Nuclear chemistry

NUCLEAR CHEMISTRY

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NUCLEAR CHEMISTRY

Nuclear chemistry is a subfield of chemistry dealing radiactivity, nuclear processes and nuclear properties. It is the chemistry of radioactive elements such as the actinides, radium and radon together with the chemistry associated with equipment (such as nuclear reactors) which are designed to perform nuclear processes. Nuclear chemistry is a branch which deals with the study of the composition of nuclei atoms and the reactions. The energy released during a nuclear reaction is called nuclear energy.

Nuclear reactions

These reactions involve changes in the no. of nucleons present in the nucleus. This results in the formation of a new species. The energy changes associated with nuclear reactions are about million times more than those associated with chemical reactions. These are determined by using Einstein’s mass–energy equation.
E= mc2

E = Energy
m = mass
c = velocity of light

Mass defect and Binding energy
Mass defect
Mass defect is the loss of mass during the formation of the nucleus of the isotope. The actual mass of an isotope of an element is less than the sum of masses of the protons, neutrons and electrons present in it. This defect is called mass defect (Δm).

Binding energy
During the formation of a nucleus from nucleons, some mass is lost. The mass loss is converted into energy. The release of energy imparts stability of the nucleus. The energy released when constituent nucleons combine to form a nucleus is called binding energy of the nucleus.
E = Δmc2
Δm = mass defect
c = 3 × 108 m/s.
Higher the binding energy of an isotope, greater is its stability.

Nuclear reactions are divided into two categories.
1. Natural radioactivity
2. Artificial radioactivity
1. Natural radioactivity
In this, nucleus undergoes a spontaneous change and only a single nucleus
is involved.
2. Artificial radioactivity
In this, the interaction of two nuclei brought about artificially. This is generally
carried out by bombarding a relatively heavier nucleus with a lighter one.
Nuclear Fission
Definition
Nuclear fission is defined as “the process of splitting of heavier nucleus into two (or) more smaller nuclei with simultaneous liberation of large amount of energy”.

4.1.2 Mechanism of nuclear fission
Nuclear fission reactions can be brought about by high, moderate or low speed neutrons without being repulsed.
When U235 is bombarded by thermal neutron (slow moving) unstable U236 is formed. The unstable U236 then divides into two approximately equal nuclei with release of neutrons and large amount of energy.

CHARACTERISTICS OF NUCLEAR FISSION REACTION

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Characteristics of Nuclear Fission reaction

1. A heavy nucleus (U235 or Pu239), when bombarded by slow moving neutrons, split into two
or more nuclei.
2. Two or more neutrons are produced by fission of each nucleus.
3. Huge amount of energy is produced as a result of nuclear fission.
4. All the fission fragments are radioactive, giving off β and radiations.
5. The atomic weights of fission products range from about 70 to 160.
6. The nuclear chain reactions can be controlled and maintained steadily by absorbing a
desired number of neutrons. This process is used in nuclear reactor.
7. All the fission reactions are self-propagating chain-reactions because fission products contain
neutrons (secondary neutrons) which further cause fission in other nuclei.
8. Every secondary neutron, released in the fission process, does not strike a nucleus, some
escape into air and hence a chain reaction cannot be maintained.
9. The number of neutrons, resulting from a single fission, is known as the multiplication factor.
When the multiplication factor is less than 1, a chain reaction does not take place.
10. The control of chain reaction is necessary in order to maintain a steady reaction. This is
carried out by absorbing a desired number of neutron by employing materials like
percentage of Cd, B or steel.
11. In a nuclear reactor, the multifactor is one. This is achieved by proper arrangement of
fissionable materials.

