Nuclear fusion and nuclear fission

Nature of science:

Trends and discrepancies—our understanding of nuclear processes came from both theoretical and experimental advances. Intermolecular forces in UF6 are anomalous and do not follow the normal trends. (3.1)

Understandings:

Nuclear fusion:

• The mass defect (Δm) is the difference between the mass of the nucleus and the sum of the masses of its individual nucleons.
• The nuclear binding energy (ΔE) is the energy required to separate a nucleus into protons and neutrons.

Nuclear fission:

• The energy produced in a fission reaction can be calculated from the mass difference between the products and reactants using the Einstein mass–energy equivalence relationship E = mc².
• The different isotopes of uranium in uranium hexafluoride can be separated, using diffusion or centrifugation causing fuel enrichment.
• The effusion rate of a gas is inversely proportional to the square root of the molar mass (Graham’s Law).
• Radioactive decay is kinetically a first order process with the half-life related to the decay constant by the equation
• The dangers of nuclear energy are due to the ionizing nature of the radiation it produces which leads to the production of oxygen free radicals such as superoxide (O₂^-), and hydroxyl (HO·). These free radicals can initiate chain reactions that can damage DNA and enzymes in living cells.

• There are only a very small number of countries that have developed nuclear weapons and the International Atomic Energy Agency strives to limit the spread of this technology. There are disputes about whether some countries are developing nuclear energy for peaceful or non-peaceful purposes.
• Nuclear incidents have a global effect; the accidents at Three Mile Island and Chernobyl and the problems at Fukushima caused by a tsunami could be discussed to illustrate the potential dangers.

Theory of knowledge:

• “There is no likelihood that humans will ever tap the power of the atom.” (Robert Millikan, Nobel Laureate Physics 1923 quoted in 1928). How can the impact of new technologies be predicted? How reliable are these predictions? How important are the opinions of experts in the search for knowledge?
• The release of energy during fission reactions can be used in times of peace to generate energy, but also can lead to destruction in time of war. Should scientists be held morally responsible for the applications of their discoveries? Is there any area of scientific knowledge the pursuit of which is morally unacceptable?

Utilization:

• Syllabus and cross-curricular links:
• Topics 4.1 and 4.3—structure and bonding
• Topic 16.1—first order reactions
• Physics topic 7.2—nuclear fusion
• Geography—the different polices and attitudes to nuclear energy are discussed in resources sections in the guide

Applications and skills:

Nuclear fusion:

• Calculation of the mass defect and binding energy of a nucleus.
• Application of the Einstein mass–energy equivalence relationship, E= mc² to determine the energy produced in a fusion reaction.

Nuclear fission:

• Application of the Einstein mass–energy equivalence relationship to determine the energy produced in a fission reaction.
• Discussion of the different properties of UO₂ and UF₆ in terms of bonding and structure.
• Solution of problems involving radioactive half-life.
• Explanation of the relationship between Graham’s law of effusion and the kinetic theory.
• Solution of problems on the relative rate of effusion using Graham’s law.

Guidance:

• Students are not expected to recall specific fission reactions.
• The workings of a nuclear power plant are not required.
• Safety and risk issues include: health, problems associated with nuclear waste, and the possibility that nuclear fuels may be used in nuclear weapons.
• Graham’s law of effusion is given in the data booklet in section 1.
• Decay relationships are given in the data booklet in section 1.
• A binding energy curve is given in the data booklet in section 36.

• Aim 7: Computer animations and simulations of radioactive decay, and nuclear fusion and fission reactions.
• Aim 8: Consideration of the advantages and disadvantages of nuclear fusion illustrates the economic and environmental implications of using science and technology. The use of fusion reactions in the hydrogen bomb can also be discussed.