A chemical reaction can be defined as the conversion of some chemical compounds to new compounds. Chemical reactions can be represented as chemical equation in which chemical compounds are written in the form of their molecular formulae. The compounds which involve in reaction are called as reactants and the newly formed compounds are called as products. So we can say that chemical reactions are conversion of reactants to product molecules which can be represented in the form of chemical equations.
$A + B \rightarrow C + D$
The molecules of reactants and products involves in chemical reaction. Some chemical reactions affect the nucleus of atom. Such reactions are known as nuclear reactions. They are different from chemical reactions as chemical reactions do not affect the nucleus of atom but only electrons take part in chemical reactions. The electrons are exchanged from one or more compounds to form a different compound during chemical reactions. On the contrary during nuclear reactions the nucleus particles are changed which results the transformation of one element to another. Nuclear reaction can be defined as the interaction of two nuclear particles to produce two or more nuclear particles with gamma rays.
Overall nuclear reactions involve the transformation of one nuclide to another. Nuclear scattering is interaction of a nucleus with another nucleus without changing the nature of any nuclide. On the basis of interaction of nuclear reactions, they can be classified as nuclear fission and nuclear fusion reactions. Nuclear fusion reactions involve fusion of light elements which results the formation of heavy nucleus with large amount of energy. This is main source of energy production in stars and the Sun. The interaction between cosmic rays and matter also causes nuclear reactions. Nuclear reactors are device which initiate and control the nuclear chain reactions. Radioactive nuclei are used in these nuclear reactors as reactants to produce new nuclei with large amount of energy. Radioactive nuclei involve in different decay processes and on the basis of them, they can classify in different types.
Neutron-rich nuclei have high neutron to proton ratio which decay by a process that can convert a neutron to a proton. It results a decrease in neutron-to-proton ratio. Another type of radioactive nuclei is neutron-poor nuclei which are placed on the lower right side of the band of stable nuclei. They have low neutron-to-proton ratio therefore decay by processes which can convert a proton to a neutron and increase the neutron-to-proton ratio. Heavy nuclei have mass number more than 200 which makes them unstable regardless of the neutron-to-proton ratio. These nuclei tend to decay by emission of alpha particle that decreases the number of protons and neutrons in the original nucleus by 2.
Nuclear reactions can be classified as nuclear fission and nuclear fusion reactions. Nuclear fission involves the splitting of large nuclei to two or more smaller units with release of energy. In other words it is fission of heavy nuclei into two or more fragments with some sub-atomic particles like neutrons and energy. The mass difference in parent and daughter nuclei is associated to energy released during the reaction. Nuclear power plants are mainly based on nuclear fission and generate power. Usually nuclei of uranium atoms undergo nuclear fission like uranium-235. Nuclear fission of U-235 is initiated by hitting it with neutrons. It results the fission of U-235 to Ba-141 and Kr-92. Another possibility is the formation of Cs-140 and Rb-92 with 200 MeV energy and two neutrons.
Nuclear fission is a chain reaction in which U-235 reacts with neutrons to form U-236 that further splits to krypton-92 and barium-141 along with 3 neutrons. No doubt there are many possible fission products. For all nuclear fission reaction, the sum of all neutrons and protons in the products is the same as the reactants. A small amount of mass is lost that is called as mass defect which is converted into energy.
Nuclear fusion is another type of nuclear reaction in which light nuclei are forced together to fuse and form heavy nuclei. It released a large amount of energy as the mass of the combination will be less than the sum of the masses of the parent nuclei. One of the most common examples of nuclear fusion is fusion of H-2 (deuterium) and H-3(tritium) to form He-4 (helium). If we compare the energy released during nuclear fusion and fission, nuclear fusion produces more energy per kilogram than fission. At the same time, nuclear fission reactor is much easier to build and requires less energy input. That is the reason all commercial nuclear reactors are fission reactors as the energy released during these process can be converted to electricity.
The deuterium-tritium fusion reaction is potential nuclear energy sources for the Earth.
D + T -> 4He + n + 17.58 MeV
Some other deuterium fusion reactions are listed below.
