Nuclear Fission and Nuclear Fusion

This lesson explains Nuclear Fission and Nuclear Fusion for Cambridge IGCSE Physics 0625. It covers Supplement content for Extended candidates. Focus on the cause-and-effect explanation and the exact quantities being compared. Read the explanation once, then attempt the worked method from a blank page.

This is Supplement content for Extended candidates.

What is nuclear fission?

Nuclear fission is the splitting of a heavy nucleus into two smaller nuclei.

A typical sequence is:

1. a heavy fissile nucleus absorbs a neutron
2. the nucleus becomes unstable
3. it splits into two smaller nuclei
4. two or three neutrons are released
5. energy is released

The emitted neutrons can be absorbed by other fissile nuclei, causing further fissions.

What is a chain reaction?

A chain reaction occurs when neutrons released in one fission cause additional fissions.

• If too many neutrons escape or are absorbed without causing fission, the reaction decreases.
• If, on average, one neutron from each fission causes another fission, the reaction can remain steady.
• If more than one neutron from each fission causes further fission, the number of reactions increases rapidly.

In a nuclear reactor, control systems regulate the number of neutrons available to continue the chain reaction. A moderator can slow neutrons, while control rods absorb neutrons.

How do you balance a fission equation?

In a balanced nuclear equation:

• the total nucleon number is the same on both sides
• the total proton number is the same on both sides

One possible fission reaction is:

${}^{235}_{92}\text{U} + {}^{1}_{0}\text{n} \rightarrow {}^{141}_{56}\text{Ba} + {}^{92}_{36}\text{Kr} + 3{}^{1}_{0}\text{n}$

Check nucleon numbers:

$235 + 1 = 141 + 92 + 3$

Check proton numbers:

$92 = 56 + 36$

Different fission product pairs are possible. Use conservation of nucleon number and proton number to complete the equation given in the question.

What is nuclear fusion?

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus.

Fusion powers stars. In stars, light nuclei can combine at extremely high temperature and pressure. Energy is released because the products have slightly less total mass than the starting nuclei.

A simple example using hydrogen isotopes is:

${}^{2}_{1}\text{H} + {}^{3}_{1}\text{H} \rightarrow {}^{4}_{2}\text{He} + {}^{1}_{0}\text{n}$

Check nucleon numbers:

$2 + 3 = 4 + 1$

Check proton numbers:

$1 + 1 = 2 + 0$

Why is controlled fusion difficult on Earth?

Positively charged nuclei repel one another. They must move fast enough to come very close before the strong nuclear force can bind them.

This requires extremely high temperatures. The hot material must also be confined long enough and at sufficient density for useful numbers of fusion reactions to occur.

The syllabus requires awareness that research is investigating fusion as a possible large-scale source of electricity. Do not claim that commercial fusion power is already a standard source of grid electricity.

Fission compared with fusion

Nuclear fission	Nuclear fusion
a heavy nucleus splits	light nuclei join
commonly initiated by neutron absorption	requires extremely high temperature and suitable confinement
releases further neutrons	can release a neutron depending on the reaction
can form a chain reaction	powers stars
produces smaller nuclei and energy	produces a heavier nucleus and energy

Both processes conserve nucleon number and proton number in a balanced equation.

Original fission question

A uranium-235 nucleus absorbs a neutron and splits as shown:

${}^{235}_{92}\text{U} + {}^{1}_{0}\text{n} \rightarrow {}^{144}_{56}\text{Ba} + {}^{89}_{36}\text{Kr} + x{}^{1}_{0}\text{n}$

Calculate $x$. [2]

Conserve nucleon number:

$235 + 1 = 144 + 89 + x$

$236 = 233 + x$

$x = 3$

The proton number is already balanced because $56 + 36 = 92$.

Original fusion question

Explain why nuclear fusion can release energy even though nucleon number and proton number are conserved. [2]

A complete answer is:

The total mass of the product particles is slightly smaller than the total mass of the starting nuclei. The decrease in mass is associated with energy released by the reaction.

Original marking guidance

• one mark for the products having a smaller total mass
• one mark for the mass decrease being associated with released energy

Common mistakes

• Reversing the definitions. Fission splits a heavy nucleus; fusion joins light nuclei.
• Balancing only the top numbers. Both nucleon number and proton number must balance.
• Saying fission destroys neutrons. Neutrons may be absorbed or released, but the complete nuclear equation must conserve nucleon number.
• Saying stars use fission. Stars release energy mainly through fusion.
• Calling a nuclear chain reaction a chemical combustion reaction. It is a sequence of nuclear fissions initiated by neutrons.
• Claiming fusion needs no special conditions. Very high temperature and confinement are central difficulties.
• Saying matter simply disappears. A small decrease in mass is associated with released energy.

Exam technique

For every nuclide equation, write two totals below it: nucleon number and proton number. For a comparison question, state the starting nuclei, the product type, the necessary conditions and whether released neutrons can continue the process.

How this is examined

Extended Papers 2 and 4 can test definitions, chain reactions, balanced nuclide equations, qualitative mass-energy changes and the conditions needed for fusion. The topic also links to energy resources and to the Sun and stars. Practical papers do not require a fission or fusion experiment, but they can use nuclear data to assess calculations, graphs or evaluation.