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Natural uranium (NU) is a mixture of about 0.7% $\ ^{235}U$ (fissile) and 99.3% $\ ^{238}U$ (non-fissile). To create uranium-based nuclear weapons NU has to undergo a process called enrichment in order to increase the ratio of fissile uranium.

Today the ratio of fissile uranium in NU is about 0.7%. However, $\ ^{235}U$ decays faster than the non-fissile $\ ^{238}U$:

$\ ^{235}U$ has a half-life of $7.04\cdot10^8$ years.
$\ ^{238}U$ has a half-life of $4.46\cdot10^9$ years.

and perhaps a few billion years ago natural uranium was already weapons-usable without the need for enrichment.


Questions:

  • Is my assumption correct? Would it be possible to have weapons-grade NU in the past? Say 4.5 billion years ago, when our planet was created.

  • Could nuclear terrorism be a threat to the existence of extraterrestrial civilizations that form on a planet where NU is already weapons-usable without need for enrichment?

Note: I'm asking because I'm wondering if it's possible that some extraterrestrial civs have evolved in planets with enriched NU. This question is not about humans; our past is history.

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    $\begingroup$ For the purposes of this question, what's the difference between "terrorism" and the actions of something like a state military actor? I'm serious. The word "terrorism" has, perhaps over the last 20 years or so in particular, been used and abused to such an extent that I have yet to come across a single coherent, consistently applied definition. If you want terrorism to be a central point to what you're asking about, then please tell us which definition of "terrorism" you prefer. $\endgroup$ – a CVn Jan 30 '18 at 20:02
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    $\begingroup$ Little known fact*, this is actually what wiped out the dinosaurs. Herd of sauropods was walking through an area with a bunch of uranium ore near the surface. Stepped in the wrong place, and boom, it goes critical. * Not actually a fact. $\endgroup$ – AndyD273 Jan 30 '18 at 20:02
  • $\begingroup$ @MichaelKjörling I agree, it has been abused a lot. For the purpose of this question we could define "terrorism" as "massive violence from non-state groups". $\endgroup$ – Fermi paradox Jan 30 '18 at 20:05
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    $\begingroup$ Well, that definition brings the obvious follow-up, then; what's a "state group"? You are asking about a time at which even single-cell life didn't exist on Earth, possibly about a planet that's not Earth, yet you're using terms that have only had their meanings established in recent time frames in a human, let alone geological, perspective. Take an example of a nomadic group of proto-humans on the African savannah; what's a state group to them? To a group of elephants? Penguins in present-day Antarctica? A village on the outskirts of the Roman empire around 100 AD? Mesopotamia 15k years ago? $\endgroup$ – a CVn Jan 30 '18 at 20:16
  • $\begingroup$ @MichaelKjörling Not sure why you bring up proto-humans, penguins and elephants. They are obviously not capable of creating nuclear weapons. Yes, the question does not refer to earth. I'm asking about similar planets that for whatever reason had intelligent life capable of using U for weapons and had HEU NU(if that's even possible given how long it took for life on earth to evolve). $\endgroup$ – Fermi paradox Jan 30 '18 at 20:50
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TL;DR

Probably yes, natural uranium (NU) is usable in weapons in "young" [note 1] solar systems. (not weapons-grade, but weapons-usable)
Probably yes, terrorists would have a much easier time creating (crude) nuclear weapons. Whether not needing enrichment would hinder or help civilizations prosper and survive is uncertain since there are both problems (terrorism, nuclear proliferation) and benefits (cheaper nuclear energy).


NU enrichment levels

2 billion years ago NU was enriched enough to sustain a nuclear chain reaction (Oklo cave was probably a natural nuclear reactor)

4.5 billion years ago natural uranium was already enriched enough (~23%) to be used in weapons without the need for further enrichment (although enrichment would still be beneficial).

Using the code at the bottom we get the following results:

2 billion years ago natural uranium was 3.7% enriched.   
4 billion years ago, 16.7%.    
5 billion years ago (about the age of our solar system) 31%.   
6 billion years ago, 51%.   

On the other hand we are not quite certain how life on earth formed and perhaps there is a time threshold on how fast life can form in a freshly created solar system, eg. requiring at least 1 billion years for multicellular organisms to form etc.

By going into an even earlier period higher enrichments are possible but probably unlikely, since $\ ^{235}U$ and $\ ^{238}U$ are created through the r-process and their abundance when created is probably not that different (about 1.5 $\frac{\ ^{235}U}{\ ^{238}U}$ = 60% enrichement in supernovas).

