Theory of Nuclear Power Plant

INTRODUCTION :

Matter consists of minute particles known as atoms. These atoms contain enormous `binding energy'. Electrons, Protons and Neutrons are sub-atomic particles. The central part of atom is nucleus and negatively charged electrons revolve around this nucleus in an orbit. Nucleus contains positively charged protons and neutrons. Neutrons are neutral in charge.

When a neutron is induced to enter the nuclei of a certain heavy atom, they will fission into two smaller nuclei. They eject two or three neutrals and fragments emit a gamut of rays such as alpha, beta and gamma rays. During this process of fission enormous amount of heat energy is liberated and it is called 'Nuclear Energy'. Controlled fission of heavier unstable atoms such as U-235 (Uranium), Th-232 (Thorium) and artificial elements Pu-239 liberate large amount of heat energy which can be used to heat water and generate steam. Steam is used to drive turbines which when coupled to electrical generators produce power.

THEORY OF NUCLEAR POWER :

Atomic Structure:

Atom of an element is smallest particle that can exist. An atom consists of a nucleus at its centre and electrons revolve around it in well defined orbits. Nucleus consists of protons and neutrons. The electrons are negatively charged sub - atomic particles and having very insignificant mass as compared to the mass of protons and neutrons. Electron is identified as a β - particle.

Proton is positively charged. Neutron is identified as α - particle and it possesses no electrical charge. Neutrons are present in all atoms except in hydrogen atoms. Protons and Neutrons arc collectively called nucleons and are tightly bound together. On the whole atoms is a neutrally charged smallest particle of matter,

Atomic Mass Unit (a.m.u.) :

Atomic mass of a substance is the number of times its atom is heavier than one atom of hydrogen or 1/16th of an oxygen atom or 1/12th of a carbon atom. Atomic masses are measured accurately by mass spectrographs.

Isotopes :

Atoms of the same element are alike in size, shape and weight. But atoms of certain elements possess different masses. They are called Isotopes. In other words, atoms of same element possessing different masses arc called isotopes.

For example, there are two types of chlorine atoms possessing masses of 35 and 37 atomic masses. Atomic number (Z) is the total number of protons in a nucleus. It is also equal to the total number of electrons in a neutral atom. Mass number (A) is the total number of protons (p) and neutrons (n) in a nucleus. Thus A = p+n = Z+n.

Radio Activity : It is defined as the spontaneous disintegration of the nucleus of one or more items.

Some of the radioactive elements are radium, thorium and uranium. They disintegrate spontaneously and give off electromagnetic radiations. The phenomenon of emission of these waves by unstable elements Is called radioactivity The substances which emit such radiation arc called radioactive elements, Some elements are by nature radioactive, while some can be rendered radioactive artificially.

α, β and γ Radiations

Rutherford showed that the radiations emanated by radioactive substances are of three kinds.

(a) Alpha rays (b) Beta rays (c) Gamma rays

(a)Alpha rays : Each α - particle comprises of two protons and two neutrons i.e helium nuclei. α - particles are heavy particles carrying positive charge, equal to the charge of 2 protons. These rays are the most ionizing but the least penetrative of three radiations.

An example of α - emission is the decay of plutonium.

(b) Beta rays : β - particles are electrons with negligible mass and negative charge, They travel at the speed of light. β - radiations are 100 times more penetrating than α - radiations. Over exposure to β - radiations may cause skin burns. They need better shielding than that required for α - particles, Because of their smaller mass, β - rays are easily deflected in a magnetic field and in as direction opposite to that of alphas.

(c) Gamma rays: γ - rays are high energy photons. They are electromagnetic waves of very short wave length varying from 0.001 A° to 0.1 A°. They originate in the nucleus and have a very low ionizing power but a very high penetrating power. γ - rays are not deflected by electric field or magnetic field and hence they are not charged. Because of very high penetrating power, thick lead shielding is needed as a protection from their effects.

