Nuclear Reactions
The process by which the identity of a nucleus is changed when it is bombarded by an energetic particle is called nuclear reaction.
Let a nucleus X (called parent nucleus) be bombarded by an energetic particle c. The resultant nucleus after the interaction between X and a is Y (called daughter nucleus) and an outgoing particle d is emitted. The nuclear reaction may be represented as follows:
X + c → Y + d
Here, X and c are known as reactants and Y and d are known as products.
Q-value or Energy of Nuclear Reaction
The energy absorbed or released during nuclear reaction is known as Q-value of Nuclear reaction.
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Q-value is defined as the difference between the mass of reactants and the mass of products
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i.e.
Q-value = (Mass of Reactants – Mass of products) c² joule
or
Q-value = (Mass of reactants – Mass of products) a.m.u.
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Q value of a nuclear reaction may also be defined as the difference between the kinetic energy of reactants and products.
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If Q < 0, the nuclear reaction is known as endothermic. The energy is absorbed in the reaction.
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If Q > 0, the nuclear reaction is known as exothermic. The energy is released in the reaction.
Note:
(i) Total charge or atomic number (Z) before and after the reaction is conserved. That is ∑Z= 0 For example, in a nuclear reaction
(ii) Mass number (A) is conserved. That is ∑A = 0.
(iii) Linear momentum of the particle before the reaction is equal to the linear momentum of the particles after the reaction. That is, ∑p= 0.
(iv)Angular momentum of the particle before the reaction is equal to the angular momentum of the particles after the reaction. That is, ∑L= 0.
Nuclear Fission
Nuclear fission is a pro
cess in which the nucleus of an atom splits into two or more smaller nuclei along with the emission of large amount of energy.
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Nuclear fission was discovered by Otto Hahn and Strassmann in 1939.
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They bombarded uranium (92U235) nucleus with a thermal neutron and found that the products were barium and krypton plus three neutrons along with large amount of energy. The fission of 92U235 is represented as
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The nuclear fission was explained by Bohr and Wheeler using liquid drop model of the nucleus.
Chain Reaction in Nuclear Fission
Types of Chain reactions
(i) Uncontrolled chain reaction: If more than one neutrons produced in a fission cause further fissions at each stage, then the number of fissions and energy released multiply rapidly. Such a chain reaction is called uncontrolled chain reaction. In such a chain reaction, huge amount of energy is released within a fraction of a second. This is the underlying principle of Atom bomb.
(ii) Controlled chain reaction: If only one neutron is available to cause further fission at each stage, then a constant amount of energy is released. Such a reaction is called controlled chain reaction.
Nuclear reactor
A nuclear reactor is a system that contains and controls sustained nuclear chain reactions.It was formerly known as atomic pile.
Reactors are used for generating electricity, moving aircraft carriers and submarines, producing medical isotopes for imaging and cancer treatment, and for conducting research.
The schematic diagram of nuclear reactor is shown in figure below.
Main Components of Nuclear Reactor
1. Fuel:The fissionable material used in the reactor is called as fuel. The commonly used fuels are Uranium, Plutonium or Thorium. It can be U-235, U-238, Pu-236 or Th-232. Uranium is mostly preferred as it has high melting point.
2. Coolant:Coolant removes heat from the fuel elements and transfers it to water.A good coolant should not absorb neutrons,should be non-oxidising , non toxic and non-corrosie and have high chemical and radiation stability.It should have good heat transfer capability.
Typical coolants are water,heavy water, carbon dioxide gas and liquid sodium.
3. Canning Materials:The fuel elements,in a nuclear reactor,is canned so that the fuel does not contaminate the coolant.Canning eliminates radiation hazard.Aluminium,magnesium,beryllium and stainless steel are used as canning material.
4. Moderators:The purpose of moderator material is to slow down the fast neutrons that are produced during the fission process. Moderator reduces the speed of the neutron by absorbing its energy but not absorb neutron.It must not react with neutrons.Graphite, Heavy water and Beryllium are common moderators.
