Nuclear fission chain reaction

Explore how neutron production, absorption, leakage and control rods determine whether a chain reaction fades, stabilises or grows.

Controlled critical reactorApproximately critical

Fissile nucleusNeutronFission productsControl rod

Active neutrons8

Total fissions0

Estimated kₑff—

Generation0

Absorbed / escaped0 / 0

Schematic energy0 units

Neutron population

Generation multiplication

Neutron-loss breakdown

What fission starts

A neutron can make a fissile nucleus split, releasing energy and more neutrons. Those secondary neutrons may trigger further fissions, be absorbed or leak away.

Reading kₑff

When kₑff is below 1 the chain fades; near 1 it remains steady; above 1 it grows. “Critical” means a steady chain reaction—not an explosion.

Control and shutdown

Control rods absorb neutrons. A SCRAM inserts them rapidly, making the system subcritical. Moderation and delayed neutrons make controlled reactor response possible.

Model limitation: Interaction probabilities are invented dimensionless teaching parameters, not isotope cross-sections. The simulation does not calculate enrichment, critical mass, weapon configurations or real reactor safety.