NUCLEAR CHAIN REACTIONS

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NUCLEAR CHAIN REACTIONS
Definition
A fission reaction, where the neutrons from the previous step continue to propagate and repeat the reaction is called nuclear chain reaction.
Suppose if we initiate fission of 92U235 nucleus by one neutron (obtained either by cosmic rays or by radioactive source) the reaction will liberate three neutrons. These three neutrons will in turn causes the fission of other three 92U235 nucleus and will liberate three more neutrons in the fission of each 92U235 i.e., nine neutrons will be obtained and further it give rise to 27 neutrons and so on.
A nuclear chain reaction continues till the whole of 92U235 nucleus are fissioned. It is also accompanied by the liberation of huge amount of energy called nuclear energy or atomic energy.
Some of the neutrons, released in the fission of U235, may escape from the surface to the surroundings or may be absorbed by U235 present as impurity. This will result in breaking of the chain and the amount of energy released will be less than expected.
For a nuclear chain reaction to continue, sufficient amount of U235 must be present to capture the neutrons, otherwise neutrons will escape from the surface.

Critical Mass
The minimum amount of fissionable material (U235) required to continue the nuclear chain reactor is called critical mass.
The critical mass of U235 lies between 1 kg to 100kg.

a. Super critical mass
If the mass of the fissionable material (U235) is more than the critical mass, it is called super critical mass.

b. Sub critical mass
If the mass of the fissionable material is smaller than the critical mass, it is called sub critical mass. Thus the mass greater or lesser than the critical mass will hinder the propagation of the chain reaction.

Nuclear energy
The enormous amount of energy released during the nuclear chain reaction of heavy isotope like U235 or Pu239 is called nuclear fission energy or nuclear energy.
The fission of U235 or Pu239 occurs instantaneously, producing enormous amount of energy in the form of heat and radiation.

Cause of the release of energy
The enormous amount of energy released during the nuclear fission is due to the loss in some mass, when the reaction takes place. It has been observed that during nuclear fission, the sum of the masses of the products formed is slightly less than the sum of masses of target species and bombarding neutron. The loss in mass gets converted into energy according to Einstein equation
E = mc2
where
C = velocity, m = loss in mass and E = energy.

TYPES OF NUCLEAR FISSION REACTION

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TYPES OF NUCLEAR FISSION REACTION
The nuclear fission reactions are of two types.

1. Uncontrolled fission reactions – Atom Bomb
2. Controlled fission reactions – Nuclear reactor

ATOM BOMB
Definition
A bomb which on the principle of a fast nuclear chain reaction is called the atom bomb.
If the nuclear chain reaction is allowed to go out of control, it will lead to powerful explosion with the liberation of enormous amount of energy. When sufficient fissionable material (critical mass) is contained in a small volume, on uncontrolled explosive chain reaction occurs.
An atom bomb contains two fissionable materials (U235 or Pu239) of sub – critical mass (less than critical mass). One of these pieces constitutes wedge while the other as target. It has a mass of TNT (trinitrotoluene) in a separate pocket. When TNT is detonated, it drives one mass of U235 into other. A super critical mass (higher than critical mass) of the fissionable material is obtained. As a result of this instantaneous chain reaction, the bomb explodes with the release of enormous amount of heat energy (10 million°C) (temperature of the sun) which is much greater than that of the most powerful TNT bomb.

92 U235 + 0n1---> 56Ba140 + 36Kr93 + 30n1 + energy

The first atom bomb used in Hiroshima (Japan) on 6-8-1945 utilized U-235 as the main reacting substance and second bomb in Nagasaki on 9-8-1945 made use of Pu-239. The fission in both the cases is similar and controlled.

Nuclear Fusion
The combination of two or more nuclei is very difficult because all the nuclei are positively charged. In order to make them combine, they must move high enough to overcome the strong forces of electrostatic repulsion. The hydrogen to helium fusion reaction in the sun takes place at about 100 million degree centigrade.

Thermonuclear reaction
In order to bring about fusion reaction, temperature of high order is required. Hence the fusion reaction is called thermonuclear reaction.
The highly destructive hydrogen bomb is based on nuclear fusion reactions of isotopes of hydrogen to form helium producing large amount of energy. The very high temperature required for this uncontrolled thermonuclear reaction is obtained by the detonation of an atom bomb. It consists of a small plutonium fission bomb with a container of isotopes of hydrogen. The fission bomb produces a very high temperature at which thermonuclear reactions start resulting in the fusion of hydrogen isotopes of deuterium and tritium to form helium.