When two atomic nuclei are brought close to each other, they fuse in spite of the repellent positive nucleus charges. For example in fusion of deuterium and tritium; isotopes of hydrogen atom, both nuclei must fly with high speed to overcome their mutual repulsion and high temperature is required to achieve that speeds of the particles.
The plasma physics is used to find a suitable procedure that allows a controlled nuclear fusion reaction and enables the use of the released energy. For example; for the formation of 1 kg of helium from deuterium and tritium, an energy of about 120 million kWh is released which is equal to a gross calorific value of 12 million kilograms of coal.
Some other examples of nuclear fusion with their energy values are listed below.
â¢ D + T -> 4He + n + 17.58 MeV
â¢ D + D -> 3He + n + 3.27 MeV
â¢ D + D -> T + p + 4.03 MeV
â¢ D + 3He -> 4He + p + 18.35 MeV
â¢ p + 11B -> 3 4He + 8.7 MeV.
The deuterium-tritium fusion reaction is the easiest fusion reactions because Deuterium is available in sufficient quantity in the oceans and tritium can be bred from the Li by neutrons generated and Li is also available in abundance.
Tritium can be produced in breeding reactors by neutron bombardment on lithium nuclei.
â¢ 7Li + n -> 4He + T + n -2.47 MeV
â¢ 6Li + n -> 4He + T + 4.78 MeV
The deuterium fusion requires extensive laboratory conditions which are difficult to achieve and maintain for long time. The fusion cycle of deuterium and tritium is given below.
In nuclear fusion reactors, the fuel mixture of deuterium and tritium is required in gaseous state. The merging of deuterium and tritium atoms releases large amount of energy in a thermo-nuclear plasma.
Highly energetic neutrons and alpha particles or helium nuclei are produced whose energy will eventually be transformed into useful energy like in electricity. The generation of electricity in nuclear fusion reactors is very difficult and cannot be done yet. One of the major issues is to control and sustain the conditions of hot plasma for nuclear fusion. Many research projects like ITER (International Thermonuclear Experimental Reactor) research project was founded to demonstrate the scientific and technical feasibility of nuclear fusion in reactors so that we can use this energy as the future power source.
Like in deuterium- tritium fusion reactor, the energy generation process can only consume 3% of the injected gases. At the same time the total quantity of tritium has to be kept to a minimum inside a reactor for safety reasons.
Therefore for quick process of fuel, a closed cycle is essential for fusion reactors. At the exhaust of nuclear fusion reactor, the extraction of tritium tritium from the tritiated gases is necessary. It includes the cryogenic separation of hydrogen isotopes and the reconditioning of heavy water which is formed as side product.
Another operational fusion experiment is JET which is capable of producing fusion energy with deuterium only. The deuterium fusion (D-D fusion) needs much higher temperatures in the range 400 â 500 million Â°C compare to plasma temperatures of 150 â 200 million Â°C for D-T fusion. In D-D fusion there are two possibilities. One is the production of Tritium and a proton and another is the production of He-3 and a neutron.
As we know that nuclear reactions produce much more energy per kg compare to normal chemical reactions. This is because during nuclear reactions, the fuel is the mass loss between parent and daughter nuclei whereas in chemical reactions, the fuel is due to formation and cleavage of chemical bonds. Some of the nuclear reactions with their heat energy values are listed below.
| Heat Energy
| H-2 + H-3 He-4 + neutrons
|| -340,000,000,000 kJ/kg
| U-235 + n Rb-90 + Cs-143 + 3 neutrons
| -88,000,000,000 kJ/kg
| CH4 + 2 O2 CO2 + 2 H2O
|| -56,000 kJ/kg
| H2 + 1/2 O2 H2O
|| -140,000 kJ/kg
The reaction coordinate diagrams for nuclear fusion and nuclear fission are given below.
In nuclear fusion reaction, the parent nuclei are hydrogen isotopes 2H and 3H at a particular energy. They react and reach to activation energy level by heating the reaction mixture at very high temperature and pressure.
Here products are helium nuclei and a neutron which have a much lower energy than the parent nuclei. Hence we can say that the energy difference is E and the energy released as heat of reaction must be equal to E + Ea. The effect of temperature with reactivity for D-T, D-He-3 and D-D fusion processes are given below.