fertile materials.

Nuclear chain reaction : With a small lump of \[{{U}^{235}}\], most of the neutrons emitted during fission escape but if the amount of \[{{U}^{235}}\] exceeds a few kilograms (critical mass), neutrons emitted during fission are absorbed by adjacent nuclei causing further fission and so producing more neutrons. Now since each fission releases a considerable amount of energy, vast quantities of energy will be released during the chain reaction caused by \[{{U}^{235}}\] fission.

Atomic bomb : An atomic bomb is based upon the process of that nuclear fission in which no secondary neutron escapes the lump of a fissile material for which the size of the fissile material should not be less than a minimum size called the critical size. There is accordingly a sudden release of a tremendous amount of energy, which represents an explosive force much greater than that of the most powerful TNT bomb. In the world war II in 1945 two atom bombs were used against the Japanese cities of Hiroshima and Nagasaki, the former contained U-235 and the latter contained Pu-239.

Atomic pile or Nuclear reactor : It is a device to obtain the nuclear energy in a controlled way to be used for peaceful purposes. The most common reactor consists of a large assembly of graphite (an allotropic form of carbon) blocks having rods of uranium metal (fuel). Many of the neutrons formed by the fission of nuclei of \[_{92}{{U}^{235}}\] escape into the graphite, where they are very much slow down (from a speed of about 6000 or more miles/sec to a mile/sec) and now when these low speed neutrons come back into the uranium metal they are more likely to cause additional fissions. Such a substance like graphite, which slow down the neutrons without absorbing them is known as a moderator. Heavy water, D₂O is another important moderator where the nuclear reactor consists of rods of uranium metal suspended in a big tank of heavy water (swimming pool type reactor). Cadmium or boron are used  as control rods for absorbing excess neutrons.

Plutonium from a nuclear reactor : For such purposes the fissile material used in nuclear reactors is the natural uranium which consists mainly (99.3%) of U-238. In a nuclear reactor some of the neutrons produced in U-235 (present in natural uranium) fission converts U-238 to a long-lived plutonium isotope, Pu-239 (another fissionable material). Plutonium is an important nuclear fuel. Such reactors in which neutrons produced from fission are partly used to carry out further fission and partly used to produce some other fissionable material are called Breeder reactors.

Nuclear reactors in India : India is equipped with the five nuclear reactors, namely

(i) Apsara (1952)         (ii) Cirus (1960)

(iii) Zerlina (1961)       (iv) Purnima (1972) and R-5 

    Purnima uses plutonium fuel while the others utilize uranium as fuel.

(2) Nuclear fusion : “Oposite to nuclear fission, nuclear fusion is defined as a process in which lighter nuclei fuse together to form a heavier nuclei. However, such processes can take place at reasonable rates only at very high temperatures of the order of several million degrees, which exist only in the interior of stars. Such processes are, therefore, called Thermonuclear reactions (temperature dependent reactions). Once a fusion reaction initiates, the energy released in the process is sufficient to maintain the temperature and to keep the process going on.

\[\underset{\text{Hydrogen}}{\mathop{4{{\,}_{1}}{{H}^{1}}}}\,\,\,\,\to \,\,\,\,\,\underset{\text{Helium}}{\mathop{_{2}H{{e}^{4}}}}\,\,\,+\,\,\,\,\underset{\text{Positron}}{\mathop{2{{\,}_{+1}}{{e}^{0}}}}\,\,\,\,+\,\,\,\text{ Energy}\]

This is not a simple reaction but involves a set of the thermonuclear reactions, which take place in stars including sun. In other words, energy of sun is derived due to nuclear fission.

Controlled nuclear fusion : Unlike the fission process, the fusion process could not be controlled. Since there are estimated to be some \[{{10}^{17}}\] pounds of deuterium \[({{\,}_{1}}{{H}^{2}})\] in the water of the earth, and since each pound is equivalent in energy to 2500 tonnes of coal, a controlled fusion reactor would provide a virtually inexhaustible supply of energy.

Hydrogen bomb : Hydrogen bomb is based on the fusion of hydrogen nuclei into heavier ones by the thermonuclear reactions with release of enormous energy.

As mentioned earlier the above nuclear reactions can take place only at very high temperatures. Therefore, it is necessary to have an external source of energy to provide the required high temperature. For this purpose, the atom bomb, (i.e., fission bomb) is used as a primer, which by exploding provides the high temperature necessary for successful working of hydrogen bomb (i.e., fusion bomb). In the preparation of a hydrogen bomb, a suitable quantity of deuterium or tritium or a mixture of both is enclosed in a space surrounding an ordinary atomic bomb. The first hydrogen bomb was exploded in November 1952 in Marshall Islands; in 1953 Russia exploded a powerful hydrogen bomb having power of 1 million tonnes of TNT

A hydrogen bomb is far more powerful than an atom bomb. Thus if it is possible to have sufficiently high temperatures required for nuclear fusion, the deuterium present in sea (as D₂O) is sufficient to provide all energy requirements of the world for millions of years. The first nuclear reactor was assembled by Fermi in 1942.

 Difference between Nuclear fission and fusion