Fission and Fusion (Open Book paper 2008)

In reactions 8 and 9, enough hydrogen and helium nuclei are produced to allow the synthesis of more elements. Lithium has two routes by which it can be made:

Route 1: 4He + 3H → 7Li

Route 2: 4He + 3He → 7Be

 7Be + 0e- → 7Li

Reactions 10-12 – synthesis of lithium through fusion reactions [6]

In the first route and the first reaction of the second route, the nuclei fuse, simply adding together the protons and the neutrons together to form a larger nucleus. However, the second reaction of the second route is more complicated. The negative electron joins with one of the positive protons in beryllium-7, forming a neutron, [7] in a way that can be thought of as the reverse of beta-decay. This decreases the atomic number by 1, so lithium is formed, but the atomic mass remains the same.

Image 1 – route 2 in the synthesis of lithium

Source [8]

190 words

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Describe, with the use of examples, the main characteristics of fission and fusion reactions. Explain how each type of reaction produces energy and describe how these reactions are controlled. Outline the main advantages and disadvantages of using fission and fusion processes for generating electricity.

Fission

Image 2 – fission

Source [9]

Both of the new nuclei are approximately half the size of the original nucleus. The neutrons given off go on to trigger more fission reactions, in a chain reaction.

Fission reactions give off energy through conversion of some nuclear mass into energy. There is a difference in mass between the reactants and the products, as seen in reaction 13.

1n + 235U → 91Kr + 142Ba + 3 1n

Reaction 13 – a possible fission reaction of uranium-235

Source [10]

The mass of the uranium + the neutron is 0.1866971 amu [10] (atomic mass unit) greater than the mass of the products combined. The rule of conservation of mass-energy, according to Einstein’s E=mc2 (where E is energy in Joules, m is mass in kilograms, and c is the speed of light in a vacuum, 2.99792 x 108m/s), explains that the lost mass has in fact been converted into energy – approximately 1.68 x 1010 kJ/mole [10].

If uncontrolled, fission will accelerate out of control due to more and more neutrons being given off. Nuclear reactors generate electricity using a mixture of 238U and 235U. 238U does not undergo fission; it absorbs neutrons, interrupting the chain reaction so helping to control it.

Image 3 – reactor and heat exchanger

Source [11]

The controlling features in the reactor:

• The graphite moderator slows the neutrons from their high speeds (at which they are less likely to cause a fission reaction on collision with 235U). The high energy of neutrons produced in fission led to the development of the first nuclear bombs.
• The control rods absorb neutrons, taking them out of the chain reaction. The rods can be moved to different depths to absorb different amounts of neutrons, controlling the rate of reaction. If they are fully inserted, all of the neutrons are absorbed and the reactor shuts down.

The reactor is cooled by passing fluids (often molten sodium metal or carbon dioxide) through pipes around the reactor. This then boils water to form steam, which is used to turn turbines, generating electricity.

Table 2 – advantages and disadvantages of fission as an electricity source

Source [12]

Fusion

Image 4 – deuterium-tritium fusion reaction

Source [13]

Under the right conditions (see section 4), the repellent forces between the positive nuclei are overcome, and the strong nuclear forces fuse the nuclei together. For experimental electricity production on Earth, the reaction chosen by scientists is the one shown in image 4 – deuterium (hydrogen-2) and tritium (hydrogen-3) forming helium-4 and a neutron.

2H + 3H → 4He + 1n

Reaction 13 – deuterium-tritium fusion reaction

Source [14]

This will release energy because the helium nuclei have a lower binding energy (the energy holding the neutrons and protons together in the nucleus) than those in deuterium and tritium. In a similar way to fission, the mass of the products is less than the mass of the reactants, and the apparently “missing” mass has been converted to energy [14].

Fusion reactions do not happen naturally in the conditions on Earth – they require the high temperatures found in stars. The deuterium-tritium reaction needs to occur in a 150 million °C plasma. In a plasma (ionised gas), the electrons are delocalised, forming a “sea” around the positive nuclei. This temperature is reached using a variety of heating methods, and by keeping the plasma away from the cooler vessel walls. In a Tokamak (the structure which is most likely to be used for a fusion power station, more detail in section 4), magnetic fields are generated by large coils, which isolate the plasma from the walls, allowing it to keep its energy for a longer time.

Table 3 – advantages and disadvantages of fusion as an electricity source

Source [15]

490 words

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Outline the main challenges that scientists face in developing fusion power stations.

