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Monday, 19 March 2012

Nuclear Fission and Fusion

When the nucleus of an atom breaks up , the pieces into which it breaks fly apart with great speed. The kinetic energy (energy of motion) of these fragments is huge. When the particles heat matter, their kinetic energy is changed into heat energy . This heat can be put to useful work in power stations. It can also be used for destructive purposes in the stom bomb.

To make this energy available it is neccessary to start a chain reaction in a radioactive material. A chain reaction starts when the fragments from one atomic nucleus collide with another nucleus causing it to spilt. The fragments from this
nucleus cause other nuclei to spilt. and so on. A large radioactive nucleus, like uranium, is spilt by bombarding it with
atomic particles. This splitting is called fission. A neutron is the atomic particle ususally used as it has no charge and
is therefore not repelled by the nucleus. When a neutron strikes the nucleus of certain isotopes of uranium, the uranium
nucleus splits into two approximately equal large fragments and either two or three neutrons. These neutrons fly off and
cause two or three more uranium nuclei to break up .This is how the chain reaction starts.

In the atom bomb all the energy of this nuclear reaction is released in a fraction of a second, in the form of tremendous
explosion. To make this happen the fuel must consist of pure uranium-235 or plutonium-239. Both of these isotopes are
fissile, that is, they split up when struck by a neutron.

In a nuclear reactor arrangements have to be made to slow down the devastating explosion that takes place in a bomb.
This is done by using a mixture of the fissile isotope of uranium and the more plentiful, but much more stable isotope,

Natural uranium is one of the most valuable and sought after minerals on earth. When it comes from the ground. It
contains only 7 fissile uranium-235 atoms in every 1000. It is not possible to build up a chain reaction in this material.
In order to start a chain reaction it is neccessary to increase the percentage of uranium-235 atoms in natural uranium
or to add plutonium to it. This is called enriching the natural fuel.


There are various types of reactor. In thermal reactors the fuel is mixed with a substance called  a moderator. This is a
substance consisting of light atoms. such as carbon or water. The neutrons emitted during fission collide with the atoms
of the moderator. These collisions slow down the neutrons so that more are able to cause fission of the uranium-235 isotope .This process genetrates a large amount of heat.

The core of a nuclear reactor contains the uranium fuel and the moderator. The heat produced is use to heat up a liquid
called the coolant. The coolant becomes very hot and is able to make water boil.The steam produced is used, as in coal
or oil-fired power stations, to rotate a turbine which drives a generator. Many countries today have nuclear power stations to supply the growing demand of electricity.

Fast reactors involve the interaction of fast neutrons with the nuclei. Steps are being taken to develop fast breeder
reactors. These use plutonium-239 as fuel mixed with uranium-238 . Some of the fast neutrons produced by the plutonium-239 fission are absorbed by the uranium-239. In this way the much more common uranium-238 can be used
to produce energy . This would enable uranium reserves to last far longer.

Albert Einstein(1879 - 1955). The great German physicist who settled in America. His theory of relativity showed that mass and energy are different forms of each other .In thermonuclear reactions some of the mass of the deuterium atoms is converted into energy .This is the source of the energy of the sun and also the H-bomb.  

A chain reaction in Uranium-235. One neutron causes a nucleus to split. The split nucleus breaks into two large fragments and either two or three fresh neutrons. These neutrons each collide with further nuclei causing more fissions.

Fissile Uranium-235 and plutonium -239 can be stored safely if the quantity is small (Less than a few kilograms). This is because so many neuttrons escape from the surface of the material that the chain reaction cannot continue. This smallest
quantity capable of maintaining a chain reaction is called the critical mass. Above the critical mass enough neutrons are kept inside the material to keep the chain reaction going .In an atom bomb two masses smaller than the critical mass are brought together tom make one lump above the critical mass.

