Neutrons do not fly straight — they stumble through matter, bouncing and slowing until they fission, get absorbed, or leak out. That random walk sets a reactor's fate.
Neutron TransportLive
random walk & moderation
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Neutrons do not travel straight — they random-walk through matter, scattering every mean free path and losing energy at each collision. This moderation slows fast neutrons to thermal speeds where they fission efficiently. A short mean free path (dense material) keeps them inside; a long one lets them leak out, which is why reactor size and geometry set criticality.
Reading this result: A moderate mean free path balances moderation against leakage — enough scattering to thermalize neutrons, but geometry still governs whether the core goes critical.
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How it works
A neutron scatters roughly every mean free path, losing energy at each collision. This moderation slows fast neutrons to thermal speeds where they fission efficiently. A short mean free path keeps neutrons inside the fuel; a long one lets them leak away, which is why the size and geometry of a reactor or bomb core determine whether a chain reaction can sustain itself.
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The solver uses established numerical methods, but results are for research and educational purposes and should be validated against experiment or professional review before you rely on them.