By comparison U used in nuclear reactors is about 3-5%, meaning that both state-owned nuclear weapons and nuclear reactors are widespread in planets that are "young".

So, yes. Natural uranium was weapons-usable and perhaps is weapons-usable in other "young" solar systems as we speak. Is the relative age the only way to have enriched NU? If uranium can be transfered to a planet by asteroids or comets just like water on Earth, then "young" age is not the only way to have planets with enriched NU.

Nuclear terrorism

When it comes to terrorists (today):

once they have [the nuclear material], 80% or more of the way [to a crude nuclear bomb]

Also,

once enriched from 0.7% U-235 to 4.5%, ~3/4 of the work of going to 90% U-235 is done

having a stock of LEU could allow a country to enrich to HEU more quickly, or with a smaller, easier-to-hide facility

Meaning that the greatest obstacle is U enrichment. With that out of the way, everything becomes much easier! Both nuclear proliferation and terrorism would be increased.

Perhaps that's an extra great filter for civilizations on young solar systems. Once they've solved the terrorism/wars issue naturally enriched U is a blessing instead. Perhaps one day we'll be able to detect nuclear detonations from distant solar systems and we'll find out.

Note 1: "young" compared to when the nearby supernova exploded creating its elements (including the U).
Note 2: I answered my own question so that you can check it. If you find any mistakes, let me know.
Edit: meant to use "weapons-usable" not "weapons-grade", thanks @kingledion


Python code used for the above results:

SECONDS_PER_YEAR = 365*24*60*60
BILLION_YEARS_TO_SECONDS = SECONDS_PER_YEAR * 1e9


U235_halflife = 7.04e8 * SECONDS_PER_YEAR   # http://www.nndc.bnl.gov/nudat2/reCenter.jsp?z=92&n=143
U238_halflife = 4.468e9 * SECONDS_PER_YEAR  # http://www.nndc.bnl.gov/nudat2/reCenter.jsp?z=92&n=146

U235_TO_U238_RATIO = 0.720 / 100   # https://physics.nist.gov/PhysRefData/Handbook/Tables/uraniumtable1.htm

# --------------------------------
#      CHANGE THIS VALUE:
bil_years = 5
#
# --------------------------------


u235_halflives = BILLION_YEARS_TO_SECONDS * bil_years / U235_halflife
u238_halflives = BILLION_YEARS_TO_SECONDS * bil_years / U238_halflife

# If there are 0.0072 parts of U-235 for every 1 part of natural U
# then we calculate the "initial" quantity (a few billion years ago)
quantity_235 = U235_TO_U238_RATIO * 2 ** u235_halflives
quantity_238 = (1-U235_TO_U238_RATIO) * 2 ** u238_halflives
ratio_235_238 = quantity_235 / quantity_238

print('Billion years:  {}\n'.format(bil_years))
print('235 quantity:   {:.3}'.format(quantity_235))
print('238 quantity:   {:.3}'.format(quantity_238))
print('ratio_235_238:  {:.3}'.format(ratio_235_238))

enrichment = quantity_235 / (quantity_235 + quantity_238)
print('enrichment:     {:.1%}'.format(enrichment))
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    $\begingroup$ If I am not mistaken, 5 billion years ago the earth did not exist yet. According to Wikipedia the earth did not have a magnetic field up until 3.5 billion years ago, which is the least thing required to sustain any kind of life (that could build weapons). $\endgroup$ – YSelf Jan 30 '18 at 19:04
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    $\begingroup$ @YSelf It's still a myth that life needs a magnetic field. Earth's field changes polarisation every now and then, with a few hundred years of very low magnetic field, and this has never been accompanied by a mass extinction. $\endgroup$ – Karl Jan 30 '18 at 19:16
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    $\begingroup$ Good answer, but if you wanted to generate discussion, you probably should have just posted the question, and let it percolate for a few days before throwing your own answer on. $\endgroup$ – kingledion Jan 30 '18 at 19:22
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    $\begingroup$ @YSelf magnetic field != atmosphere. The field helps protect the atmosphere from the solar wind which probably helps planets retain atmosphere but the main force retaining the atmosphere is still gravity. Venus is smaller and hotter with much weaker magnetic field and still has thicker atmosphere than Earth has. Maybe it didn't have its volatiles blasted to space by a huge collision? $\endgroup$ – Ville Niemi Jan 30 '18 at 20:11
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    $\begingroup$ Great answer Enrico! but a quick ELI5 question: You are assuming that none of the U is the result of decay from other elements. Is this a reasonable assumption? $\endgroup$ – smatterer Jan 31 '18 at 0:50
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No

Weapons grade Uranium-235 is enriched ~90%.

