The fundamental difference between radioactive decay and nuclear fission is that, whereas radioactive decay is spontaneous, nuclear fission must be induced. In nuclear fission, when an unstable nucleus absorbs a neutron, it splits, emitting more neutrons and setting off a continuous chain reaction. This leads to products with nuclear masses around half those of the initial nuclei, whereas in radioactive decay, the initial and final nuclear masses are relatively close together. The other major difference is that fission releases considerably more energy than decay. This energy comes from mass lost in fission, according to the equation E = mc2, where E is energy, m is mass and c is the speed of light.
Synthesis of Elements in Stars
Stars produce their energy from nuclear fusion, in which nuclei join together to make larger nuclei. Hydrogen is used in normal-sized stars:
(e+ represents a positively charged electron, and ?e is a neutrino) This process requires temperatures of around 13 million K and pressures of around 300 billion atmospheres.6 When almost all of the hydrogen has fused, the helium nuclei can collide to make nuclei such as beryllium:7
This leads to the creation of further nuclei containing four nucleons: carbon, oxygen, neon and magnesium. Once all the helium has fused, further collisions take place between the created nuclei. This leads to the production of small amounts of hydrogen and helium, producing most of the first 18 elements, such as lithium:8
Lithium can also be produced by the collision of a Beryllium-7 nucleus and an electron. The nuclear process that takes place here is electron capture, in which an atom captures an electron, turning a proton into a neutron and releasing a neutrino. This happens by the weak interaction, like -decay:9
Smaller amounts of lithium can also be produced in the fission of some nuclei by cosmic rays and in supernovae, when heavy stars become unstable and explode.10
Producing Energy through Nuclear Fission and Fusion
In nuclear fission, an unstable nucleus absorbs a neutron, exciting the nucleus, causing it to oscillate and split into two smaller nuclei. This process releases more neutrons, causing more nuclei to split, and so on. This is shown in Fig. 211 with Uranium-235.
The energy produced by nuclear fission, by E = mc2, is 3.2×10-11 J per fission.12
Uranium-235 is used to produce energy by fission – see Fig. 313.
Controlling this reaction:
* Uranium-238 is mixed with uranium-235. Uranium-238 nuclei absorb neutrons but do not react by fission, breaking the chain in the reaction.
* Graphite moderators placed in between the uranium rods reduce the kinetic energy of the neutrons produced so they can induce fission.
* Boron-coated steel control rods absorb neutrons, and can be moved in and out of the reactor. If they are fully in, the reaction stops.
Nuclear fusion takes place when, under certain conditions, two nuclei fuse together. For example, with deuterium and tritium:14
The energy produced comes from the mass lost – 3.17×10-29 kg 15 16. By E = mc2, this gives out 2.86×10-12 J per fusion. On earth, for this to happen the nuclei must be in ionised plasma at temperatures of 15×108 C. The problem with this is that it must be kept away from the walls of the container to minimise heat loss. To do this, a tokamak is used. This uses magnetic currents to keep the plasma from touching the walls (see Fig. 417). The walls are made of graphite, which is not harmed by the temperature.
Both fission and fusion have several advantages and disadvantages for use in producing electricity:
* Uranium-235 produces 3.7 million times the energy per unit mass as coal18
* Uranium-235 will not run out on the same timescale as fossil fuels19
* It produces no gases that directly cause global warming
* Fission produces waste radioactive actinides, which are dangerous for thousands of years
* Fission has led to disasters such as Chernobyl in 1986, which caused over 4000 deaths20
* The fuel – hydrogen – is abundant
* The radioactive waste products have half-lives hundreds of years less than those of fission
* Fusion is safer than fission, as only small amounts of products are used
* It produces no gases that directly cause global warming
* The conditions required for fission are hard to produce
* The process used to produce energy by fusion is not yet perfect – see below
Challenges Facing the Development of Fusion Power Stations
The major problem with fusion is generating and containing the conditions required for the reaction. As detailed above, a tokamak is used, this has some problems. The plasma still touches the bottom of the chamber, and where it does this; hydrogen reacts with the walls forming hydrocarbon radicals. These can form a film, which flakes away into the plasma, affecting performance21. Possible solutions include removing the film with lasers22 or using tungsten walls, which would not erode23.
A probable source of a solution is the International Tokamak Experimental Researcher, currently being built in France. It will be used as a prototype to test the reaction on the necessary levels required. Fusion should be available to produce commercial power by 2040.
