Nuclear Power Station Notes

NUCLEAR POWER STATION

nuclear fission

     When atoms are bombarded with neutrons, their nuclei splits into 2 parts which are roughly equal in size. Nuclear fission in the process whereby a nucleus, with a high mass number, splits into 2 nuclei which have roughly equal smaller mass numbers. During nuclear fission, neutrons are released.

Ø   There are 2 types of fission that exist:

Ø   Spontaneous Fission

Ø   Some radioisotopes contain nuclei which are highly unstable and decay spontaneously by splitting into 2 smaller nuclei.

Ø   Such spontaneous decays are accompanied by the release of neutrons.

Ø   Induced Fission

Ø   Nuclear fission can be induced by bombarding atoms with neutrons.

Ø  The nuclei of the atoms then split into 2 equal parts

Ø  Induced fission decays are also accompanied by the release of neutrons.

Ø  The Fission Process

Ø   A neutron travels at high speed towards a uranium-235 nucleus.

Ø  The neutron strikes the nucleus which then captures the neutron.

Ø  The nucleus changes from being uranium-235 to uranium-236 as it has captured a neutron.

Ø  The uranium-236 nucleus formed is very unstable. It transforms into an elongated shape for a short time.

Ø  It then splits into 2 fission fragments and releases neutrons.

Ø  Energy from Fission :

Ø  Both the fission fragments and neutrons travel at high speed. The kinetic energy of the products of fission are far greater than that of the bombarding neutron and target atom.

Ø  Energy is being released as a result of the fission reaction.

Ø   The energy released can be calculated using the equation E = mc²

Where E = energy released (J)

                      m = mass difference (kg)

          c = speed of light in a vacuum (3 x 10⁸ ms^-1)

Nuclear fusion:-

Ø  In nuclear fusion, two nuclei with low mass numbers combine to produce a single nucleus with a higher mass number.

Classification of Nuclear Reactors:-

            

            (1) On The Basis of Neutron of Energy

      (i) Thermal Reactor      (ii) Fast Reactors       (iii) Intermediate Reactor

(2) On The Basis of Type of Coolant Used

     (i)  Gas                          (ii) Light Water           (iii) Heavy Water

     (iv) Liquid Metal            (v) Hydro Carbon

(3) On The Basis of Moderators Used

     (i) Light Water                 (ii) Heavy Water        (iii) Graphite

(4) On The Basis of Fuel & Moderator Used

     (i) Homogeneous            (ii) Heterogeneous

(5) On The Basis of Application

     (i) Research Reactors     (ii) Pu- production Reactors

     (iii) Power Reactors         (iv) Space Reactors

Types of Nuclear Reactors:-

(1) Pressurized Water Reactor (PWR)

 (2) Boiling Water Reactor (BWR)

 (3) Fast Breeder Reactor (FBR)

 (4) Sodium-Graphite Reactor

 (5) Homogeneous Reactor

 (6) Gas Cooled Reactor

 (7) High Temperature Gas Cooled Reactor

Pressurized Water Reactor:-

Ø   PWR is a thermal reactor. It uses uranium oxide as fuel. Light or heavy water is used as moderator as well as reflectors

Ø  Advantages:

§   Cooling system is simple    

§   Light or heavy water is used as moderator which are cheap

§   The reactor is compact in size

§   Fission product remains contained in the reactor and are not circulated

§   Disadvantages:

§   High pressure requires a costly reactor vessel and a leak proof container

§   High pressure and high temperature is water increases corrosion and erosion problem  

§  Steam is produced at relatively low temperature and pressure 

Boiling Water Reactor (BWR):-

Ø   BWR is a thermal reactor. Light is used as moderator as well as reflectors. Steam is produced in the reactor itself instead of the heat exchanger.

Ø  Advantages :

§   Overall cost of operation decreases

§   Overall efficiency of the plant increases

§   Thermal efficiency is 30% as compared to 20-22% in PWR

§   The reactor is capable  of promptly meeting load fluctuation requirements

Ø  Disadvantages :

§   More biological protection is required

§   More elaborated safety precaution is required

Fast Breeder Reactor (FBR):-

Ø   It sues highly enriched U-235 as fuel. The reactor has no moderator. Metal liquid is used as coolant. The U-235 is surrounded by U-238.

Ø   The neutron released from the nuclear fission of the U-235 is absorbed by U-238, giving rise to Pu 239 which can be used as fuel. This is known as breeding technology.

