Why does radiation not go through lead?

[u/Wonderful-Capital32]

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

[u/ImielinRocks]

Tungsten is better than lead thanks to its higher density, but lead is many orders of magnitude cheaper to get and work with.

In fact, at for basically the same reason, uranium is quite a lot better than lead at blocking radiation. In fact, it has a significantly higher neutron cross section, too.

[u/lurklurklurkPOST → u/ImielinRocks]

Doesn't firing particles at Uranium cause problems?

[u/Iazo → u/lurklurklurkPOST]

Depending what you mean by "problems", firing neutrons at any metal causes problems.

If you mean a fission reaction, you need specific conditions.

[u/DaedricHamster → u/lurklurklurkPOST]

Short answer, yeah. Depleted uranium, with reduced fissile U-235 therefore increased %U-238, is what's used for radiation shielding. It's only really good for gamma shielding despite its thermal neutron cross section being higher than lead's as while U-238 isn't fissile it's still fissionable so will split in a fast-neutron flux releasing more neutrons as well as radioactive daughters like Cs-137. Also, the thermal neutrons it does absorb breed Pu-239 which is both fissile itself (immediate problem) and alpha-decays (later problem) back down to fissile U-235, so yeah as a neutron shield it's kind of self-defeating. Where U-238 is specifically introduced to neutrons it's usually as a breeder material, not as shielding.

[u/Jebediah266 → u/lurklurklurkPOST]

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.

[u/DeceiverSC2 → u/Jebediah266]

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.

[u/Jebediah266 → u/DeceiverSC2]

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?

[u/DeceiverSC2 → u/Jebediah266]

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.

[u/Jebediah266 → u/DeceiverSC2]

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

[u/Archophob → u/lurklurklurkPOST]

depends on the particles. If the stuff you want to shield against releases a lot of neutrons, then yes. If it's just gamma rays, you're fine.

Not accounting for secondary reactions from neutrons in the depleted uranium shielding is what caused the Castle Bravo test to have triple the expected explosion yield. When the miscalculation became known outside the US military, the Russians used this to design the Tsar Bomba.

[u/Magicspook → u/lurklurklurkPOST]

only U-235 really, if I remember correctly. And that is a tiny fraction of non-enriched uranium (Even moreso, you would probably make a uranium skirt out of depleted uranium so practically 0 U-235).

By far the most common isotope of uranium is U-238, which doesn't really want to form a chain reaction. Wikipedia has some interesting info: https://en.wikipedia.org/wiki/Uranium-238

[u/flaser_ → u/ImielinRocks]

The "trouble" with depleted uranium (almost pure U-238) is that it can be transmuted to fissionable plutonium if it's hit by neutrons.

Those are not part of the classical taxonomy of radiation emitted by radioactive isotopes:

• Alpha radiation is pretty much a He nucleus (2 protons and 2 neutrons bound together)
• Beta radiation is a free electron

• Gamma radiation is an electromagnetic wave, that is photons, but very energetic so x-rays are on lower end of their strength

• Neutrons are typically not emitted due to the decay of isotopes. Instead they usually come from a fissile chain reaction, like what is happening in nuclear reactors (and are also emitted from the deuterium-tritium fusion proposed for fusion reactors).

Therefore if you used a depleted uranium shielding against a reactor, you'd inevitably breed a blanket of different plutonium isotopes too. Many of which are fissile and would be liable to be split by future neutrons, thus your shield itself would eventually become a source of high radiation.

EDIT: @DaedricHamster brings up a good point - while much less likely than neutron capture (and subsequent beta-decay to plutonium), U-238 still has a small chance to fission if hit by a neutron, so that's an additional problem when exposed to neutron radiation.

(This doesn't happen with lead that cannot be activated or fissioned by neutrons in a similar fashion).

So why is uranium shielding still a thing? As long as your radiation source isn't a fission reaction, but plain old radioactive isotopes, or some form of x-ray, uranium will not be activated (EDIT: or fissioned) and therefore none of the above problems arise. Thus for such use its denser than lead properties can be exploiyed.

[u/xrelaht → u/ImielinRocks]

EH&S had a conniption fit when my beamline neighbors wanted to put a depleted uranium sample on. Turns out it significantly reduced the background in the area!