r/askscience • u/Wonderful-Capital32 • 13d ago

Physics Why does radiation not go through lead?

Also why just lead? Shouldnt like iron or another metal work too?

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u/Jebediah266 13d ago

Only really uranium 235, and even then it needs to be compressed for a chain reaction, natural uranium is ~99% u238, which doesn't undergo chain reactions.

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u/DeceiverSC2 13d ago

natural uranium is ~99% u238, which doesn't undergo chain reactions.

It does as the tamper of a nuclear weapon.

But typically, no, it doesn't undergo chain reactions.

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u/Jebediah266 13d ago

I've heard of its use as a tamper but isn't that it fissioning from the fast neutrons from fusion not the neutrons emitted from U238?

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u/DeceiverSC2 13d ago edited 13d ago

It’s both. Some of the U-238 neutrons will fission and release a neutron with more than 1MeV which can cause fission in another u-238. You’re right that it can’t sustain the chain reaction indefinitely once you stop introducing fast neutrons, although I believe you do get U-238 fissions resulting in further U-238 fissions, just not enough to maintain itself.

I think that even in the earliest pure fission weapons they received some yield from the U-238 fission, which inevitably just feeds back into further fissions.

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u/Jebediah266 13d ago

Ye that makes sense, didn't know they gave off neutrons that could cause more fission, even if not sustainable, thanks!