A thermal reactor .The core consist of fuel elements and moderator .The rate of reaction is controlled by control rods made of a subtance (Such as Cadmium or Boron) that absorbs neutrons. The control rods are raised and lowered into the core.The coolant extracts the heat from the core and raises steam in a separate circuit. The steam drives the turbogenerator.  
The fission of heavy nuclei is one way to obtain nuclear energy. Another way is the fusion(or joining together) of light nuclei. Deuterium is an isotope of hydrogen. Sometimes called heavy hydrogen. Two atoms of deuterium can join together to form helium. This releases an enormous amount of energy. When 1 kg of deuterium is converted to helium  the energy released is six times greater than the energy released by the fission of 1 kg of uranium.

These thermonuclear reactions, as they are called , will only take place if the deuterium atoms collide with great energy. This means that they must be moving very rapidly which, in turn , means that they must be at a very high temperature
- millions of degree centrigrade. Temperatures as high as this do not  normally occur on earth. However they do occur
in the interior of the stars.

The energy of the sun and other stars is derived from thermonuclear reactions. In the interior of the sun , hydrogen is being converted into deuterium, and deuterium into helium . The gravitational attraction in the centre of the sun forces all the hydrogen atom (protons) close together . This creates the high pressure and high temperature needed for fusion.

When a neutron collides with a proton a deuterium nucleus is formed . When two deuterium nuclei collide. A series of
reactions occur leading to the formation of helium. The total mass of helium formed is less than the total mass of deuterium. This difference in mass, called the mass defect, is converted into energy .Einstein showed that a small amount of mass is equivalent to a huge amount of energy . This is why thermonuclear reactions produce so much of energy.
It is extremely rare for the high temperatures required for thermonuclear reactions to occur on the earth. The atom (fission) bomb is one way of producing such temperatures. It is used to create the conditions for fusion in the hydrogen

If it were possible to make a fusion reactor this would have very great advantages over the fission reactor. Hydrogen is
much easier and cheaper to obtain than uranium . Two out of every three atoms of the water are hydrogen and 15 out of
every 100,000 hydrogen atoms are deuterium atoms. Sea water would therefore provide plenty of fuel.

There is another great advantage. A fission reactor produces very dangerous radioactive waste. This is at present stored in tanks until a way to store it permanently is developed. A fusion reactor would produce very little radioactive waste.

Scientists in many countries are trying to make fusion reactors work but none so far have succeeded for more than the
smallest fraction of a second. A temperature of about 100 million degrees is needed for fusion to happen . At this temperature the material is a plasma and would vaporize any cointainer.So the problem is not only reaching a high
enough temperature, but also containing the material. To overcome this, people have tried containing the plasma inside
a magnetic field. Many of the reactors are ring shaped . They are called Takomak reactors after the first one developedin the USSR.
Thermonuclear reactions occur between deuterium nuclei . A deuterium nucleus consists of  one neutron and one proton .In the first reaction two deuterium nuclei fuse to produce  a hellium- 3 nucleus (two protons and one neutron) and one spare neutron in the second reaction , which also can occur , the hydrogen isotope called tritium (one proton and two neutrons) is formed. In the last two reactions tritium and Helium-3 react with deuterium to form helium-4.

One problem of thermonuclear reactions is to keep them in a vessel .One solution relies on magnetic fields to keep the reaction away from the walls of the tube .At the high temperature of a thermonuclear reaction the atom loose their electrons and become a collection of electrons and positively charged nuclei. A gas in this state called a plasma . In a toroid (ring-shaped) reaction vessel the plasma is created by passing an enormous current through the gas. This current also produces a strong magnetic field which keeps the plasma to the centre of the tube and away from the walls.
Some experimental reaction vessels are straight .The plasma is stopped from touching the ends by magnetic mirrors (strong magnetic fields round the end of the tube). In some experiments the material is turned into a plasma using the energy of a laser beam.

The extraction of energy from a thermonuclear reaction is still in the experimental stage . One possibility is to use a substance like liquid lithium to extract the heat from the reaction tube .This would be use to produce steam to drive a turbogenerator.

In the fusion process , atoms of two light elements are squeezed together and fuse to form a heavier element.They release energy as they do so.

Animation of Nuclear Fission and Chain Reaction in Nuclear Thermal Power Station:


 Animation of Fusion Reaction:


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