Fission and Fusion

 

Synthesising new elements in Stars

    Hydrogen and helium nuclei are very significant in the synthesis of elements. They make heavier elements in a variety of processes know as nucleogenesis.  This usually happens in stars. Stars are mainly made of hydrogen and in the middle of the star; it is hot and dense that it causes hydrogen atoms to fuse with each other forming helium. A large quantity of energy is required to do this and it also releases lots of energy. It’s known as nuclear fusion.

41H                       4He

    The stars use up most of their hydrogen and then start nuclear fusion with helium.  This creates heavier nuclei atoms such as Beryllium, Carbon and oxygen.

24He                  8Be

                                                                   4He    +    8Be                   12Be

                                           

 

  This nuclear fusion carries on until the helium is used up and starts a new series of fusion with carbon. Therefore hydrogen and helium nuclei are very important in the synthesis of new elements.

    Nevertheless, nuclear changes happen when lithium and other elements in the first three periods of the periodic table are being formed. The production of the small amounts of hydrogen and helium nuclei in stars makes it possible to synthesise these elements. There are two routes for the generation of elements. Firstly, helium nuclei can fuse with a hydrogen nuclei forming lithium:

4He    +    3H                    7Li

    Another way is if a helium atom fuses with a tritium atom making Beryllium isotope. However, then Beryllium reacts with an electron to from Lithium because this would cause the proton number and the structure of the nucleus to change:

4He    +    3H                  7Be

7Be    +     e-                  7Li

Lithium can also be produce by supernovas or the action of cosmic rays on elements such as carbon and nitrogen.

Fission and Fusion

    The main characteristics of fission, is that when an atom absorbs a neutron, it causes the nucleus to oscillate and become unstable. This causes an atom to split, giving out three neutrons and causes this process again. On possible fission reaction is Uranium-235:

 

n       +        U           Kr      +      Ba     +   3   n

     

   

   

The main characteristics of fusion are when two atoms collide together to form a heavier atom. It usually happens in stars. An example of a fusion reaction is deuterium and tritium fuse to form a helium atom and a neutron.

H      +        H           He      +      n

     

 

   

   

 In fission reactions there is a lot of energy produced and it’s from the direct conversion of some of the nuclear mass into energy. It is a very exothermic reaction and can be used to generate electricity.  Nuclear fission reactions are controlled in a nuclear reactor. It ha two mechanisms for controlling the reaction, which is the graphite moderator and the control rods. The neutrons that are produced when the nucleus splits are rapid and the moderator slows this down. The control rods absorb the neutrons so can control the rate of the fission reaction.

    Energy is released from a fusion reaction, because of the lower binding energy of the atom made compared to the two that fused, the combined mass of the products, is less that the mass of the reactants. A tokamak is used to control fusion reactions as it keeps the plasma away from the vessel walls. It keeps the changed particles away by using a magnetic field.

    The advantages for nuclear fission to generate electricity:

• It releases hardly any carbon dioxide that contributes to the greenhouse effect.
•  The price of uranium hasn’t changed price unlike fossil fuels; therefore it is a secure option.

A major disadvantage is that:

•  The waste produced is very harmful and it remains harmful for thousands of years as its radiation

    The advantages for nuclear fusion:

• The fuel (hydrogen) is abundant and it is a long-term source of power.
•  The radioactivity if the structure of a fusion power station is short lived, as it doesn’t take a long time to decay.
•  It also is highly safe because only small amounts of fuel is used so cannot get out of control.

A disadvantage is that:

• It is very difficult to be able to create the conditions that happen in our Sun on Earth and to control then.

   

Fusion Power Stations

    Scientists will encounter problems with using fusion power stations as it is difficult to recreate the reaction that happen in stars on Earth. It’s hard to reproduce fusion under the same conditions as stars on Earth and to safely manage them. Fusion on nuclei requires extremely high temperatures. At normal temperatures on Earth, the repulsion between the two nuclei would be too great for the nuclei to fuse, and its very difficult to obtain these temperatures on Earth. It has to be really dense to form plasma, however this plasma must be kept away from the vessel walls, otherwise the energy would escape.

    Overall, I have explained radioactivity and fission and fusion reactions.