This simulation models a neutron-induced fission chain reaction in a lattice of nuclei. A single neutron is fired into the field; when it strikes a uranium-235 nucleus it may trigger fission, releasing the chosen number of fresh neutrons that go on to find more nuclei. By adjusting enrichment, neutrons per fission and moderation you steer the reaction towards subcritical, critical or supercritical behaviour, tracked by the live multiplication factor k.
Each green dot is a fissile-or-not nucleus on a randomised grid. Neutrons travel ballistically and bounce off the walls. On contact a Monte-Carlo draw decides whether the nucleus is U-235 (probability set by enrichment) and whether it fissions; thermal (slowed) neutrons use an 0.85 fission probability versus 0.55 for fast ones, while non-fissile nuclei mostly just absorb. Fission spawns ν daughter neutrons, and the panel estimates the multiplication factor k from the active neutron population.
Press Start to fire the seed neutron and watch the cascade; Start again pauses, and Reset rebuilds the lattice. The Enrichment slider (1-100%) sets the fraction of nuclei that are fissile U-235, Neutrons/fission (1-4) sets ν released per split, and Moderation (0-100%) sets the chance a neutron is slowed to thermal speed. The Neutrons, Fissions and k readouts update every frame.
Natural uranium is only about 0.72% U-235, which is why reactors enrich it to roughly 3-5% and weapons require far higher levels. A slow thermal neutron is hundreds of times more likely to fission U-235 than a fast one, so moderators such as water or graphite are essential to sustaining a controlled chain reaction.
It is a self-sustaining process in which a neutron splits a heavy nucleus such as uranium-235, releasing energy and two or more new neutrons. Those neutrons can split further nuclei, so one fission can lead to many. The simulation shows this branching as a cascade of moving neutron dots and glowing fissioned nuclei.
k is the average number of further fissions caused by the neutrons from one fission. If k is below 1 the reaction dies out (subcritical), at k roughly 1 it holds steady (critical), and above 1 it grows exponentially (supercritical). The panel shows a live estimate of k derived from the active neutron count.
Enrichment sets the percentage of nuclei that are fissile U-235, so higher values make collisions far more likely to cause fission. Neutrons/fission sets ν, the number of fresh neutrons each split releases. Moderation sets the probability that a neutron is slowed to thermal speed, which sharply raises its chance of causing fission.
The fission cross-section of U-235 is much larger for slow thermal neutrons than for fast ones, so a moderated neutron is far more likely to be captured and cause a split. The model reflects this by using a higher fission probability for slowed neutrons, which is why raising moderation can tip the reaction supercritical.
It is a qualitative teaching model, not a reactor-grade transport code. It captures the right cause-and-effect relationships between enrichment, neutron multiplication, moderation and criticality, but it uses simplified probabilities, two-dimensional ballistic neutrons and a finite lattice rather than real cross-section data, geometry or delayed-neutron timing.