24 April 2012

Nuclear Reactor


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.