1H2 + 1H3 2He4 + 0n1 + 17.6 MeV
The explosion of hydrogen bomb is much more powerful than an atom bomb.

Differences between Nuclear Fission and Nuclear Fusion

1. In nuclear fission process, heavy nuclei split into two or more lighter nuclei with the liberation of large amount of energy.
In nuclear fusion process, two light nuclei fuse or combine to form single nuclei with the liberation of energy.

2. The atomic number and mass number of the daughter element is less than the parent element in nuclear fission process.
The atomic number and mass number of the product nuclei is greater than the starting elements in nuclear fusion process.

3. Radioactive rays are emitted in nuclear fission process.
Radioactive rays are not emitted in nuclear fusion process.

4. Nuclear fission reaction can be controlled
Nuclear fusion cannot be Controlled

5. It is a spontaneous process occurring at ordinary temperature. Occurs at very high temperature.

6. Neutrons are emitted during fission process.
Positrons are emitted during fusion process.

7. Fission will result in chain reaction
Fusion will not result in chain reaction

NUCLEAR REACTOR OR PILE

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NUCLEAR REACTOR OR PILE
If a nuclear fission reaction is made to occur in a controlled manner, then the energy released can be used for many constructive purposes.

Definition
The arrangement or equipment used to carry out fission reaction under controlled conditions is called a nuclear reactor.
Ex. The energy released (due to the controlled fission of U-235 in a nuclear reactor) can be used to produce steam which can run turbines and produce electricity.

Components of a Nuclear reactor
The reactor core generally has a shape of circular cylinder with a diameter ranging from 5-15 m.

The main components of the nuclear reactor are
1. Fuel rods
2. control rods
3. Moderators
4. Coolants
5. Reflector
6. Pressure vessel
6. Protective shield
8. Heat exchanger
9. Turbine

1. Fuel rods
The fissionable materials used in the nuclear reactor are enriched U235 or Pu239. It should be properly used in the reactor in the form of rods or strips.
Ex: U235; Pu239 (obtained from U238)

Function:
It produces heat energy and neutrons which initiates the nuclear chain reaction. The heat should be removed efficiently during the fission process.

2. Control rods
To control the fission reaction (rate), movable rods, made of cadmium (or) boron, are suspended between fuel rods. These rods can be lowered or raised. They control the fission reaction by absorbing excess neutrons.
If the rods are deeply inserted inside the reactor, they will absorb more neutrons and the reaction become very slow. On the other hand, if the rods are pushed outwards, they will absorb less neutrons and the reaction will be very fast.
Ex. 48Cd113 and 5B10
48Cd113+ 0n1 ---> 48Cd114 + -ray
5B10 + 0n1 ---> 5B11 + -ray

Function:
It controls the nuclear chain reaction and avoids the damage of the reactors.

3. Moderators
It is a material which is used to reduce the kinetic energy of fast fission neutrons (1 Mev or 13,200 km/s) to slow neutrons (0.25 eV or 22,000 m/s) and is done in a fraction of second. The fission chain reaction in the nuclear reactor is maintained by these slow neutrons.
When fast moving neutrons collide with moderator, they lose energy and goes slow down.
Ex. Ordinary water, Heavy water,Graphite,Beryllium
Function:
The kinetic energy of fast neutrons (1 Mev) is reduced to slow neutrons (0.25 eV)

Properties of a good moderator
High slowing power
High resistance to corrosion
High melting point (solid) and low melting point (liquid)
Cheap and abundant in nature.
Good thermal conductivity
Chemical and radiation stability

4. Coolants
In order to absorb the heat products during fission, a liquid called coolant is circulated in the reactor core. It enters the base of the reactor and leaves at the top. The heat carried by out-going liquid is used to produce steam.
Ex. Water and heavy water- They are good moderator and coolants.