Einstein's Equation :

Energy is released as a result of interaction involving atomic nuclei. Protons and neutrons (constituents of nuclei called nucleons) rearrange themselves in a nuclear reaction liberating energy followed by loss of certain mass. This relation is given by Einstein's equation, E = mC2

where E = energy released (Joules)

m = decrease in mass (kg)

C = speed of light (m/s)

Since nuclear masses are given in atomic mass units (a.m.u.), C in m/s, and E is obtained in electron - volts (ev).

The electron - volt is the energy gained by a particle carrying through a unit electric charge when it is accelerated by a potential of 1 volt.

Mass Defect and Binding Energy :

The mass of an atom is less than the sum of the masses of its protons, neutrons and electrons, and the difference is as the mass defect.

Ex. Consider the formation of a helium nucleus out of two and two neutrons. The mass of neutron is 1.00893 amu, the mass of proton is 100785 amu and the mass of He nucleus is 4.00282 amu.

The decrement in mass is called mass defect. If particles are brought together quickly to form an atom, a mass equal to mass defect, is suddenly converted into energy. This amount of mass has been converted into potential energy which holds the nucleus together. This potential energy which is equivalent to mass defect is called binding energy.

On the otherhand, to break up an atom into its individual particles, energy equivalent to its mass defect must be supplied. The mass defect, therefore, indicates the existence of binding energy in the Binding energy thus, is the energy required to break up a nucleus into its constituent parts (protons and neutrons) and place them at an infinite distance from one another.

The binding energy of nucleons is a direct measure of nuclear stability. The binding energy per nucleon differs from one element to another depending on its mass number.

Nuclear Reaction :

Rutherford (1919) while bombarding nitrogen gas with energetic alpha particles from Radium - C, discovered the first ever artificial transmutation of an element. Nitrogen was found to be converted into Oxygen through emission of a proton.

(Note : Sum of superscripts and subscripts on LHS and RHS are to be equated)

If heat energy is released in a nuclear reaction it is called exo - ergic reaction and if heat energy is absorbed, the reaction is called endo - ergic

Nuclear Fission and Chain Reaction :

Fission is a process of splitting a heavy nucleus into two other nuclei. For example, when Uranium nucleus (Z = 92) is bombarded with a neutron, it is split up into two fragments called Barium (Z = 65) and Krypton (Z = 36). In this process three additional neutrons are liberated from the nucleus of Uranium atom followed by the release of enormous heat energy This nuclear reaction is represented as follows.

Z = Atomic number

= no: of protons in the nucleus (p+)

= no: of electrons (e-)

Now Z = p, A = n + p, n = A — Z

A = Atomic mass

Atomic mass number = total no: of nucleons

= no: of neutrons + no: of protons = n + p

In this process of fission of one atom of Uranium there is a decrease of mass of about 0.2155. This decrease in mass results in energy equivalent of 200 MeV i.e., certain quantity of mass is converted into energy. If 1 gm of Uranium undergoes fission, the energy released would be equal to 2.28 x 104 kWh. Further, the energy release would take place in a very short interval of time i.e., 10 second, Resulting temperature would be 10,000,000°C.

The three additional neutrons emitted during above nuclear reaction are called secondary neutrons. They, in turn, can cause fission in the neighbouring nuclei, producing more and more secondary neutrons. This is called chain reaction (figure) which continues until whole of the fissionable material undergoes fission. Chain reaction produces tremendous amount of energy in a very short interval of time.

The result of fission is an increase in the binding energy per nucleon. The change is binding energy develops kinetic energy and heat. The nuclear fission process is used in nuclear power plants for generation of energy and power.

Nuclear Fusion :

Fusion is the process of forcing together two light atoms to make a heavier atom. Fusion produces much more heat than fission. It is the believed to be the main source of energy for the sun and stars.

Fusion reactions are possible only at very high temperatures and are also known as thermonuclear reactions. We have, at present, nuclear plants adopting nuclear fission only.