5. Control Rods:These rods absorb neutrons and stop the chain reaction to proceed further. These are made up of steel containing a high percentage of material like cadmium or boron which can absorb neutrons. When control rods are completely inserted into the moderator block then all the neutrons is absorbed and reaction comes to halt.
6. Shielding:Shielding prevents radiations to reach outside the reactor. Lead blocks and concrete enclosure that is strong enough of several meters thickness are used for shielding.
7. Turbines:Steam produced in the boiler is now passes to a turbine. The force of the steam jet causes the turbine to rotate. Heat energy (steam) is converted to mechanical energy (moving turbine).
8. Generator:The generator is a device that convert mechanical energy into electrical energy. The turbine moves and the change in magnetic flux cause electricity. This is transmitted to substations for distribution of electric power.
Working of Nuclear Power Station
A generating station in which nuclear energy is converted into electrical energy is known as nuclear power station. The main components of this station are nuclear reactor, heat exchanger or steam generator, steam or gas turbine, AC generator and exciter and condenser.
The heat liberated in the reactor due to the nuclear fission of the fuel is taken up by the coolant circulating in the reactor. A hot coolant leaves the reactor at top and then flows through the tubes of heat exchanger and transfers its heat to the feed water on its way. The steam produced in the heat exchanger is passed through the turbine and after the work has done by the expansion of steam in the turbine, steam leaves the turbine and flows to the condenser. The mechanical or rotating energy developed by the turbine is transferred to the generator which in turn generates the electrical energy and supplies to the transmission line. Pumps are provided to maintain the flow of coolant, condensate, and feed water.
Types of Nuclear Reactors
Reactors can be classified in many ways according to type of fission,fuel used,moderator material,distribution of fuel and moderator,etc.However,only a few of the possible combinations are appropriate and practicable which is given in table below.
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Reactor Type
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Coolant
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Moderator
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Fuel
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Pressurised water reactors (PWR)
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Light water
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Light water
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Enriched uranium
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Boiling water reactors (BWR)
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Light water
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Light water
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Enriched uranium
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Pressurised heavy water reactor (PHWR)
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Heavy water
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Heavy water
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Natural uranium
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Gas-cooled reactors (Magnox, AGR)
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CO2
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Graphite
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Natural or enriched uranium
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Light water graphite reactors
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Pressurised boiling water
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Graphite
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Enriched uranium
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Nuclear Fusion
The nuclear reaction in which two or more atomic nuclei collide at a very high speed and join to form a heavy nucleus and energy is released is called Nuclear Fusion.
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During this process, matter is not conserved because some of the matter of the fusing nuclei is converted to photons (energy).
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The fusion of two nuclei with lower masses than iron (see binding energy per nucleon vs mass number plot) generally releases energy, while the fusion of nuclei heavier than iron absorbs energy.
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Fusion generally occurs for lighter elements only.
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Fission normally occurs only for heavier elements.
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The most common nuclear fusion reaction is the merging of hydrogen nuclei to form helium nuclei. This is the process that occurs in the interiors of stars including the sun.This is called proton proton chain.
Condition for Nuclear fusion
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Fusion requires high temperatures about 100 million Kelvin.
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High pressure
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The electrostatic force between the positively charged nuclei is repulsive(the protons in each nucleus will tend to repel each other because they have the same charge), but when the separation is small enough, the attractive nuclear force is stronger. Therefore, the requirement for fusion is that the nuclei have enough kinetic energy that they can approach each other despite the electrostatic repulsion.
Proton Proton Chain
The reaction involves three steps.
Step-I:Two protons combine to form a deuterium nucleus, which consists of one proton and one neutron . A positron (also called an anti-electron) and a neutrino are generated during this part of the process.
Step-II:The deuterium nucleus combines with another proton, forming a nucleus of helium 3, which consists of two protons and one neutron. An energetic photon is produced during this part of the process, with a wavelength in the gamma-ray portion of the electromagnetic spectrum .
Step-III:Two nuclei of helium 3 combine to form a nucleus of helium 4, which consists of two protons and two neutrons. In this part of the process, two protons (ordinary hydrogen nuclei) are released. These protons can eventually become involved in another fusion reaction.