Disadvantage:
Need pressurization. They become corrosive at high temperatures.
Liquid metals like Na, K are excellent coolants at high temperature due to their high thermal conductivity and low vapour pressure.
Sodium is highly reactive with hydrogen and also becomes radioactive due to neutron capture. Hence it needs shielding.

Air – It is a good coolant but suitable only for low – power reactors. At high temperatures, it reacts with materials like Al, Mg which are used in the water.
The most widely used gaseous coolant is CO2. It is cheap and does not attack metals at high temperature .But it reacts with graphite at high temperature.
Function: It cools the fuel core.

5. Reflector
These are usually placed around the core to reflect back some of the neutrons that leak out from the surface of the core. They are made up of the same materials as the moderator. Good properties include

Low absorption and high reflection for neutrons.
High resistance to oxidation and irradiation.
High radiation stability
Ex. H2O, D2O and graphite.

6. Pressure Vessel
It encloses the core and also provides the entrance and exit passage for coolant. Holes at the top of the vessel are provided to insert or pull out the control rods.

Functions:
It withstands the pressure as high as 200 kg/cm2.

7. Protective shield
The nuclear reactor is enclosed in a thick massive concrete shield (more than 10 meters thick)

1. Thermal shield : Generally, a thermal shield consists of a 50-60 cm thick iron or steel covering and placed very near to the reactor core.
By absorbing most of the γ rays, it becomes heated and prevents the adjacent wall of the pressure vessel from becoming hot. This thermal shield is cooled by a circulation of water.

2. Biological shield: It is made up of a layer of concrete of a few diameter thickness around the thermal shield. Its function is to absorb any γ rays and neutrons coming out from the inner thermal shield.

8. Heat exchanger
This process involves transfer of the heat liberated from the reactor core to the boiling water and get steam at about 400 kg/cm2.

9. Turbine
The steam generated in the heat exchanger is used to operate a steam turbine, which drives a generator to produce electricity.

LIGHT WATER NUCLEAR POWER PLANT

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LIGHT WATER NUCLEAR POWER PLANT
Definition
Light-water nuclear – power plant is a nuclear reactor in which U235 fuel rods are submerged in water. Here, water acts as coolant and moderator.
The fission reaction is controlled by inserting or removing the control rods of B10 automatically from the spaces in between the fuel rods. The heat emitted by fission of U-235 in the fuel core is absorbed by the coolant (light water). The heated coolant (water at 300°C) then goes to the heat exchanger containing sea water. The coolant here, transfers heat to sea water, which is converted into steam. The steam then drives the turbines, generating electricity.

Pollution
Though nuclear power plants are very important for production of electricity, they will cause a serious danger to environments.

Problem on disposal of reactor waste
Disposal of reactor waste is another important problem because the fission products viz., Ba139 & Kr92 are themselves radioactive. They emit dangerous radiation for several hundred years. So the waste is packed in concrete barrels, which are buried deep in the sea.

BREEDER REACTOR

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BREEDER REACTOR

Definition
Breeder reactor is the one which convert non-fissionable material. (U-238, Th-232) into fissionable materials U-233, Pu-239). Thus the reactor produces more fissionable materials than it consumes.
92U235+ 0n1 --> 94Pu239 + 2e-
Non – fissionable Fissionable
94Pu239 + 0n1 --> Fission products + 30n1

In breeder reactor, of the three neutrons emitted in the fission of U-235, only one is used in propagating the fission chain with U-235. The other two are allowed to react with U-238. Thus, two fissionable atoms of Pu-239 are produced for each atom of U-235 consumed. Therefore, the breeder reactor produces more fissionable material than it uses.

In general,
1. The fissionable nucleides such as U-235 & Pu-239 are called fissile nucleides.
2. The non-fissionable nucleides such as U-238 & Th-232 are called fertile nucleides.