Physics 001.002.016 Nuclear Fission and Nuclear Fusion

Alignment

Learning Intentions

By the end of the lesson, students will be able to:

• Describe nuclear fission as the splitting of a large unstable nucleus into smaller nuclei, usually after neutron absorption.
• Describe nuclear fusion as the joining of small nuclei to form a larger nucleus.
• Represent nuclear fission and nuclear fusion using balanced nuclear equations.
• Identify conservation of nucleon number and proton number in nuclear equations.

Success Criteria

By the end of the lesson, students have successfully:

• Defined nuclear fission and nuclear fusion in their own words.
• Labelled reactants and products in fission and fusion equations.
• Checked that mass number and atomic number are conserved in nuclear equations.
• Explained why both fission and fusion can release energy.
• Compared fission and fusion using at least three features.

Syllabus Reference

• Unit 1: Thermal, Nuclear and Electrical Physics
• Topic 2: Ionising Radiation and Nuclear Reactions
• Energy and Mass Defect
• Describe nuclear fission and nuclear fusion with the aid of nuclear equations.

Phenomenon

The Sun releases enormous amounts of energy through nuclear fusion, while nuclear power stations release energy through nuclear fission. Both processes involve changes to atomic nuclei, not changes to electrons or chemical bonds.

A small amount of mass is converted into energy during these nuclear reactions. This is why nuclear reactions can release far more energy per kilogram than chemical reactions such as burning coal or petrol.

Key Idea

Nuclear fission and nuclear fusion are nuclear reactions that change the nucleus of atoms. They can be represented using balanced nuclear equations.

In any balanced nuclear equation:

• total mass number is conserved
• total atomic number is conserved
• charge is conserved
• energy may be released if the products have less mass than the reactants

Concept

Nuclear fission occurs when a heavy nucleus splits into two or more smaller nuclei. A neutron is often absorbed first, making the nucleus unstable.

Example:

Nuclear fusion occurs when two light nuclei combine to form a heavier nucleus. Fusion requires extremely high temperature and pressure so that nuclei can overcome electrostatic repulsion.

Example:

Convention

Nuclear equations use nuclide notation:

where:

• is the mass number, equal to protons plus neutrons
• is the atomic number, equal to the number of protons
• is the chemical symbol

When balancing nuclear equations:

• add mass numbers across the top
• add atomic numbers across the bottom
• make sure totals are equal on both sides

For example, in the fusion equation:

Mass number check:

Atomic number check:

The equation is balanced.

Misconceptions

Common misconceptions students have regarding the concept when applying to various situations and solving problems. It could be a conceptual, mathematical or logical misconception.

• Fission and fusion are the same because both release nuclear energy.
• Nuclear equations are balanced using ordinary chemical equation rules only.
• Energy release means mass number is not conserved.
• Fusion is easy to start because it releases large amounts of energy.
• Fission always produces the same daughter nuclei.

Further Reading

• QCAA Physics General Senior Syllabus: Unit 1, Topic 2
• Nuclear fission in power generation
• Nuclear fusion in stars
• Binding energy and mass defect
• Mass-energy equivalence using

Explicit Instruction

Nuclear reactions involve the nucleus. They are different from chemical reactions, which involve electrons.

There are two major nuclear reactions used to explain large-scale energy production: fission and fusion.

Fission means splitting.

A large nucleus, such as uranium-235, absorbs a neutron. The uranium nucleus becomes unstable and splits into smaller nuclei. Extra neutrons and energy are released.

A common fission equation is:

Check mass number:

Check atomic number:

Fusion means joining.

Small nuclei, such as hydrogen isotopes, join to form a larger nucleus. Fusion occurs naturally in stars. The Sun produces energy mainly through fusion reactions involving hydrogen nuclei.

A common fusion equation is:

Check mass number:

Check atomic number:

Both fission and fusion can release energy because the total mass of the products is slightly less than the total mass of the reactants. The missing mass has been transformed into energy.

