Saturday, March 17, 2012

Work, energy and power part 3:How a nuclear reactor works


Today we'll learn :


    1. Nuclear energy  
2. The conservation of energy and mass


  3. Thermonuclear energy

  4. Power




1. Nuclear energy



In the previous post ( Work,energy and power part 2) we have discussed the energy changes which lead to the generation of electricity in a coal-burning electric power station. In a nuclear electricity generating installation the heat required for raising steam is provided by a nuclear reactor instead of coal furnace. 


  


How a nuclear reactor works 


    

It consists ( as simply shown above )  of a strong steel pressure vessel enclosing a core made of graphite bricks. This graphite core has a number of vertical channels which are filled with rods of a very heavy metal called uranium. interspersed among the uranium rods are a set of boron steel rods which may be raised or lowered in similar channels in the graphite. These are the control rods, and their function will be explained in the next few paragraphs.





   

The uranium used in the reactor consists of a mixture of two different kinds of atoms, of which the most important are called uranium-235. Quite spontaneously, some of these uranium-235 atoms explode or disintegrate to form other atoms of smaller mass. When this happens, energy is radiated from the central core or nucleus of the atom together with small high-speed particles called neutrons. If one of the neutrons happens to strike the the nucleus of a neighboring atom this may also disintegrate, with a further evolution of energy and the production of more neutrons. This splitting up of the nucleus is called fission.



 


The graphite of which the core is composed is called a moderator. Its function is to slow down the speed of the neutrons, as it found that fission of uranium-235 is more likely to occur with slow neutrons than with fast ones.


            In a small piece of uranium mixed with moderator most of the neutrons escape through the surface. If, however, the amount of material is increased the chances that a neutron will collide with an atomic nucleus will also increase, since there are more atoms present.


    Each nuclear fission which occurs produces two or three fresh neutrons which are, in turn, capable of promoting the fission of further nuclei. When the lump of uranium and moderator is above a certain critical size the fission process proceeds cumulatively in what is called a chain reaction ( as shown above ).




    This is where the above-mentioned boron steel rods play their part. Before the uranium rods are loaded into the graphite core the boron rods are already in position, and these have the property of being able to absorb neutrons which are shot out from the uranium, and so prevent the chain reaction from starting.


When sufficient uranium rods have been added to effect critical conditions the pressure vessel is sealed and the boron rods raised out of the core. The uranium rods are now freely bombarded by one another's neutrons and the chain reaction begins.


The rate at which fission occurs can, of course, be controlled by raising or lowering the boron rods. If these are fully inserted into the graphite core the reaction shuts down completely, and only the normal spontaneous nuclear fission takes place.


 


       

The heat energy released by the fission process is carried away as internal energy in a steam of high-pressure carbon dioxide gas which is continuously pumped through the pressure vessel. This hot gas circulates through a special steam boiler, and the steam so raised is used to drive an electric turbo-generator in the usual way.- Science is amazing!-.



The uranium bomb


       If pure uranium-235 is used a chain reaction is possible without the need for a moderator. The first atomic bomb contained two pieces of uranium-235, each of which was not large enough to sustain a chain reaction on its own. If two such pieces are forced into contact with one another they form a lump which exceeds the critical mass. As soon as this occurs a chain reaction starts and proceeds at such a rate that a large proportion of the nuclei explode almost instantaneously.


2. The conservation of energy and mass


I do not want to repeat myself. Please :) ( my dear reader ) review : Equivalence of mass and energy .



3.

Thermonuclear energy



  Scientists also obtained energy by the fusion of hydrogen nuclei to form heavier ones. This is called a thermonuclear reaction and is the source of the sun's energy. Under the extremely high-temperature conditions in the interior of the sun, hydrogen nuclei fuse together to form helium nuclei, and the resulting loss in mass is poured out in the form of radiation.



4. Power

      

Machines may be classified by the speed with which they do work; thus there are motor-car engines of small " power", as they are rated, or large power.





Power is defined as the rate of doing work


    or


        average power = work done / time taken

 

        The SI unit of power is called the watt ( W ) and is a rate of working of 1 joule per second


        Thus,                         1 W = 1 J / s


        Larger units used are the kilowatt ( kW) and the megawatt ( MW)



 


      

Solved problems :




         

1- Calculate the power of a pump which can lift 200 kg of water through a vertical height of 6 m in 10 s. ( Assume g = 9.8 m / s² .)


           Solution:


Force overcome = ma = 200 × 9.8 N ( Newton's second law )


            Distance            = 6m


            Work done        =Force× distance= Fd = 200 × 9.8 × 6 J


            Time taken       = 10 s


            Power               = work done / time taken = Fd / t = 200 × 9.8 × 6  / 10


                                    = 11176 W = 1.18 kW.

 

     2- Calculate the power of a boy whose mass is 40 kg finds that he can run up a flight of 45 steps, each 16 cm high, in 5.2 s ( Assume g = 9.8 m / s² .)



           Solution:


            Force overcome = ma = 40 × 9.8 N ( Newton's second law )


            Distance            = 45 × 16 = 720 cm = 7.2 m


            Work done        = Force× distance= Fd =  40 × 9.8 × 7.2 J


            Time taken       = 5.2 s


            Power               = work done / time taken =  Fd / t


                                     = 543 W = 0.54 kw.


   The result is creditable to the boy, but it must be remembered that he can maintain this very high power only for a comparatively short time.

 

     Experiment shows that the average power of a man walking upstairs at an ordinary pace is only about 0.33 kW.