Nuclear Fission Reaction Process - Nuclear Fission Reactors

All about nuclear fission Reaction , Nuclear reactors - controlled chain reaction, Energy Generated in Nuclear Fission Process.  

Concept of Nuclear Fission Reaction Process | How does a Nuclear Reactor Work? 

Definition of Nuclear Fission Reaction: 

    Nuclear fission is the nuclear process by which a heavy nucleus splits into two or more lighter nuclides of intermediate mass number with release of a large amount of energy. 

Characteristics of nuclear fission Reaction:   

1. Most of the heavier isotopes of the elements with Z>82 show spontaneous fission. In such a case, the number of protons within the nucleus is very large, so that the electrostatic force of repulsion exceeds the nuclear binding force.
2. When the energy is supplied to a heavy nucleus by an impinging particle to produce nuclear fission, this is called induced fission reaction.
3. The neutrons are the most effective fission producing particle as they carry no charge.
4. Heavy nuclei with even number of protons and odd number of neutrons can undergo fission by slow neutrons as these are less stable.
5. Heavy nuclei with even number of protons and even number of neutrons can undergo fission by fast neutrons as these are most stable.
6. If the neutrons are ejected as a result of fission within a time interval of 10^-14 second, then they are called prompt neutrons.
7. If the neutrons are ejected as a result of fission with a time interval of greater than 10^-14 second, they are called delayed neutrons.
8. The fission induced by γ -rays is termed as Photo-fission. 

Liquid drop model for Nuclear fission Process:

    In 1939 Bohr and Wheeler proposed the theory of nuclear fission depending on the liquid drop model of the nucleus. They assumed that the nucleus is a liquid drop of  nuclear fluid such that it is -

1. Incompressible
2. Electrically charged 

    So the drop is spherical in shape without any external forces. Though the coulomb repulsive forces among the protons try to distort the shape.

    When some energy is imparted to the liquid drop (Let, by capture of neutron), oscillations are set up in the drop which tend to distort the spherical shape of the nucleus, while the surface force due to surface tension tries to restore it.

    When the excitation energy is large then the spherical shape changes to ellipsoidal shape . For greater excitation energy shape changes to the dumb-bell shape. For sufficiently large excitation energy the nucleus breaks into two fragments i.e. nuclear fission takes place.

    The minimum excitation energy for complete separation of the fragments is known as fission threshold energy or activation energy.

Transuranic elements or transuranics :

    The elements beyond Uranium (238,92) in the periodic table are known as transuranic elements. These elements do not occur in nature but are man made in the laboratory. They are very radioactive and possess very short half-lives. 

    The first transuranic element is Neptunium, discovered by McMillan and produced by Fermi. 

    The other transuranic elements are given by :

1. Plutonium(Pu)
2. Americium(Am)
3. Curium(Cm)
4. Berkelium(Bk)
5. Californium(Cf)
6. Einsteinium(Es)
7. Fermium(Fm)
8. Mendelevium(Md)
9. Nobelium(No)
10. Hafnium(Hm)      etc.

Of the above transuranics Neptunium(Np)  and Plutonium(Pu) have a major role in the production of nuclear energy by nuclear fission process.  

Example of Nuclear Fission Reaction - :

Nuclear fission Reaction equation of uranium-235 is shown below -

 

    The value of Q in the above uranium-235 fission equation is the amount of energy released in the fission process.

    

    The fission fragments show β-minus  activity to gain Stability.

OR

Energy Generated in Nuclear Fission Process of Uranium-235:

The energy released in the fission reaction can be calculated as the following -

Then we can say that in Nuclear Fission reaction we get about 200 MeV energy.

Nuclear Fission Chain Reaction: 

     In each event of fission reaction with heavy nuclei, the highly excited nuclei emit two or more prompt neutrons. These neutrons interact with the neighbouring nuclei and produce in turn fissions, so that more neutrons are emitted. The result is an avalanche like build up of fission events. This is called a chain reaction.

    The important point in the possibility of achieving a chain reaction is the size of the core. 

