Physics 001.002.018 Nucleon-Induced Nuclear Fission Reaction

Alignment

Learning Intentions

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

• Explain neutron-induced nuclear fission as a reaction where a neutron is absorbed by a heavy nucleus, making it unstable.
• Represent neutron-induced fission using nuclear equations.
• Identify that many fission reactions produce extra neutrons.
• Explain why these extra neutrons are important for further fission reactions.

Success Criteria

By the end of the lesson, students have successfully:

• Described the sequence of events in neutron-induced fission.
• Balanced nuclear fission equations using conservation of mass number and atomic number.
• Identified the parent nucleus, incoming neutron, fission fragments and extra neutrons in a fission equation.
• Explained that extra neutrons can induce further fission reactions if absorbed by other fissile nuclei.

Syllabus Reference

• Unit 1: Thermal, Nuclear and Electrical Physics
• Topic 2: Ionising Radiation and Nuclear Reactions
• Energy and Mass Defect
• Explain a neutron-induced nuclear fission reaction, including references to extra neutrons produced from many of these reactions.

Phenomenon

A single neutron enters the nucleus of a uranium-235 atom. The uranium nucleus becomes unstable, splits into two smaller nuclei, releases energy, and produces additional neutrons.

This is the process used in nuclear fission reactors. A very small amount of nuclear fuel can release a large amount of energy because the nucleus itself is being rearranged.

Key Idea

A neutron-induced fission reaction occurs when a neutron is absorbed by a heavy, unstable or fissile nucleus. The nucleus becomes highly excited and splits into two smaller nuclei, releasing energy and often producing extra neutrons.

These extra neutrons are important because they can be absorbed by other fissile nuclei, causing further fission reactions.

Concept

In neutron-induced nuclear fission:

1. A neutron approaches a heavy nucleus such as uranium-235.
2. The neutron is absorbed by the nucleus.
3. The nucleus becomes an excited compound nucleus.
4. The excited nucleus becomes unstable and splits into two smaller nuclei.
5. Energy is released.
6. Extra neutrons are produced in many fission reactions.

A common school-level example is:

The star symbol means the nucleus is in an excited, unstable state.

The excited nucleus may then split:

Combining these gives:

This equation shows that one incoming neutron can produce three outgoing neutrons.

Convention

The key conventions associated with the concept and in the branch of established knowledge is below.

• Nuclear reactions must conserve mass number, .
• Nuclear reactions must conserve atomic number, .
• A neutron is written as .
• Uranium-235 is written as .
• The top number is the mass number, , which counts protons and neutrons.
• The bottom number is the atomic number, , which counts protons.
• Fission fragments are the smaller nuclei produced when the heavy nucleus splits.
• The products of fission can vary; uranium-235 does not always split into the same pair of smaller nuclei.
• Many fission reactions release extra neutrons, commonly two or three in school-level examples.

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.

• Students may think the incoming neutron simply “breaks” the nucleus by impact. More accurately, the neutron is absorbed, forming an unstable excited nucleus.
• Students may think uranium-235 always splits into barium and krypton. In reality, several different pairs of fission fragments are possible.
• Students may think the extra neutrons are optional particles added to balance the equation. They are real products of many fission reactions.
• Students may think atomic number does not need to be conserved because new elements are formed. Atomic number is still conserved across the whole reaction.
• Students may confuse neutron-induced fission with natural radioactive decay. Fission requires an incoming neutron, while natural decay occurs spontaneously.

Further Reading

• Nuclear reactors use controlled neutron-induced fission.
• Nuclear weapons involve uncontrolled rapid fission chain reactions.
• Fission products are often radioactive and may decay further.
• The energy released comes from a decrease in mass, described by mass-energy equivalence.

Explicit Instruction

Neutron-induced fission begins with a neutron and a fissile nucleus.

A fissile nucleus is a nucleus that can undergo fission after absorbing a neutron. Uranium-235 and plutonium-239 are common examples.

The general structure is:

For uranium-235:

The uranium-236 nucleus is unstable because it has absorbed energy and an extra neutron. It then splits into smaller nuclei.

Example:

Check conservation of mass number:

Check conservation of atomic number:

The reaction is balanced.

The important feature is that the reaction starts with one neutron but produces three neutrons. These extra neutrons may go on to induce fission in other uranium-235 nuclei.

This leads naturally to the idea of a fission chain reaction, which is the next topic.

Worked Examples

Worked Example 1

Balance the following neutron-induced fission equation:

Step 1: Balance mass number.