Generally, enriched uranium for research is kept below 20% to prevent weapons proliferation. Commercial reactors will use maybe 5% enriched uranium. However, even at 20% enrichment, the critical mass of U is about 400 kg.

From the World Nuclear Association, Uranium must be enriched to at least 90% in specialized facilities to be used for bomb-making. Most of Little Boy was enriched to 89%, while the average enrichment was 80%. These are the enrichment levels you will need to make a bomb.

Finally, this paper provides evidence that 50% enriched Uranium is not a significant danger for weapons proliferation.

Conclusion

Even with 31% enriched Uranium 5 billion years ago, any terrorist would still need access to an enrichment facility or a breeder reactor (to create Plutonium). In other words, while the work that needs to be done to create a bomb is lower in this 'young' world, the facilities needed do not change, so being a nuclear terrorist is no easier.

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    $\begingroup$ 400kg U is about 20 liters, or 20 boxes of milk. You are right about "weapons-grade U" though, I used the wrong term; meant to say usable for weapons. $\endgroup$ – Fermi paradox Jan 30 '18 at 19:49
  • $\begingroup$ 1."evidence" is quite a strong word. Can you quote the relevant part of the paper? 2. correct me if I'm wrong but the paper states that 20% U-235 is a good threshold, and argues that current plans for using HEU in research reactors is a bad idea since it will increase proliferation risks ("this trend cannot be justified with technical arguments as it clearly reduces the proliferation resistance of the nuclear fuel cycle"). Nowhere does it state that 50% HEU is not a significant danger for weapons proliferation. $\endgroup$ – Fermi paradox Jan 31 '18 at 6:58
  • $\begingroup$ 3. "Uranium must be enriched to at least 90% in specialized facilities to be used for bomb-making" - No, that's not what it says. It says it "would have to be enriched", which doesn't mean that lower enrichment U can't be used. They (probably) prefer WGU because it is more efficient when it comes to yield, reliability and delivery. $\endgroup$ – Fermi paradox Jan 31 '18 at 7:05
  • $\begingroup$ 4. You didn't address nuclear proliferation in question-2. Also, I will edit the question to make it clear that I mean "weapons-usable", but will retain the term "weapons-grade" in question-1 to keep your answer relevant. If you think i should revert the edit, or edit differently, let me know. $\endgroup$ – Fermi paradox Jan 31 '18 at 7:24
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Also consider Plutonium-244

With a half life of 80 million years, you will have to evolve quickly indeed, but it could probably be made into nuclear bomb material without any isotope separation, just chemical separation (I'm unable to get information on its fissile properties).

So what could have happened is that in your solar system, there was a supernova 50 million years ago nearby, which sent some massively-Plutonium and Uranium enriched asteroids into your system..

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  • $\begingroup$ Hmm sounds quite interesting, I haven't consider Pu in young solar systems. You might be right, I'll check later on if that would be possible or there are obstacles (some plutonium isotopes can make the material unusable) $\endgroup$ – Fermi paradox Jan 31 '18 at 16:53
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A 50% U-235(straight from supernova) can be made into a bomb, as shown in this answer.

But 4by ago it had about 16.7%(number stolen from Fermi_paradox answer). Critical mass for 20% is 800 kg, 245 kg with good Be reflector!

So I guess you could make a bomb out of 16.7% U with big reflector, but device would be very heavy and not as reliable.

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    $\begingroup$ No. The mechanics of making this bomb are much more complex than you are admitting to. There is too much U-238, which means too much resonance absorption. As temperatures increased, doppler broadening would decrease the fission cross section of U-235 while increasing the capture energies of the U-238. You would have a very incomplete chain reaction and a fizzled bomb (hi, North Korea). This and your linked answer are both wrong. $\endgroup$ – kingledion Jan 31 '18 at 1:35
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    $\begingroup$ @kingledion Interesting, I will look into it. Could you point to the sources? Your link at princeton.edu/~aglaser/2005aglaser_why20percent.pdf sets lower limit at 20% "Uranium fuel below 20% virtually eliminates the possibility" $\endgroup$ – Vashu Jan 31 '18 at 3:07

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