This is described by mass-energy equivalence:

In nuclear reactions, the energy released is related to the mass defect:

Worked Examples

Worked Example 1

Question:

Describe the following reaction as fission or fusion.

Solution:

The reactant is a large uranium nucleus. It absorbs a neutron and splits into two smaller nuclei, barium and krypton.

Therefore, this is nuclear fission.

Check mass number:

Check atomic number:

Final answer:

This reaction is nuclear fission because a heavy nucleus splits into smaller nuclei and releases neutrons and energy.

Worked Example 2

Question:

Describe the following reaction as fission or fusion.

Solution:

The reactants are two small hydrogen nuclei. They combine to form a larger helium nucleus.

Therefore, this is nuclear fusion.

Check mass number:

Check atomic number:

Final answer:

This reaction is nuclear fusion because two light nuclei combine to form a heavier nucleus and release energy.

Worked Example 3

Question:

Complete the missing particle in the nuclear equation.

Solution:

Balance mass number:

Balance atomic number:

Final answer:

The missing number is , so three neutrons are released.

Check for Understanding

Check 1

Classify each reaction as fission or fusion.

a.

b.

Expected answers:

a. Fusion, because two small nuclei combine.

b. Fission, because a large nucleus splits into smaller nuclei.

Check 2

Explain why this equation is balanced.

Expected answer:

Mass number is conserved:

Atomic number is conserved:

Therefore, the equation is balanced.

Check 3

A student says, “Fusion and fission are chemical reactions because they release energy.”

Explain why this is incorrect.

Expected answer:

Fusion and fission are nuclear reactions because they involve changes to the nucleus. Chemical reactions involve rearrangements of electrons and bonds, not changes to the nucleus.

Investigation (Alternative to Explicit)

Hypothesis

If nuclear equations represent real nuclear reactions, then the total mass number and atomic number should be conserved across the reaction.

Data Collection

Students are given a set of nuclear equations, including fission and fusion reactions. For each equation, students record:

• reactant nuclei
• product nuclei
• total mass number on each side
• total atomic number on each side
• whether the equation shows fission or fusion

Example data table:

Reaction	Total before	Total after	Total before	Total after	Fission or fusion
					Fission
					Fusion

Analysis

Students compare the totals before and after each reaction.

They answer:

1. Is mass number conserved?
2. Is atomic number conserved?
3. What features show that a reaction is fission?
4. What features show that a reaction is fusion?
5. Why can energy be released even though mass number is conserved?

Evaluation

Students evaluate the model of nuclear equations by considering:

• nuclear equations show conservation of nucleon number and charge
• nuclear equations do not show the exact mass defect unless masses are supplied
• the same fission reactant can produce different daughter nuclei
• fusion equations show the overall reaction but not the extreme conditions required

Problems

The following problems are designed to build fluency in describing fission and fusion using nuclear equations.

1. Define nuclear fission.

2. Define nuclear fusion.

3. State one similarity between nuclear fission and nuclear fusion.

4. State one difference between nuclear fission and nuclear fusion.

5. Classify the reaction below as fission or fusion.

6. Classify the reaction below as fission or fusion.

7. Check whether the following equation is balanced.

8. Complete the missing mass number.

9. Complete the missing atomic number.

10. A nuclear equation is shown below.

Explain whether this is more closely related to fission or fusion.

11. Explain why high temperature is required for fusion.

12. Explain why extra neutrons released in fission are important for nuclear reactors.

13. Compare nuclear fission and nuclear fusion in a table.

14. Explain why nuclear equations must conserve atomic number.

15. Explain how both fission and fusion can release energy even though they are different processes.

Followup

Self-check

Students should be able to answer the following without notes:

• Can I define fission and fusion?
• Can I identify fission and fusion from a nuclear equation?
• Can I balance mass number and atomic number in a nuclear equation?
• Can I explain why energy is released in nuclear reactions?
• Can I describe one real-world example of fission and one real-world example of fusion?

Next Topic

The next topic is neutron-induced nuclear fission, including the production of extra neutrons and how these neutrons can lead to a fission chain reaction.