Fission chain reaction and fusion sim
Drag the control rods of a cut-away reactor core model and run the chain reaction one generation per click, watching U-235 nuclei split and graphing your own run, until the chain is steady; then turn the heating dial of a cut-away tokamak model and fire hydrogen-2 and hydrogen-3 nuclei together to see why fusion needs a very high temperature; balance the fission and fusion equations.
Radioactivity: nuclear fission, the chain reaction and nuclear fusion · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Model: each fission of uranium-235 gives out 2 or 3 neutrons (2.5 on average); half are lost, and the control rods absorb up to half of the rest, in proportion to how far in they are, so the chain is steady (one fission causes one more, k = 1) at one rod setting, grows above it and dies below it. The page follows the average number of fissions per generation, rounded, one generation per click. Fusion: ²H + ³H → ⁴He + n; a pair fuses if it meets fast enough, with a simple spread of energies. About 200 MeV per fission and 17.6 MeV per fusion, from memory. Nothing runs on its own.
- The part you move: the control rods' crosshead (fission), the heating dial (fusion)
- Teaching models: a cut-away reactor core with its coolant loop, and a cut-away tokamak
Use it in your lessons
- Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
- Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
- Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.
Free guide: 12 practicals as sims, and how to run one in a lesson →
Need a version for your exact set-up? Have one made: US$49 a figure in an order of 5.
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