3D HKDSE · Radioactivity & Nuclear Energy

Nuclear Fission Chain Reaction & Reactor

Fire a neutron into uranium-235, tune density, moderator and control rods, and watch subcritical, critical or supercritical chain reactions in a reactor cross-section.

Controls

U-235 density80

Moderator70%

Control rod insertion40%

Anim. speed1.5×

Show neutron trails

Presets

Focus

Watch how inserting control rods or increasing the moderator changes the multiplication factor k and the number of neutrons in each generation.

Live data

Reactor state Idle

Subcritical (k < 1)

Critical (k ≈ 1)

Supercritical (k > 1)

Est. multiplication k —

Free neutrons0

Intact U-23580

Fissions0

Energy released0 MeV

Generation0

Key ideas

\(^{235}\mathrm{U} + n \rightarrow\) fragments \(+ 2\text{–}3\,n + \sim 200\,\mathrm{MeV}\)
Mass defect: \(E = mc^{2}\)
\(k < 1\) subcritical · \(k \approx 1\) critical · \(k > 1\) supercritical

U-235 nucleus Neutron (v) Fission fragment Energy flash Control rod

Criticality k = —

Idle

k < 1

k ≈ 1

k > 1

Abstract overheating warning — energy rising rapidly (supercritical). Not a realistic blast scene.

Drag • Wheel • Pinch

Neutrons per generation & cumulative energy Neutrons · Energy

Physics notes (for teachers & students)

A thermal neutron can be captured by \(^{235}\mathrm{U}\), splitting it into two fission fragments and releasing typically 2–3 neutrons plus about 200 MeV, mainly as kinetic energy of the fragments (mass defect → \(E=mc^{2}\)).

A moderator (e.g. water or graphite) slows fast fission neutrons so they are more likely to cause further fissions. Control rods absorb neutrons and keep the multiplication factor \(k\) near 1 in a power plant.

This model is highly simplified: nuclei and neutrons are not to scale; escape, absorption and fission probabilities are tuned for classroom visibility; the uncontrolled-chain preset only shows abstract supercritical growth — no blast or casualty imagery.