NUCLEAR REACTOR

In a nuclear reactor heat is produced by the fissioning or splitting of uranium atoms.

Conventional thermal power stations use oil or coal as the source as the source of energy. The reserves of these fuels are becoming depleted in many countries and thus there is a tendency to seek alternative sources of energy. In a nuclear power station instead of a furnace there is a nuclear reactor, in which heat is generated by splitting atoms of radioactive material under suitable conditions. For economical use in a power system a nuclear power station generally has to be large and where large units are justifiable.

The main parts of a nuclear power station are

• Nuclear reactor
• Heat exchanger
• Steam turbine
• Condenser
• Generator

                                                                                    Fig- Nuclear Reactor

Working:

In a nuclear reactor heat is produced by the fissioning or splitting of uranium atoms. A cooling medium takes up this heat and delivers it to the heat exchanger, where steam for the turbine is raised. When the uranium atoms split, there is radiation as well, the reactor and its cooling circuit must be heavily shielded against radiation hazards.

Large electrical generating plants which provide most of our electricity all work on the same principle - they are giant steam engines. Power plants use heat supplied by a fuel to boil water and make steam, which drives a generator to make electricity. A generating plant's fuel, whether it is coal, gas, oil or uranium, heats water and turns it into steam. The pressure of the steam spins the blades of a giant rotating metal fan called a turbine. That turbine turns the shaft of a huge generator. Inside the generator, coils of wire and magnetic fields interact - and electricity is produced.

Parts of Nuclear Reactor:

1.       nuclear fuel

2.       reactor core

3.       moderator

4.       control rods

5.       reflector

6.       reactor vessel

7.       biological shielding

8.       coolant

Nuclear fuel:

Fuel of a reactor should be fissionable material which can be defined as a fissionable material which can be defined as an element or isotope whose nuclei can be caused to undergo nuclear fission nuclear bombardment and to produce a fission chain reaction.

The fuels used are: U²³⁸, U²³⁵, U ²³⁴, UO₂

Fertile materials, those which can be transformed into fissile materials, cannot sustain chain reactions. When  a fertile material is hit by neutrons and absorbs some of them, it is converted to fissile material.U²³⁸ and Th ²³² are examples of fertile materials used for reactor purposes. 

Reactor core:

This contains a number of fuel rods made of fissile material.

Moderator:

This material in the reactor core is used to moderate or to reduce the neutron speeds to a value that increases the probability of fission occurring.

Control rods:

The energy inside the reactor is controlled by the control rod. These are in cylindrical or sheet form made of boron or cadmium.

These rods can be moved in and out of the holes in the reactor core assembly.

Reflector:

This completely surrounds the reactor core within the thermal shielding arrangement and helps to bounce escaping neutrons back into the core. This conserves the nuclear fuel.

Reactor vessel:

It is a strong walled container housing the core of the power reactor. It contains moderate, reflector, thermal shielding and control rods.

Biological shielding:

Shielding helps in giving protection from the deadly α- and β-particle radiations and γ-rays as well as neutrons given off by the process of fission within the reactor.

Coolant:

This removes heat from the core produced by nuclear reaction. The types of coolants used are carbon dioxide, air, hydrogen, helium, sodium or sodium potassium.

Principle of reactor control:

When a nucleus captures a neutron the resulting compound nucleus is unstable. It splits into two fragments, releases energy and ejects some neutrons. If conditions are favorable, neutrons ejected by the first fission may be captured by other nuclei and the chain reaction begins. If the energy output from a reactor is to be maintained constant, one neutron and not more than one from each fission must split another nucleus(multiplication factor, k=1)

Otherwise control of chain reaction will not be possible.

 The principal law of nuclear energy is   E = mc²

                             Where   W-Energy (joules)                        

                                m- Mass (kilograms)

                                c- Speed of light (3*10⁸m/sec)

The main reactions inside a reactor are

²³⁸U₉₂    +   ¹n₀     à   ²³⁹U₉₂   +  γ

²³⁹U₉₂   has a half life period of 23.5 min only and hence it is    unstable.

²³⁹U₉₂    +   ⁰e_-1       à   ²³⁹Np₉₃

²³⁹Np₉₃ again has a short half life and emits β-particles.

²³⁹Np₉₃   +   ⁰e_-1     à  ²³⁹Pu₉₄  

Advantages of Nuclear Power Plant:

1. Space requirement of a nuclear power plant is less as compared to other conventional power plants of equal size.
2. A nuclear power plant consumes very small quantity of fuel. Thus fuel transportation cost is less and large fuel storage facility is not needed.
3. There is increased reliability of operation.
4. Nuclear power plants are not affected by adverse weather   conditions.
5. Nuclear power plants are well suited to meet large power demands. They give better performance at higher load factors (80-90%).
6. Materials expenditure on metal structures, piping, storage mechanisms are much lower for a nuclear power plant than a coal burning power plant.
7. It does not require large quantity of water.

Disadvantages:

1. Initial cost of nuclear power plant is higher as compared to hydro or steam power plant.
2. Nuclear power plants are not well suited for varying load conditions.
3. Radioactive wastes if not disposed carefully may have bad effect on the health of workers and other population.
4. Maintenance cost of the plant is high.
5. It requires trained personnel to handle nuclear power plants.