[Natsu]

It's always confused me how mined lead somehow has more radioactivity. Shouldn't the lead in the ground also have decayed over time?

Also, it makes me wonder why someone enterprising hasn't stockpiled a few tons of the stuff somewhere to let it become low background lead for the future. You'd think that some government or another would be able to drop a million dollars on putting a lead stockpile somewhere safe for the future.

[toast0]

> It's always confused me how mined lead somehow has more radioactivity. Shouldn't the lead in the ground also have decayed over time?

Looking into this a bit, it seems that the radiation in refined lead isn't coming from the lead ore, but from the other materials used in the smelting process. Old lead would have had time for all those things to decay.

[oh_sigh]

I wonder why manufacturers don't just use filtered atmospheric air in the process to create low-background steel, if it is infact valuable.

[chrisbrandow]

Extremely difficult to separate different molecules that differ by only the weight of a single neutron.

[ars]

They are using Oxygen, Oxygen can't become radioactive for longer than a few seconds anyway.

All they need to do is filter for pure oxygen, nitrogen is fine as well.

It's really just CO2 that's the problem.

[awalton]

So this is all kinds of wrong. When you make steel, you typically use 99+% pure oxygen - modern mills do air separation and reject nitrogen and argon (which makes steel brittle when it's dissolved in) and basically anything they can reasonably separate out but oxygen.

But furthermore, it's not radioactive oxygen isotopes that get into the steel in the first place - they're scant to non-existent in nature, since all three common isotopes of oxygen are stable and most of the rest decay in seconds. It's other radioactive isotopes in the air from the bomb tests.

99+% isn't 100%, and it turns out those tiny fractions of a percent of junk contain the isotopes that are the real problem, namely Cobalt-60 created by the nuclear tests. Carbon-14 isn't nearly as big of a problem, since its decay mode is just beta and can be designed around, but the gamma decay from Cobalt-60 contamination is much harder to deal with.

Furthermore, because of Cobalt's position on the periodic table and the desire to have a small amount of cobalt in steel anyways to give it better working properties, it's not something that's easily filtered out, even in processes that reform steel like vacuum remelting which exist to make mechanically harder and better quality steel by slow melting and recrystalization. Once the Cobalt's in there, it's in there - you just have to wait for it to decay.

As it turns out, we're in luck, most of the fallout from those bomb tests has passed through numerous half-lives and is much less of a problem today than it was in the 1980s and 1990s when the low background stuff became such a hot commodity. So it doesn't really matter as much that we're running out. Furthermore, oxygen separation technologies and cryogenic liquid handling have improved, so we can do an even better job keeping contamination out. If someone wanted to set up a low background mill, they probably could do it today with commodity molecular sieves and centrifugation of the oxygen rejecting all but the light fraction...

[Natsu]

How is the cobalt getting mixed with the stuff mined from the ground? I guess that's the part that confuses me.

Also, what's in the lead? I didn't think we wanted CO2 in lead but I'm really not sure what impurities it might have naturally.

[catalogia]

I'm not sure about this; smelting lead is generally done in a blast furnace, fed with coke. Burning that coke is going to produce a ton of CO2. Is that good enough because the carbon in the coke is all very old? (Searching for information relating to C-14 and coal lead me to a bunch of wacko new earth creationist websites claiming that carbon dating indicates coal is young...)

Also, maybe this isn't really a concern, but "high temperature and pure oxygen" makes me think "metal fire."

[saagarjha]

Not valuable enough to bother.

[nix23]

So salvage it from sunken ships is cheaper then?

[DonHopkins]

Just cheaper for whoever salvages the ship -- somebody else had to pay for the sunken cost of building and sinking the ship.

[rrrrrrrrrrrryan]

> sunken cost

Nice.

[nix23]

> somebody else had to pay for the sunken cost of building and sinking the ship

Na remember it was war, so all the money went back to the economy :/

[awalton]

Yes. The demand for low background steel and lead is in the few dozen tons per year max, not millions of tons per year. Salvaging it is probably at least an order of magnitude cheaper than making new low background steel once you realize the amount of hassle involved. Furthermore, it's fairly easily recycled if you're not launching it into space (which is a common use case for the stuff, since building space probes to study the cosmic background is all the rage these days).

[garmaine]

How do you suggest filtering by isotope?

[gruturo]

As others said, simply separating the CO2 away would mostly get rid of the problem - all non-stable isotopes of Oxygen have a max half life of 122 seconds, but, for the record, isotopic separation is easy when there is a very large difference in atomic weight.

In Uranium's case it's difficult because it's 235/238 = 1.28% (actually way worse - Uranium hexafluoride is used, which adds 114 units, bringing the ratio down to 0.86%)

In Oxygen's case the ratio would be at least 6.6% (15 vs 16).

Most importantly in Uranium you are interested in the tiny amount of U235, while in Oxygen you'd be interested in the huge bulk of O16, O17, O18, which are the stable isotopes. O16 alone is 99.762% of all Oxygen, and you can afford to lose half in your centrifuge if it spares you a few cycles, it's not exactly hard to come by.

[awalton]

Great. None of these do anything for the real problem: Cobalt-60.

[ars]

Just filter for oxygen. Radioactive oxygen doesn't last more than a few seconds.

The issue is contamination with Carbon (CO2). Nitrogen also is not a problem.

[TimSchumann]

I mean, there are ways to do this. None of which are remotely cost effective.

[number6]

Centrifuges

[renewiltord]

This stuff is useful, but it isn't that useful that you're going to sift and build an entire toolchain.

[mcculley]

What would be the timeframe over which one would make a profit? Our global financial system is based on the 30 year U.S. Treasury bond. There are no economic incentives to plan beyond 30 years.

[Natsu]

Probably a few hundred years, so yeah, it wouldn't be worth it to the people doing it at all within their lifetime. That's why I was suspecting a far-sighted government or such with enough money to squirrel it away for future use.

The US sorta has a few things like that already--the oil reserve & the stockpile of helium, though I understand the latter is winding down still. Given the importance of those materials to science, I would think that there might be some scientifically-motivated project to protect our access to such things.

[lukas099]

This is interesting. Can you point me to anything that will expand on/help me understand this?

[mcculley]

I don't know of any good sources. But my understanding is that there are very few "Methuselah bonds" by which it is meant instruments that have lifespans of 50 years or more. So anybody with a profit motive has no advantage for putting money into something that would have a longer term focus. This explains to me why, for example, banks are still loaning money for buildings on Miami Beach. I would love to understand the dynamics better. It seems to me that only governments can be planning for longer terms. China, maybe?

[missedthecue]

Dykes are cheap.

[mcculley]

I'm assuming you mean "dike" and not "dyke".

Dikes will not protect property in Florida. Our buildings are sitting on top of porous limestone.

[missedthecue]

oof yes. Dyke is not what i meant.

But florida already has a dike system. Hundreds of miles of them and they work quite well.

[sandworm101]

Forget filters. It would have to be done in a vacuum and you would need to centrifuge the required gasses/carbon. It is possible in small batches, but the sunken ships do exist ready to harvest. Maybe in the future the market will come.