Physics 11–12 · Year 12

Modelling a fission chain reaction with dominoes, mousetraps and a simulation

Module 8: From the Universe to the Atom (Properties of the Nucleus)

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Each fission releases neutrons that can trigger further fissions, so the number of events per generation multiplies unless enough neutrons are absorbed or escape.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

mousetraps snapping on fingers
balls flying out

Controls

teacher sets traps with a setting tool, clear-walled box with a lid, safety glasses

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

What you need

Dominoes set up in a branching pattern (one topples two, each topples two more), stopwatch
Teacher demonstration: 20 to 50 set mousetraps in a clear-walled box, two table-tennis balls loaded on each, one ball dropped in
PhET Nuclear Fission simulation for the neutron-triggered version

How to do it

Set the dominoes as a branching tree; topple the first and count how many fall in each generation.
Set them in a single line for comparison: one topples one (a controlled reaction).
Remove some dominoes (absorbers) from branches and find how many removals stop the reaction spreading.
Teacher: drop one ball into the mousetrap box; film in slow motion and count the balls in the air in successive frames.
Simulation: fire one neutron into a lump of uranium-235 and vary the size and the control-rod setting until the reaction just sustains itself.

What you should see

With two per generation the counts run 1, 2, 4, 8, 16, 32; with three per generation 1, 3, 9, 27, 81, 243, 729 by the seventh generation. Removing enough branches so that fewer than one domino per domino falls stops the spread within a few generations. In the mousetrap box one ball sets off traps whose balls set off more, so the slow-motion film shows the number of balls in the air growing each generation until the traps run out.

What changes

What you change: number of successors per event and the fraction absorbed
What you measure: events per generation and whether the reaction sustains
What you keep the same: same domino spacing
same starting event

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

A chain reaction always becomes an explosion; with k held at 1 it runs steadily, as in a reactor.
Control rods slow the neutrons; they absorb them, and the moderator is what slows them.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH12-15 PH11/12-6 PH11/12-7
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-04 PY-12WS-06
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities