What is a nuclear reactor?
It is an apparatus in which heat is produced
due nuclear fission chain reaction for the
generation of electricity.
1. Reactor Core
It consists of fuel elements, control rods,
coolant, moderator and pressure vessels. Cores
generally have shapes of right circular cylinders
with diameters ranging from .5 to 15 metres.
Fuel rods made of uranium rods clod in thin
sheath of stainless steel, zirconium or
It is placed round the core, to reflect back
some of the neutrons that leak out from core
3. Control Rods
It is made up of heavy mass element.It
simply absorb the neutrons so that it can
either maintain or stop a reaction.
It has following purposes-
1.For starting the reactor.
2.For maintaining at that level.
3.For shutting the reactor down under normal
or emergency conditions.
For starting the reactor-
To start a reactor,a neutron from a source is
ejected through thermal means and the
control rods are taken upwards so that the
control rods can not disturb the reaction.
Hence neutron hits the fuel rods,break it into
lighter nuclie,energy is released,number of
neutron keeps on increasing since K will be
greater than 1 for this time period and hence
reaction starts and its rate also increases.
For maintaining the reaction at constant
When rate of reaction achieves a
permissible value then control rods are
inserted between the fuel rods in such
away that K becomes equal to 1.
Hence the rate of reaction achieves a finite
For shutting down the reactor
To shut down the reactor either in normal
or emergency condtions,the control rods
are inserted in such away that K becomes
less than 1.
Hence the number of neutrons keeps on
decreasing,ie rate of reaction
decreases,hence the reaction stops after a
certain interval of time.
To slow down neutrons from high velocities and
hence high energy level which they have on being
released from fission process so that probability of
neutron to hit the fuel rods increases.
Main moderator used:
2.Heavy water D2O.
Properties of Moderator
High slowing down power
High MP for solids and low for liquids
Chemical and radiation stability
High thermal stability
D2O is best for moderator.
Coolant is used to remove intense heat from the produced in the
reactor and that heat can be transferred to water in a seprate vessel
which is converted into steam and runs the turbine.
1. Low Melting point
2. high Boiling point
3.chemical and radiation stability
Commonly used Coolant
Mostly used gaseous Coolant is CO2
Shielding is required to protect the working
men from the harmful effects of the
In fission,alpha particles,beta
particles,gamma rays,slow and fast
neutrons are formed in which gamma rays
and fast neutrons are of main significance.
To provide protection against them,thick
layers of lead or concrete provided round
Thick layers of metals or plastics are
sufficient to stop alpha and beta particles.