While studying radioactivity,we have seen that an α-particle is emitted from radium-226 and radon-222 is obtained. This nuclear is change is represented by the following equation:
Such an equation represents a nuclear reaction. Above mentioned nuclear reaction takes place on its own accord. However, it was Rutherford who, first of all, expressed his opinion that besides natural radioactivity decay processes,other nuclear reactions can also occur. A particle x is bombarded on any nucleus X and his process yield a nucleus Y and a light object y as given below:
X + x → Y +y
Rutherford performed an experiment on the nuclear reaction in 1918. He bombarded α-particles on nitrogen. He observed that as a result of this reaction, oxygen is obtained and a proton is emitted. That is
This reaction indicated that when α-particle enters the nucleus of ,then an excitation is produced in it. And as a result of it and a proton are produced. Since the experiment of Rutherford, innumerable nuclear reactions have been observed. For nuclear reactions to take place the fulfillment of certain conditions is a must.
Before and after any nuclear reaction the number of protons and neutrons must remain the same because protons and neutrons can neither be destroyed or can they be created. We elaborate this point from the example of Rutherford’s nuclear reaction of and here:
A nuclear reaction can take place only when the total energy of the reactants including the rest mass energy is equal to the total energy of the products. For its explanations we again take the example of the nuclear reaction of Rutherford involving and . In this reaction the mass of the reactants is :
Mass of =14.0031 u
Mass of =4.0026 u
Total mass of the reactants = 18.0057 u
In the same way the mass of the products is
Mass of = 16.9991 u
Mass of = 1.0078 u
Total mass of the products after the reaction = 18.0069 μ.
This shows that the total mass after the reaction is greater than the total mass before the reaction by 0.0012 μ. We known that a 1μ mass = 931 MeV energy, therefore a mass difference of 0.0012μ is equivalent to an energy of 931 MeV × 0.0012μ = 1.13 MeV. Hence this reaction is possible only when a additional mass of 0.0012 μ is added to the reactants to the minimum kinetic energy of the α-particle is 1.13 MeV such as obtained from . The energy these α-particle is equal to 7.7 MeV which is greater than 13 MeV. Had these α-particles been obtained from a source that give out α-particles whose energy was less than 13 MeV then this reaction would not have taken place.
From the conditions described above we can tell whether any nuclear reaction is posible or not. There is an interesting aspect in a nuclear reaction that it can take place in the opposite direction also. We known that is obtained by the reaction with an α-particle of appropriate energy. If we accelerate protons, with the help of a machine like cyclotron, and increase their velocity and then bombard these high velocity protons on n , Rutherford’s nuclear reaction of and will proceed in the backward direction as:
By bombarding different elements with α-particles, protons and neutrons, many nuclear reactions have been produced.Now we describe one such nuclear reaction with the help of which James Chadwick discovered neutron in 1932. When was bombarded with α-particles emitting out of ,then as a result of a nuclear reaction and a neutron were obtained. This reaction is shown below with an equation:
As neutron carries no charge, therefore it presented a greater amount of difficulty for its identification. Anyhow when neutron were passed through a block of proffin, fast moving protons were ejected out and these were easily identified. It may be remembered that a large amount of hydrogen is present in proffin and the nuclei of hydrogen atoms are protons. The emission of protons is the consequence of elastic collisions between the neutrons and the protons. This indicates that the mass of neutron is equal to the mass of the proton. It may be remembered that when an object of certain mass collides with another object of equal to mass at rest, then as a result of elastic collisions, the moving object comes to rest and the stationary objects begin to move with velocity of the colliding object. The discovery of neutron has brought in a revolution in the nuclear reactions as the neutrons carry no charge so that can easily enter the nucleus.
The arrangement of Chadwick’s experiment for the discovery of neutron.
