Nuclear Fission Chain Reaction
Load a sample of some isotope — adjust the isotope, purity, fuel amount, moderator, and cooling/control rods to see the chain reaction fizzle out, self-sustain, or run away, generation by generation.
Preview — sign up free to change the numbers yourself
What you can adjust
- Isotope
- 0 – 6
- Purity
- 0 – 1
- Fuel amount
- 0 – 1
- Moderator
- 0 – 1
- Cooling / control rods
- 0 – 1
- Starting neutrons
- 1 – 10
0 = U-235, 1 = U-238 (fertile, won't chain-react on thermal neutrons alone), 2 = Pu-239, 3 = U-233, 4 = Pu-241, 5 = Th-232 (fertile), 6 = Boron-10 (a neutron absorber — never reacts).
Fraction of the sample that is the chosen isotope, vs. inert filler. 0 = no active material at all, 1 = pure sample.
How much fuel is loaded (size/density of the assembly). More fuel leaks fewer neutrons out to the surroundings, raising the multiplication factor — and draws more atoms in the visualization.
Neutron-slowing catalyst (e.g. water/graphite) mixed into the assembly. Raising it thermalizes more neutrons, raising the multiplication factor.
Control-rod / neutron-poison insertion. Raising it absorbs more neutrons without fissioning them, lowering the multiplication factor — the fine knob for dialing a runaway setup down to a stable critical reaction.
How many neutrons are fired into the sample to start the chain reaction.
About this simulation
What it shows
A sample of fissile material is hit by neutrons, and the neutron population is followed generation by generation. It dies away, holds steady or runs away, shown with a neutron count and a multiplication factor k.
The key idea
Each fission releases neutrons that can cause more fissions. If on average each neutron causes less than one further fission (k below 1) the reaction fades, at k = 1 it holds steady, and above 1 it grows.
Try this
With Isotope on U-235, raise Cooling / control rods in steps: k falls, and once it drops below 1 the neutron count dies away instead of growing.
The chain reaction is modelled with a single average multiplication factor k for the whole run. It is a teaching model, not a reactor design.