NUCLEAR REACTOR OR PILE

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.