“Such a reaction in which a heavy nucleus like that of uranium splits up into two nuclei of equal size along with the emission of energy during the reaction is called fission reaction”
Otto Hahn and Fritz Strassmann of Germany while working upon the nuclear reactions made a starling discovery. They observed that when slow moving neutrons are bombarded on , then as a result of a nuclear reaction , an and average of three neutrons are obtained. It may be remembered that the mas of both Krypton and barium is less than that the mass of uranium. This nuclear reaction was different from hither to studied other nuclear reaction in two ways. First as a result of the breakage of the uranium nucleus two nuclei of almost equal size are obtained, whereas in the other nuclear reactions the difference between the masses of the reactants and the products was not large. Secondly a very large amount of energy is give out in this reaction.Fission reaction of can be represented by the equation:
Here Q is the energy give out in this reaction, By comparing the total energy on the left side of the equation with total energy on the right side, we find that in the fission of one uranium nucleus about 200 MeV energy is given out. It may be kept in mind that three is no difference between the sum of the mass and the charge number on the both sides of the equation. Fission reaction can be easily explained with the help of graph. This graph shows that the bending energy per nucleon is greatest for the middle elements of the periodic table and this bending energy per nucleon is a little less for the light or very heavy elements i.e., then nucleons in the light or very heavy elements are not so rigidly bound. For example the binding energy per nucleon for uranium is about 7.7 MeV and the products of the fission reaction of uranium, namely barium and krypton, have a total mass less than the mass of uranium equal to 8.5 -7.6 = 0.9 MeV per nucleon. Thus when a uranium nucleus breaks up, as a result of fission reaction,into barium and krypton, then an energy at the rate of 0.9 MeV per nucleon is given out. This means that an energy 235 × 0.9 = 211.5 MeV is given out in the fission of one uranium nucleus.
The fission process of uranium does not always produce the same fragments (Ba,Kr). In fact any of the two nuclei present the upper Horizontal part of binding energy could be produced. Two possible fission reactions of uranium are given below as an example:
Hence in the uranium fission reaction several products may be produced. All of these products (fragments) are radioactive. Fission reaction is not confined to uranium along it is posible in many other heavy elements. However, it not been observed that fission takes place very easily with the slow neutrons in uranium-235 and plutonium-239, and mostly these two are used for fission purposes.
Fission Chain Reaction:
We have observed that during fission reaction a nucleus of uranium-235 absorbs a neutron and breaks into two nuclei of almost equal masses besides emitting two or three neutrons. By properly using these neutrons fission reaction can be produced in more uranium atoms such that a fission reaction can continuously maintain itself. The process is called fission chain reaction. Suppose that we have a definite amount of and a slow neutron originating from any source produces fission reaction in one atom of uranium. Out of this reaction about three neutrons are emitted. If conditions are appropriate these neutrons produce fission in some more atoms of uranium. In this way this process rapidly proceeds and in an infinitesimal small time a large amount of energy along with huge explosion is produce, it is representation of fission chain reaction
It is posible to produce such conditions in which only or neutron, out of all the neutrons created in one fission reaction becomes the cause of further fission reaction. The other neutrons either escape out or are absorbed in any other medium except uranium. In this case the fission chain reaction proceeds with its initial speed. To understand these conditions carefully look at. The resulting neutrons scatter in the air and so the cannot produce any fission chain reaction. Some favorable conditions for chain reaction. Some of the neutrons produced in the first fission reaction produce only one more fission reaction but here also no chain reaction is produced. If the sphere sufficiently big,then most of the neutrons produced by the fission reaction get absorbed in before they escape out of the sphere and produce chain reaction. Such mass of uranium in which one neutron, out of all the neutrons produced in one fission reaction, produces further fission is called critical mass. The volume of this mass of uranium is called critical volume.