Step 2: Check atomic number.

Therefore:

This reaction produces three extra neutrons.

Worked Example 2

Explain what happens in this reaction:

Answer:

A uranium-235 nucleus absorbs an incoming neutron. This forms an unstable excited nucleus, which splits into barium-144 and krypton-89. The reaction also releases three neutrons and energy.

The incoming neutron caused the fission reaction. The three outgoing neutrons are important because they may be absorbed by other uranium-235 nuclei and cause further fission reactions.

Check mass number:

Check atomic number:

The equation is balanced.

Worked Example 3

A neutron-induced fission reaction produces two smaller nuclei and two extra neutrons:

Explain why the products are reasonable.

Step 1: Check mass number.

Step 2: Check atomic number.

Step 3: Interpret the reaction.

The uranium-235 nucleus absorbed a neutron and split into xenon-140 and strontium-94. Two extra neutrons were produced. These neutrons may continue on to induce further fission reactions.

Check for Understanding

Check 1

In the reaction below, identify:

• the incoming neutron
• the fissile nucleus
• the fission fragments
• the extra neutrons

Expected response:

• Incoming neutron: on the left-hand side
• Fissile nucleus:
• Fission fragments: and
• Extra neutrons: on the right-hand side

Check 2

Complete the sentence:

A neutron-induced fission reaction begins when a heavy nucleus __________ a neutron.

Expected response:

A neutron-induced fission reaction begins when a heavy nucleus absorbs a neutron.

Check 3

Explain why the following equation is not balanced:

Expected response:

The mass number on the left is:

The mass number on the right is:

The mass numbers are not equal, so the equation is not balanced. It should have three neutrons, not two neutrons.

Investigation (Alternative to Explicit)

Hypothesis

If a neutron is absorbed by a fissile nucleus, then the nucleus can split into two smaller nuclei and release extra neutrons because the absorbed neutron forms an unstable excited nucleus.

Data Collection

Students use a simulation, bead model, dice model, or card-sort model of neutron-induced fission.

Suggested model:

• One large card represents uranium-235.
• One small card represents an incoming neutron.
• Two medium cards represent fission fragments.
• Two or three small cards represent extra neutrons.
• Students arrange the cards into a balanced nuclear equation.

Students record:

• incoming particle
• original nucleus
• fission fragments
• number of extra neutrons
• total mass number before and after
• total atomic number before and after

Analysis

Students answer:

1. Was mass number conserved?
2. Was atomic number conserved?
3. How many extra neutrons were produced?
4. Why are the extra neutrons important?
5. How is neutron-induced fission different from natural radioactive decay?

Students should identify that fission is not just a spontaneous emission from a nucleus. It is induced when a neutron is absorbed.

Evaluation

Students evaluate the model by answering:

• What part of the model represents the incoming neutron?
• What part of the model represents the excited unstable nucleus?
• What part of the model represents the fission fragments?
• What limitation does the model have?
• Does the model show the energy released? If not, how could this be improved?

Possible limitation:

The model shows conservation of nucleon number and proton number, but it does not show the actual nuclear forces, mass defect, or kinetic energy of the fission fragments.

Problems

The following problems are designed to practise explaining neutron-induced fission and balancing nuclear equations.

1. Explain what is meant by neutron-induced nuclear fission.

2. In the reaction below, identify the fission fragments:

3. Balance the equation:

4. Balance the equation:

5. Explain why the extra neutrons produced in many fission reactions are significant.

6. A student says, “The neutron hits the nucleus and smashes it apart like a hammer.” Explain why this is an incomplete explanation.

7. A fission reaction begins with one neutron and produces three neutrons. Explain how the number of neutrons has changed.

8. Explain why neutron-induced fission is an artificial transmutation rather than natural radioactive decay.

9. Complete the missing nucleus:

10. Challenge: Explain how neutron-induced fission provides the basis for a fission chain reaction, without fully describing the conditions needed for a sustained chain reaction.

Followup

Self-check

Students should be able to answer:

• Can I explain why a neutron is needed to begin neutron-induced fission?
• Can I describe what happens after uranium-235 absorbs a neutron?
• Can I identify the fission fragments in a nuclear equation?
• Can I balance mass number and atomic number in a fission equation?
• Can I explain why extra neutrons are produced in many fission reactions?
• Can I explain why these extra neutrons matter?

Next Topic

The next topic is:

Explain a fission chain reaction.

This lesson will extend the idea that extra neutrons from one fission reaction can cause further fission reactions in nearby fissile nuclei.