TYPE OF NUCLEAR REACTION
1. Spallation reaction
These are the reactions in which high speed projectiles may chip a heavy nucleus into several fragments.
29 Cu63 + 2He4 + 400 MeV ® 17Cl37 + 14 1H1 + 16 0n1
2. Nuclear fission reaction
Nuclear fission is the process in which a heavy nucleus breaks up into two lighter nuclei of almost equal size with the release of an enormous amount of energy. This type of nuclear fission reaction was first observed by German Chemists Otto Hahn, F.Strassman and Meitner by bombarding 92U235with slow moving neutrons. The process is usually accompanied by emission of neutrons. The nuclear fission has been produced in heavy nuclei such as 235U,238U, 232Th by neutrons, protons, deuterons.
Mechanism of fission
In the fission process, the heavy nucleus absorbs a neutron and forms an unstable compound nucleus. The compound nucleus then breaks up more or less in the middle to give fission product.
A typical example of the fission process in the fission of uranium by neutrons is explained by the following equation.
92 U235 + 0n1 ® 56Ba141 + 3 0n1 + 200 MeV
Further, the neutrons released (say three) from the fission of first uranium atoms can hit three other uranium atoms. In this way a chain reaction is set up resulting into the liberation of an enormous amount of energy. In the case of nuclear fission, 92U236 formed breaks up in several ways.
This fission process is self multiplying process and hencea tremendous amount of energy is released in a very short interval of time. Therefore, explosion takes place. Atom bomb is based on nuclear fission process.
Energy released in nuclear fission reaction
92U235 + 0n1 -- > 42Mo95 + 57La139 + 20n1 + 7-1e0
The isotopic mass of U235 = 235.118 amu
The isotopic mass of 42Mo95 = 94.936 amu
The isotopic mass of 57La139 = 138.95 amu
The isotopic mass of 0n1 = 1.009 amu
∴ 235.118 + 1.009 ® 94.936 + 138.95 + 2 × 1.009
236.127 amu ® 235.906 amu
∴ The mass converted into energy is
= (236.127 - 235.906) amu
= 0.213 amu
Since 1amu (atomic mass unit) = 931 MeV, for one 235U fission energy
released = 0.213 × 931.48 = 200 MeV
The first atom bomb used in Hiroshima (Japan) utilised 92U235 isotope as the main reacting substance and second bomb in Nagasaki made use of Plutonium (239) (August 1945). The fission in both the cases is similar and uncontrolled. Enormous amount of energy equal to that produced by 20000 tons of TNT is produced accompanied by heat, light and radioactive radiations.
Nuclear Power Generator
A nuclear reactor or nuclear power generator is a kind of furnace for carrying out the controlled fission of a radioactive material like U235 for producing power.
The core of the nuclear reactor produces heat through nuclear fission. Heavy water at high pressure takes heat away from the core. In the heat exchanger, the heavy water inside the reactor gives up its heat to water outside the reactor, which boils to form steam. The steam is taken away to drive turbines that make electricity. In Tamilnadu atomic power stations generating electricity are situated at Kalpakkam and another one is being constructed at Koodamkulam.
3) Nuclear Fusion
When lighter nuclei moving at a high speed are fused together to form a heavy nucleus, the process is called nuclear fusion.
In fusion reaction, the mass of heavier nucleus formed is less than the total mass of two lighter nuclei. Thus, just like a fission reaction, the source of energy in a fusion reaction is also the disappearance of mass, which gets converted into energy.
Nuclear fusion reaction takes place at very high temperature of about 108K.
Therefore, this reaction is called thermonuclear reaction.
1H2 + 1H3 ® 2He4 + 0n1 + Energy
Deuterium Tritium Helium
The Mass loss is equal to 0.018 amu and the corresponding energy released is 1.79 × 109 KJmol-1.
The highly destructive hydrogen bomb is also based on the fusion reactions of hydrogen to form helium producing large amount of energy. Hydrogen bomb consists of an arrangement for nuclear fission in the centre surrounded by a mixture of deuterium (1H2) and lithium isotope (3Li6). Fission reaction provides the high temperature necessary to start the fusion. Fusion reactions take place in hydrogen bomb.
i) Fission ® heat + neutrons
ii) 3Li6 + 0n1 -- > 1H3 + 2He4 + 4.78 MeV
1H2 + 1H3 ---- > 2H4+ 0n1+ 17.6 MeV