Fission Chain Reaction Simulator

Fission Reactor Core

Control rods (absorption) 40%

Fuel density 55

Neutron speed 1.0×

Neutrons live

–

Fissions total

–

k (multiplication)

–

Nuclei intact

–

How it works

Each fissile uranium-235 nucleus struck by a neutron splits into two fragments and releases 2–3 new fast neutrons. Every free neutron then either escapes the core, is absorbed by a control rod, or strikes another nucleus and triggers a new fission. The neutron multiplication factor k is the average number of neutrons from one fission that go on to cause another:

k = neutrons causing new fission / neutrons from previous fission
k < 1  → subcritical, reaction dies out
k = 1  → critical, steady chain reaction
k > 1  → supercritical, exponential growth

Control rods — neutron-absorbing material; raising absorption removes more neutrons per generation, pulling k down toward or below 1.
Fuel density — how tightly packed the fissile nuclei are; denser fuel means a travelling neutron is more likely to hit one before escaping, pushing k up.
Neutron speed — playback speed of neutron flight between nuclei; does not change k itself, only how fast the reaction visibly unfolds.
Fire seed neutron — injects one neutron to start (or restart) the chain reaction from a fully intact core.

Real reactors hold k at exactly 1.000 with control rods for steady power output; a weapon is built to push k well above 1 for an instant, uncontrolled release.

k ≈ –

Intact U-235 nucleus Split fragment Free neutron Control rod

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