So, what is core? The core is the space in which the fission reaction takes place.

    The minimum size of the core at which a chain reaction can be obtained is called critical size. 

    The mass contained in the core of fissionable material in critical size is called the critical mass.

Nuclear Reactor and Controlled Chain Reaction: 

    In the earlier days, we used coal and other non-renewable sources for the purpose of power generation. But since we are running out of them at a very rapid pace we need to look at sustainable and possible ways of moving forward.

    Let's take a moment to understand the working of nuclear reactors, the modern-day devices which are used extensively for generation of power. 

    A nuclear reactor is essentially a device where a neutron-induced self-sustaining chain reaction involving fission of heavy elements takes place. 

The purpose of the reactor is to - 

1.  initiate nuclear fission reaction;
2.  control these reactions ;
3.  extract the energy produced by the fission. 

-The control of neutrons is the key to the functioning of a nuclear reactor.

The first nuclear reactor came into operation in 1941 under the leadership of Fermi. A schematic diagram of a nuclear fission reactor is shown in the figure below -

Basic Elements of a Nuclear Reactor:

    

All types of nuclear reactors contain the following essential basic components -

1. The Fuel : a material that undergoes nuclear fissions and thereby supplies neutrons for inducing further fissions is called Fuel;
2. The moderator- The moderator is used for controlling the speed of the neutrons in the controlled nuclear chain reaction;
3. the neutron reflector to prevent neutrons from escaping from the core;
4. The cooling system to control the intense temperature of the fuel elements during the controlled nuclear fission and transport the generalized heat to heat the engine.
5. the control and safety arrangements to control the chain reaction against ' running away ' and protect everyone and everything in the surrounding.
6. Mechanical framework for the various parts and parcels of it.

Nuclear Fission Process of Uranium inside a Nuclear Reactor :

The setup consists of 3 core parts - fuel components, moderator, control rods.

 

 Fuel components - consists of cylindrical rods placed into bundles . A Uranium compound ceramic is shaped into pellets and inserted into Zircaloy tubes that are bundled with one another and contain fissile nuclei of uranium-235 or U-238.

The number of rods varies greatly as per the dimensions of the reactor. The fuel components are placed in the reactor core. The fuels are immersed in water that act as a moderator.

 

Moderator paly a crucial role in slowing down the energy of the neutrons in nuclear reactors which are given out during the fission process by the fuel elements. 

 

Deuterium oxide is generally used as a moderator. Thermal neutrons produce efficient reaction with uranium-235. During this fission process, new neutrons are given out. These have the energy of about one MeV. 

 

These new neutrons, accompanied by huge energy with them may escape from other fission processes. Here, moderators come into play. Moderators slow down these high-energy neutrons. 

 

The Cooling System controls the temperature of the tube.

 

Another very crucial part of the nuclear reactor is the "control rods".

We need steady flow of energy from the reactor. For this purpose, we must control the speed of the reaction inside the reactor. We may control the fission reaction by using the control rods.

 

Every single fission reaction leaves excess neutrons which in turn are utilized for more subsequent fission reactions. Control rods absorbs the excess neutrons in the moderator to prevent any further unnecessary fission reactions.

These rods are generally made up of Boron or Cadmium. 

 

To increase the rate of reaction the control rods may be removed. A steady output of energy may be maintained by inserting or removing the control rods in the fission reactor. 

 

Apart from the above section, three other important core components of a nuclear reactor are as follows -

1.  Pressure vessel or pressure tubes 
2.  Pressurizer
3.  Steam generator

 

 Now we have understood the various elements of the nuclear reactor. 

 

Let us discuss the operations in the nuclear reactor briefly. Nuclear reactor consists of a reactor core pump and a heat sync. Due to the large amount of heat released during the fission process surrounding water gets heated up and converts into steam. This steam turns the turbine and thereby starting the generator which produces electricity for lighting up our world.

Hope you have got an idea about the Nuclear Fission reaction process and working of the Nuclear Reactors.