The requirements for sustained fusion in a tokamak (toroidal (ring-shaped) magnetic chamber) are as follows:

• Plasma temperature of 100-200 million Kelvin

This is needed to overcome the repellent forces between the two positive nuclei to be joined. When power in is equal to power out, it is called Breakeven [15]. When the power out heats the plasma so no external heating is required, it is called Ignition [15]. Breakeven has been achieved in the JET tokamak, but reaching Ignition would be the challenge in a commercially viable fusion power station.

• Energy confinement time of 4-6 seconds

This is a measure of how long energy is retained in the plasma. The plasma needs to be kept away from the cooler tokamak walls to increase the energy confinement time. This can be achieved with magnetic fields, although these require a large amount of energy:

Image 3 – the magnetic fields in the JET tokamak

Source [15]

A challenge is to find a way of keeping a high energy confinement time with a lower input of energy. This could be achieved with superconductive coils (but these have to be cooled using liquid helium) or laser-induced inertial confinement systems.

• Plasma density of 1-2 x 1020 particles/m3

The fusion power is decreased if there are impurities in the plasma, such as the helium nuclei produced from the fusion. The challenge is to remove the impurities and to replace the fuel (2H and 3H nuclei) without stopping the reaction, as for Ignition to be reached, the reaction must be continuous [15].

Scientists also need to find out ways of cleaning the plasma-facing tiles of the structure of deposits (mainly hydrogen isotopes, carbon and beryllium).

One challenge is to overcome the negative public view of nuclear power. Fusion only uses a few grams of reactants at any one time, and if there is a leak or a malfunction, the plasma will immediately cool and decay, terminating the reaction.

220 words

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Sources

[1] Salters Advanced Chemistry Chemical Ideas, Heinemann Educational Publishers, Oxford, 2nd edition, 2000. Page 20.

[2] Nuclear properties > Processes of nuclear decay > Beta-particle decay, Encyclopaedia Britannica Online. 30/03/08.

http://www.britannica.com/eb/article-81189/transuranium-element#622175.hook

[3] Lise Meitner, Radiochemist, physicist and co-discoverer of nuclear fission, Gordon Woods, Chemistry Review, Volume 16, Number 1, September 2006. Box 1 (page 3) and page 4.

[4] Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction, Nature – Physics Portal. 24/03/08

http://www.nature.com/physics/looking-back/meitner/index.html

[5] Nuclear Fission, Thinkquest. 30/03/08

http://library.thinkquest.org/3471/fission_body.html

[6] Fusion, Powering the Future? Chris Warrick, Chemistry Review, Volume 16, Number 1, September 2006. Box 1 (page 9).

[7] A Moving Model of the Beginning of the Universe, NASA. Page 5. 24/03/08

http://imagine.gsfc.nasa.gov/docs/teachers/elements/imagine/BigBang/student_handout.pdf

[8] Adapted from Primordial Nucleosynthesis, OpenLearn LabSpace. 24/03/08

http://labspace.open.ac.uk/mod/resource/view.php?id=167939

[9] Oxford Illustrated Science Encyclopaedia. 24/03/08

http://fds.oup.com/www.oup.co.uk/images/oxed/children/yoes/atoms/fission.jpg

[10] Why is energy released during a fission reaction? MadSci Network. 24/03/08

http://www.madsci.org/posts/archives/2006-08/1154618198.Ph.r.html

[11] Lise Meitner, Radiochemist, physicist and co-discoverer of nuclear fission, Gordon Woods, Chemistry Review, Volume 16, Number 1, September 2006. Box 2 (page 6).

[12] Energy Matters: Advantages and Disadvantages, Thinkquest. 24/03/08.

http://library.thinkquest.org/20331/types/fission/advant.html

[13] Nuclear Fusion, Splung Physics. 24/03/08.

http://www.splung.com/content/sid/5/page/fusion

[14] Fusion, Powering the Future? Chris Warrick, Chemistry Review, Volume 16, Number 1, September 2006. Box 2 (page 10).

[15] Comparisons of various energy sources, The Virtual Nuclear Tourist. 25/03/08.

http://www.nucleartourist.com/basics/why.htm

[16] Fusion Basics, Focus On: Fusion Technology, and Multimedia, EFDA-JET. 24/03/08

http://www.jet.efda.org/pages/fusion-basics/fusion2.html

http://www.jet.efda.org/pages/fusion-basics/fusion3.html

http://www.jet.efda.org/pages/fusion-basics/fusion4.html

http://www.jet.efda.org/pages/fusion-basics/fusion5.html

http://www.jet.efda.org/pages/fusion-basics/fusion6.html

http://www.jet.efda.org/pages/focus/fusion-tech/index.html

http://www.jet.efda.org/pages/multimedia/gallery/index.html