If the mass of uranium is much greater than the critical mass, then the chain reaction proceeds at a rapid speed and a huge explosion is produced. Atom bomb works at this principle. If the mass of uranium is less than the critical mass, the chain reaction does not proceed. If the mass of uranium is equal to the critical mass, the chain reaction proceeds at its initial speed and in this way, we get a source of energy. Energy, in an atomic reactor, is obtained according to this principle. The chain reaction is not allowed to run wild, as in an atomic bomb but is controlled by a series of rods, usually made of cadmium, that are inserted into the reactor. Cadmium is an element that is capable of absorbing a large number of neutrons without becoming unstable or radioactive. Hence, when the cadmium control rods are inserted into the reactor, they absorb neutrons to cut down on the number of neutrons that are available for the fission process. In this way the fission reaction is controlled.
In a nuclear power station the reactor plays the same part as does furnace in a thermal power station. In a furnace, coil or oil is burnt to produce heat, while in a reactor fission reaction produces heat. When fission takes place in the atom of uranium or another heavy atom, then an energy at the rate of 200 MeV per nucleon is produced. This energy appears in the form kinetic energy of the fission fragments. These fast moving fragments besides colliding with one another also collide with the uranium atom. In this way their kinetic energy gets transformed in heat energy. This heat is produce steam which in turn rotates the turbine. Turbine rotates the generator which produces electricity. A sketch of a power station are shown in figure:
A reactor usually has four important parts. These are:
- The most important and vital part of a reactor is called core. Here the fuel is kept in the shape of cylindrical tubes. Reactor fuels are of various types. Uranium was used as fuel in the elementary reactors. In this fuel the quality of is increased from 2 to 4 percent. It may be remembered that the quality of in the naturally occurring is only 0.7 percent. Now-a-day plutonium-239 and uranium-233 are also being used as fuel.
- The fuel rods are placed in a substance or small atomic weight, such as water,heavy water,carbon or hydrocarbon etc. These substance are called moderators. The friction of these moderators is to slow down the speed of the neutrons produced during the fission process and to direct them toward the fuel. Heavy water, if may be remembered is made of , a heavy isotopes of hydrogen instead of . The neutrons produced in the fission reaction are very fast and energetic and are not suitable for producing fission in reactor fuel like or etc. For this purpose slow neutrons are more useful. To achieve this moderators are used.
- Beside moderator three in an arrangement for the control of number of neutrons, so that all the neutrons produced is fission, only one neutron produces further fission reaction. The process is achieved either by cadmium or by boron because they have the property of absorbing fast neutrons. The control rods made of cadmium or boron are moved in or out of the reactor core to control the neutrons that can initiate further reaction. In this way the speed of the chain reaction is kept under control. In case of emergency or for repair purposes control rods are allowed to fall back into the reactor and thus stop the chain reaction and shut down the reactor.
- Heat is produced due to chain reaction taking place in the core of the reactor. The temperature of the core, therefore, rise to about 1200 °C. To produce steam from this heat, it is transported to heat exchanger with the help of water,heavy water or any other liquid under great pressure. In the heat exchanger this heat is used to produce steam from ordinary water. The steam is then used to run the turbine which in turn rotates the generator to produce electricity. The temperature of the steam coming out of the turbine is about 300°C. This is further cooled to convert it into water again. To cool this steam, water from some river or sea is, generally,used. Heavy water is being used as a moderator and for the transportation of heat also from the reactor core to heat exchanger, heavy is used. To cool steam coming out of the turbine sea water is being used.
The nuclear fuel once used for changing the reactor can keep on operation continuously for a new months. There after the fissile material begins to decrease. Now the used fuel is removed and fresh fuel is fed instead. In the used up fuel is intensely radioactive substance. The half-life of these radioactive remnant material is many thousand years. The radiations and the particles emitted out of this nuclear waste is very injurious and harmful to the living things. Unfortunately there is no proper arrangement of the disposal of the nuclear waste. This cannot be dumped into oceans or left in any place where they will contaminate the environment,such as through the soil or the air. They must be not allowed to get into the drinking water. The best place so far found to store these in the bottom of old salt mines,which are very dry and are thousands of the meters below the surface of the Earth. Here they can remain and decay without polluting the environment.