A nuclear reactor is
a device in which the nuclear reaction is carried on as perfectly controlled
chain reaction, in a self sustained manner.
The most common use
of nuclear reactors is for the generation of electrical power.
* In nuclear reactor,
the chain reaction is brought about under controlled conditions.
* If the chain
reaction is put under control, after some time a steady state is established.
History:
·
The first artificial nuclear reactor (Chicago Pile-1)
was constructed at the University of Chicago by a team led
by Enrico Fermi in 1942.
·
After the Chicago Pile, the U.S.
military developed nuclear reactors for the Manhattan Project
starting in 1943. The primary purpose for these reactors was the mass
production of plutonium for nuclear weapons.
·
The first commercial nuclear power
station, Calder Hall was opened in 1956 with an initial
capacity of 50 MW.
·
The first portable nuclear reactor
"Alco PM-2A" used to generate electrical power (2 MW) for Camp Century
from 1960.
Classifications
of Nuclear Reactors
1. Classification by neutron
energy
* Fast reactor:
In
these reactors the fission is affected by fast neutrons without any
use of moderators.
*
Thermal reactors.
In
this the fast neutrons are slowed by the use of moderators. The slow neutrons are absorbed by the fissionable fuel and chain
reaction is maintained. The moderator
is most essential component in these reactors.
2. Classification by
moderator material
* WATER
MODERATOR
* HEAVY
WATER MODERATOR
* GRAPHITE
MODERATOR
* BERYLLIUM
MODERATOR
3. Classification by
coolant
* WATER
COOLED REACTORS
* GAS
COOLED REACTORS
* LIQUID
METAL COOLED REACTORS
* ORGANIC
COOLED REACTORS
4. Classification by generation
5. Classification by
phase of fuel
NATURAL FUEL
In
this reactor the natural uranium is used as fuel and generally heavy water or graphite is used as a moderator.
ENRICHED
URANIUM
In this reactor, the uranium used contains 5
to 10 % U-235 and ordinary water can be
used as moderator.
6. Classification by
use
Nuclear
reactors consist of five main elements
1) The fissionable
material called fuel
2) Moderator
3) Neutron reflector
4) Cooling system
5) The safety and control
system
1) The fissionable
substance:
* Uranium isotopes U233,
U238
* Thorium isotopes Th232
*
Plutonium isotopes Pu239,
Pu240
and Pu241
2) Moderator:
* The
function of the moderator is to slow down the highly energetic neutrons
produced in the process of fission of U235
to thermal energies.
*
Heavy water (D2O), graphite,
beryllium are used as moderators.
*
It has low atomic weight and low absorption cross section for neutrons.
3) Neutron reflector:
*
Leakage of neutrons can be very much reduced and the neutron flux in the
interior can be increased.
*
Materials of high scattering cross-section and low absorption cross-section are
good reflectors.
4) Cooling system:
*
It removes the heat evolved in the reactor core.
*
The coolant or heat-transfer agent (water, steam, He, CO2, air and certain
molten metals and alloys) is pumped through the reactor core.
*
Then, through a heat exchanger, the coolant transfers heat to the secondary
thermal system of the reactor.
5) Control and safety
system:
* The control system
enable the chain reaction to be controlled and prevent it from spontaneously
running away. This is accomplished by pushing control rods into the reactor
core.
* These rods are of a
material (Boron or Cadmium) having a large neutron absorption cross section.
These rods absorb the neutrons and hence cut down the reactivity.
* By pushing in the
rods, the operation of the reactor can be made to die down, by pulling them out
to build up.
* The safety systems
protect the space surrounding the reactor against intensive neutron flux and
gamma rays existing in the reactor core.
* This is achieved by
surrounding the reactor with massive walls of concrete and lead which would
absorb neutrons and gamma rays.
