Physics
Nuclear Fission
One neutron splits one nucleus, and two or three more fly out. Each either splits the next nucleus, gets swallowed, or leaks out the side. Whether that count lands above or below one is the whole difference between a lump of metal, a power station and a bomb.
Every neutron on screen really walks until it splits a nucleus, is captured or escapes; k is counted from those events, not assumed
Multiplication k—
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Scenario
watch one neutronwatch the chain
naturalweapons
2 cm90 cm
baretamper
outin
Numbers
Multiplication k—
Generation—
Free neutrons—
Fissions—
Core mass (sphere)—
State—
A lump of natural uranium
What to observe
- In the inert lump, follow one neutron. Orange: it split a nucleus and two or three new ones burst out. Grey: it was swallowed. Blue at the rim: it leaked. k is just the score, and here it stays near zero.
- More enrichment gives a neutron more targets, a bigger core gives it less edge to leak through. A bare sphere of weapons-grade metal goes just past k = 1 at a radius of 8.7 cm, about 52 kg: the real critical mass.
- Chernobyl is the power reactor with its rods pulled out and auto control off. k sits only a little above one, yet the population graph bends and goes vertical, because the growth compounds every generation. The card below tells the story.
- Hiroshima: compact, reflected weapons-grade metal with k near two, so the population doubles every generation and the core detonates. Reactor fuel is too dilute for that: it boils and bursts instead.
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