[I], Diagram showing the half-life of phosphorus-32
Nuclear fission can create a chain reaction, which is why it can be used for providing a lot of energy, unlike radioactive decay, which releases less energy each time a atomic nucleus disintegrates and stops altogether when the element becomes stable.
Hydrogen and helium are important in the synthesis of elements as they are the most common (making up almost 89% and 11% of the universe respectively). The atomic nuclei of the hydrogen and helium are used to make bigger elements through nuclear fusion. It usually starts off with hydrogen being converted into helium:
41H → 4He + subatomic particles
Helium can also be made by combing other hydrogen isotopes, like helium-1 and tritium, or two deuterium nuclei. After the reaction, the helium nuclei is lighter than the original hydrogen particles because the reaction releases subatomic particles in the form of heat and radiation[4]. Once the helium is made, it becomes easier to make larger nuclei, like neon or magnesium:
4He + 4He → 8Be
4He + 8Be → 12C
12C + 4He → 16O
16O + 4He → 20Ne
20Ne + 4He → 24Mg
When the carbon nuclei is formed, they can combine to form larger elements, like magnesium:
12C + 12C → 24Mg
Although magnesium-24 is not an unstable isotope, it may split and form other stable nuclei:
12C + 12C → 23Na + 1H
12C + 12C → 20Ne + 4He
This produces more helium and hydrogen that can be used to make the other small nuclei, or the helium nucleus can collide with the hydrogen nucleus to make lithium:
4He + 3T → 7Li
There is also another way to make it, by using different isotopes of helium:
4He + 3He → 7Be
7Be + e- → 7Li
The second step of the reaction is a form of β-decay called electron capture[5], which occurs when there are too many protons in the nucleus. The unstable beryllium captures an electron to stabilise itself by changing the electron and proton into a neutron. The loss of the proton turns the beryllium into lithium. Some lithium-6 is also made, as it has a natural abundance of between 3.75% and 7.59%[6][7].
Nuclear fission happens when a neutron collides with an atomic nucleus. This causes the nucleus to become unstable and split into almost equal parts called fission products and release two or three neutrons. These neutrons can then go on to collide with other nuclei, causing a chain reaction. An example is with uranium-235:
1n0 + 235U92 → 89Kr36 + 144Ba56 +31n0
[II], Picture showing how fission works
The total weight of the fission products is less that the weight of the uranium-236 because of the neutrons released as energy. Isotopes that go through fission like uranium-235 can only absorb a low-energy neutron, also called a slow neutron[8]. Uranium-235 will not capture a fast electron, so in a nuclear reactor, an electron moderator made from graphite is used. Control rods in the reactor are also used to catch spare neutrons, which stop the reaction from becoming too violent. The control rods can be moved in or out, depending on how fast they want the rate of fission. The heat from the energy produced is used to turn water into steam. The steam is pressurised and is used to turn the turbines, producing electricity.
[III], diagram of a nuclear reactor
A problem with using a nuclear reactor is the fission products made afterwards. The fission products are better at absorbing neutrons than the uranium, which makes the reaction die out. Because of this, around 96% of the fuel put in is not used[9]. The radioactive waste is then stored underwater because the gamma (γ) rays reflect off the water molecules and use up their energy in the water. The waste can either be kept underwater, or they can be put into special casts deep underground. Unfortunately, the UK does not have the facilities to keep them underground.
Nuclear fusion happens when two or more atomic nuclei join together to form a heavier element. Nuclear fusion can either release or absorb energy. Fusion reactions are preferred because they are safer than fission reactions as only a small amount of fuel is put in and the length of radioactivity is shorter as well.
A disadvantage of using nuclear fusion is that is still produces greenhouse gases. Although nuclear power is supposed to be an alternative to fossil fuels, methane is formed inside the tokamak by the helium particles eroding the carbon tiles of the inner lining.
Bibliography
* Lise Meitner: Radiochemist, physicist and co-discoverer of nuclear fission
Chemistry Review
Sep 2006
* Fusion: Powering the Future?
Chemistry Review
Sep 2006
[1] http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch23/modes.php
By the Bodner Research Group
Last Updated 18th December 2007
[2] http://www-ik.fzk.de/tritium/overview/index.html
By KATRIN
Last Updated 19th September 2006
[3] http://www.chem.duke.edu/~jds/cruise_chem/nuclear/chemwindow/halflife2.html
By Dr. Diane Szaflarski et al. from the University of California, San Diego
Last Updated 3rd Dec 2007
[4] http://www.physicsforums.com/archive/index.php/t-132036.html
Found on a forum
Posted on 15th Sep 2006
[5] http://education.jlab.org/glossary/electroncapture.html
By Steve Gagnon
[6] http://en.wikipedia.org/wiki/Lithium
Last Updated 4th May 2008
[7] http://www.webelements.com/webelements/elements/text/Li/isot.html
By Mark Winter & The University of Sheffield
Last Updated 12 Jun 2007
[8] http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/fission.html
By C.R Nave, Georgia State University Department of Physics & Astronomy
Created 2005
[9] http://nakeddiscovery.com/scripts/mp3s/audio/Naked_Scientists_Show_07.02.11.mp3
The Naked Scientists Podcast- 11th Feb 2007, approx. 30 mins
Presented by Dr Chris Smith, Cambridge University
Pictures & Diagrams
[I] Based on diagram from http://www.chem.duke.edu/~jds/cruise_chem/nuclear/chemwindow/halflife2.html
[II] http://en.wikipedia.org/wiki/Image:Nuclear_fission.svg
Posted onto the “Nuclear Fission” page
Last edited 31st Dec 2005
[III] From http://www.world-nuclear.org/how/npreactors.html