Used throughout the report:
* Chemistry Review: Lise Meitner: Radiochemist, physicist and co-discoverer of nuclear fission, Gordon Woods, Volume 16 Number 1, September 2006 (Article 1); Fusion, Powering the future?, Chris Warrick, Volume 16 Number 1, September 2006; and Lithium, Chris Ennis, Volume 15 Number 31, February 2006 (Article 2)
* Salters Advanced Chemistry Chemical Ideas, George Burton et al, Heinemann Educational Publishers, Halley Court, Jordan Hill, Oxford, OX2 8EJ, ISBN 0-435-63129-9, first edition 1994, second edition 2000
1 Equation copied from page 3, Lise Meitner: Radiochemist, physicist and co-discoverer of nuclear fission; see above
2 Fig. 1 copied manually from Page 487, The exchange nature of forces, Advanced Physics, Tom Duncan, John Murray (Publishers) Ltd, 50 Albemarle Street, London, W1S 4BD, first edition 1972, ISBN 0-7195-7669-5, fifth edition 2000, reprinted 2002
3http://www.chemie.de/lexikon/e/Beta_decay, Beta Decay, (c) 1997-2008 Chemie.de Information Service
4 Equation copied from page 3, Lise Meitner: Radiochemist, physicist and co-discoverer of nuclear fission; see above
5 Table adapted from Page 20, Nuclear Reactions, Salters Advanced Chemistry Chemical Ideas; see above
6Where did the chemical elements come from?, Page 131, The Universe: A Biography, John Gribbin; published by Penguin Books Ltd, 80 Strand, London, WC2R ORL, ISBN 978-0-1410-2147-8, 2006
7Equation copied from Box 1: Nucleogenesis, Page 21, Lithium, Chris Ennis; see above
8Equation copied from Box 1: Nucleogenesis, Page 21, Lithium, Chris Ennis; see above
9 http://www.britannica.com/nobelprize/article-48278, electron capture with a Beryllium-7 nucleus, from the Encyclopaedia Britannica’s Guide to the Nobel Prizes, (c) 2008 Encyclopaedia Britannica, Inc.
10 Where do the chemical elements come from?, Page 10, Salters Advanced Chemistry Chemical Storylines, George Burton et al, Heinemann Educational Publishers, Halley Court, Jordan Hill, Oxford, OX2 8EJ, ISBN 0-435-63119-5 first edition 1994, second edition 2000
11 Fig. 2 taken from http://www.astro.bas.bg/~petrov/herter00.html, lecture notes for astronomy, Bulgarian Institute of Astronomy http://www.astro.bas.bg/
12 http://www.lancs.ac.uk/ug/bloomer/nuclearpower/theory.htm, Nuclear Fission and Nuclear Fusion, from the University of Lancaster, www.lancs.ac.uk
13 Fig. 3 copied manually from Box 2, Article 1 (see above); adapted from Chemistry Today, (c) E. Henderson, Macmillan Publishers Ltd, 1977.
14 Equation copied from Box 2, Fusion, powering the future?, Chris Warrick; see above
15 Mass of reactants and products given in proton masses in Box 2, Fusion, powering the future?, Chris Warrick; see above
16 Proton masses converted to kilograms using the mass of one proton as 1.67×10-27 kg, from Data Sheet, Page 3, AQA GCE AS Physics A Unit 1, January 2007
17 Fig. 4 taken from http://www.jet.efda.org/pages/fusion-basics/fusion3.html, Fusion Basics, (c) EFDA-JET
18 http://www.virtualnucleartourist.com/basics/reasons1.htm, a website “intended to provide you basic information about the different types of plants and their principle of operation”, (c) 2006 The Virtual Nuclear Tourist
19 http://www.abc.net.au/rn/scienceshow/stories/2007/2080110.htm, an interview with Professor Martin Sevior from the University of Melbourne by the Australian Broadcasting Corporation
20 http://www.who.int/ionizing_radiation/chernobyl/who_chernobyl_report_2006.pdf, the World Health Organisation’s report on Chernobyl
21 Chemistry Review: Fusion, powering the future?, Chris Warrick; see above
22 http://www.lasers.org.uk/paperstore/Cleaning14.pdf, Dust Removal from Next Generation Tokamaks by Laser and Flashlamp Cleaning, K. G. Watkins et al, Lasers and Laser Engineering, University of Liverpool, 2001
23 http://www.jet.efda.org/documents/articles/samm.pdf, Controlled thermonuclear fusion enters with ITER into a new era, Page 12; Ulrich Samm, EFDA-JET, 2003