Fission reactions differ from radioactive decay both in the way that the reaction must be started and in the type of products that are formed . Radioactive decay is a passive action, while fission is active. For radioactive decay, the atom is unstable; while the nuclei in the process of fission absorb a neutron, then oscillate to become unstable. Moreover, the product of radioactive decay is only an atom of other element; while the products of fission are 3 neutrons and 2 different elements. Hydrogen and helium are by far the most abundant element, which is 89% and 11% respectively .
The process of making the simple elements (like lithium, etc. ) is called nucleogenesis. Hydrogen acts as a producer, which is the start of the nucleogenesis. It converts to helium in nuclear fusion. When hydrogen is used up to produce helium, fusion of helium with other elements occurs. Helium nuclei react to form beryllium, carbon, oxygen, neon and magnesium : If helium is used up to produce the elements mentioned above, carbon nuclei combine to produce elements: Although the production of hydrogen and helium is small, it is still possible for the star to produce other elements.
There are 2 routes to generate lithium: Route 1 Route 2 The first route shows the simple rule of fusion, while the second route shows the changes of the nucleus by the collision between an atom and an electron. The proton number and the structure of the nucleus are changed when the electron reacts with it.  An unstable nucleus may become more stable through radioactive decay. Some heavy nuclei can also become more stable when they split into two smaller parts. This is called nuclear fission. For example, when a uranium-235 nucleus is bombarded by a neutron, it splits into two smaller nuclei.
Two or three neutrons and a large amount of energy are released. Energy is released when light nuclei fuse together to form a heavier nucleus. This process is called nuclear fusion. For example, a deuterium nucleus ( H) can fuse with a tritium ( H) nucleus to form a helium-4 nucleus ( He), releasing a neutron and a large amount of energy. The following shows the concept of fusion. For nuclear fission, each fission reaction release 200 million eV (where 1 eV= 1. 602? 10-19J) . By calculation, 200? 106? 1. 602? 10-19 =3. 204? 10-10 J.
For nuclear fusion, there are many combinations of fusion, while only 1 type of fusion will be considered, the typical fusion deuterium nucleus ( H) and tritium ( H) nucleus. This shows the fusion reaction of deuterium and tritium. The energy of the helium is 3. 5MeV and the energy of the neutron is 14. 1MeV, which give the total of 17. 6MeV of energy. By calculation, 17. 6? 106? 1. 602? 10-19 =2. 81952? 10-12 J The figure below shows a schematic diagram of nuclear power plant. The nuclear fuel used, uranium(U), is contained in fuel rods.
There is only 0. 7% of naturally occurring uranium is the fissionable U-235, where the rest is U-238. Because of this, the fuel used is first enriched by increasing the proportion of U-235 to about 3%.  The neutrons released during fission often travel too fast to trigger further fissions. The fuel rods are surrounded by a moderator, which may be water or graphite. The neutrons are slowed down when they collide with the nuclei in the moderator. The rate of reaction is controlled by control rods that can be moved in and out of the reactor.
They are made of materials like boron or cadmium, which can absorb neutrons. When the control rods are moved in, the rate of chain reaction decreases. When they are moved out, the reaction is speeded up. In case of emergency, all control rods are moved in to shut the reactor. Fission Fusion Advantage Disadvantage Advantage Disadvantage -chain reaction can take place -highly radioactive used fuel -unlimited supply of fuel -uncontrollable -can be controlled -long half-lives -product of fusion is stable -requires high temperature -2 loops to avoid radioactive substances reach the turbine -formation of bi-products -product is non-radioactive -product with short half-lives.
Nuclear Fusion can be a good energy supply, however, there are some challenges that scientists are facing. Firstly, nuclear fusion is uncontrollable, once it is out of control, explosion of hydrogen bomb would occur. Secondly, it requires high and sustainable temperature to generate electricity or energy, it costs so much. Lastly, the fusion power station will be occupied for military use, which may lead a catastrophic war.
Reference: 1. Articles(given) Lise Meitner Fusion Powering the future?2. Physics in Life Electricity, Magnetism and Atomic Physics Longman S. S. Tong, K. S. Lam, S. C. Chiu P. 176 range in air P. 177 penetrating power P. 178 deflection of nuclear radiation in electric and magnetic field P. 221-225 Nuclear energy 3. Wikipedia Fusion http://en. wikipedia. org/wiki/Nuclear_fusion Fission http://en. wikipedia. org/wiki/Nuclear_fission Power Plant http://en. wikipedia. org/wiki/Talk:Fossil_fuel_power_plant 4. Advancing Physics AS P. 172 eV 5. Salters Chemical Ideas P. 20 properties of ? ,? and ? emission.