Ø   Advantages:

§   High breeding gain is possible

§   No moderator is required

§   Low absorption of high energy neutron permits high fuel burn up

Ø   Disadvantages :

§   Reactor need enriched fuel

§   Liquid sodium is highly corrosive

§   Specific  power is not as in thermal reactor

§   Neutron flux is high at the centre of the core

Nuclear Fuels:-

Ø   U-235, U-233 and PU-239 are the nuclear fuels used in the reactors. U-235 is available in nature but U-233 and PU-239 are obtained from Th-234 and U-238 using breeding technology. Natural uranium contains 0.006%of U-234, 0.712% of U-235 and 99.28% of U-238

 Desirable Properties of Nuclear Fuel:-

§  High tensile strength at high temperature

§   High conductivity and high ductility

§   It must be corrosion resistive

Ø   Enriched Uranium: The enrichment process increases the concentration of U-235 in natural uranium. Enriched uranium is used in FBR. Use of enriched fuel reduces the size of the nuclear reactor to be designed.

Ø  Methods of Uranium Enrichment :

   (i) Gaseous Diffusion Method                   (ii) Thermal Diffusion Method

   (iii) Centrifugal Method                             (iv) Electro-magnetic Method

Coolants:

Ø   Coolants are used to remove the heat generate in the reactor chamber, which isto be used for steam generation.

Ø   Liquid (H₂O, D₂O), Metal (Na, Bi, Li, Na-K alloy), Gas (H₂, N₂, He, CO₂) are used as coolants as per the suitability.

Ø   Desirable Properties of Coolants:

§   High melting point

§   High boiling point

§   It must be considerably cheap 

§   It should have low corrosion property

§   High co-efficient of heat transfer

Moderators and Reflecting Material

Ø   Moderators can be used as reflectors since both have the same properties

Ø   H2O, D2O, He, Be, Graphite can be used as moderators

Ø   Desirable Properties of Moderators :

Ø      (i) High slowing down property   (ii) High resistance to corrosion

Ø      (iii) Good thermal conductivity    (iv) Chemical & radiation stability

Ø      (v) It should be cheap and abundant

Ø      (vi) Low melting point and high boiling point

Control Rod Materials:-

Ø   Control rods are used to control the chain reaction in the process of fission

Ø   Boron (B), Cadmium (Cd), Hafnium (Hf) are used as control rod materials

Ø   Desirable Properties of Control Rod Material:

Ø      (i) Cheap    (ii) High melting point  (iii) High corrosion resistive

Ø      (iv) High chemical and radiation stability

Gas Turbine Power Plant:-

Ø  Components : Compressor, Combustion chamber and Turbine-Generator set

Ø   Types of Gas Turbine Plant:

    (1)  Open Cycle Gas Turbine Plant

    (2) Closed Cycle Gas Turbine Plant

Ø   Open Cycle Gas Turbine Plant:

    The ambient air enters into the compressor and the gas coming out of the turbine are exhausted into the atmosphere and the working medium has to be replaced continuously.

•    Advantages :

 (i) Negligible warm up time                (ii) Any hydrocarbon can be used as fuel

  (iii) Less weight and small in size    (iv) Used as peak plant

•  Disadvantages :

 (i) High loss of heat in exhaust gas    (ii) Sensitive to system component efficiency

 (iii) Deposition of carbon and ash on turbine reduces the turbine efficiency

Ø  Closed Cycle Gas Turbine Plant

     The working fluid is continuously used in the system without its change in phase and the required heat is given to the working fluid in the heat exchanger

•  Advantages ;

  (i) The machines used are cheaper and smaller

 (ii) No corrosion of the turbine blades

 (iii) No need of filtration of the incoming air

 (iv) Inferior oil and solid fuel can be used

 (v) Maintenance cost is low and reliability is high due to longer useful life

•  Disadvantages :

   (i) Large amount of cooling water is required

   (ii) Components used are complicated in design

   (iii) The response to the variation of load is poor

   (iv) It requires a big heat exchanger

Advantages of Nuclear Power Plant:

 (i) Reduces the demand for fossil fuels i. e. coal, gas and oil

 (ii) No problem in transportation and storage of fuel

 (iii) Requires less place

 (iv) The can produce secondary fuels

 (v) Used as base plant

 (vi) Operation not affected by weather condition

Disadvantages of Nuclear Power Plant:

    (i)          High initial capital cost

  (ii)          Greater technical knowledge is required

(iii)           Radio-active pollution

(iv)           Poor response to the fluctuation of the load

  (v)           The fuel used is rare and expensive

(vi)           Disposal of nuclear waste  is a big problem