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If a much older solar system - say 10 billion years with a slow burning star - would sprout intelligent life at around that age, given the half-life of U-235 and natural Uranium reserves, according to math U-238, although twice as hard to find, would still be available. But U-235 would be nearly all decayed and difficult if not impossible to find in even trace amounts.

If such a civilization became aware of the physics required for nuclear bombs - like we did decades before building ours - my understanding is that they would not be able to since they cannot obtain any fissile isotopes, and U-235 is the lowest hanging fruit.

Their easiest path would be to convert the U-238 into Plutonium-239 but without a fissile isotope and chain reaction to begin with, they would have no way to generate neutrons in the amounts needed to create a fissile critical mass.

Would they have to wait until they can build viable fusion reactors and use those as the neutron source with which to produce Plutonium from the Uranium? Or is there another path they can take to synthesize their first critical mass in the absence of a fissile isotope on the planet.

If yes, I am particularly interested if such a technology would viably be produced -before- fusion reactors.

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    $\begingroup$ "wait until they can build viable fusion reactors" - if they are going to just wait, they will never succeed ;) $\endgroup$ – Mołot Mar 7 '17 at 12:18
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    $\begingroup$ U may be the "lowest hanging fruit", but there are radioisotopes of other elements below Z=92 - in fact, IIRC, Z=84 (Polonium) or higher have no non-radioactive isotopes, and neither do Z=43 (Technetium) or Z=61 (Promethium). $\endgroup$ – Jeff Zeitlin Mar 7 '17 at 12:36
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    $\begingroup$ They might even be smart enough never to build nuclear bombs at all. But that is just a side note. $\endgroup$ – Burki Mar 7 '17 at 12:44
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    $\begingroup$ @b.Lorenz - IIRC, there are designs for Thorium-fueled (Z=90) reactors. That certainly implies fissile enough for nuclear fuel. Th may not be weaponizable, and that was a primary consideration in most reactors presently commissioned. $\endgroup$ – Jeff Zeitlin Mar 7 '17 at 13:01
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    $\begingroup$ @JeffZeitlin Throium reactors depend on a plutonium core to start the reaction. Once the plutonium reactor is running, the thorium in the core starts turning into the fissile isotope U-233, which then powers the reactor. Thorium directly will not run the reaction. $\endgroup$ – kingledion Mar 7 '17 at 14:33
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There are lot of neutron sources:

https://en.wikipedia.org/wiki/Neutron_source

Not all of them is useful for large scale transmutation since their neutron flux is many orders of magnitude smaller than that of nuclear reactors. However, spallation neutron sources have higher flux than reactors and don't require radioisotopes (They are high energy particle accelerators bombing high-Z materials with protons). Thus, they can transform U-238 into Pu-239, or Thorium-232 (which has half life even exceeding U-238) into U-233.

Since the Thorium fuel cycle has many disadvantages (including especially nastily radioactive daughter elements), and their spallation source will need to have extreme luminosity and, therefore, extreme energy requirements, the production of their first tons of nuclear fuel would require significant effort. (Enriching uranium is, in fact, also quite difficult).

After they have their first critical reactor, they can use it to breed more and more fuel and construct other reactors and nukes.

Thus, if they really want, they can achieve fission before fusion, but they may deem it impractical.

EDIT: It was suggested that without natural fissionables they will never discover fission and have no motive to produce artificial fissionables, but I disagree with this. If their science advances roughly on the same way as ours, the following will happen:

First they discover radioactive decay in long-lived natural isotopes (or in those which are continuously recreated by cosmic radiation) and, thus, get to know that a nucleus is composed of smaller parts. Then they invent cyclotrons and linear accelerators and start to systematically smash together all available isotopes to see what happens. This way they discover the neutron and start to formulate their theories and models about the inner workings of the nucleii. If they are clever enough, they may predict fission purely theoretically and start to search paths to nucleii with the required Z and N in order to test their models.

Even if they can't discover it on paper, one day a diligent scientist will test U-238 or Th-232 against neutron radiation and, thus, discover Plutonium and U-233. The elated researcher will isolate these atoms and test them again against neutrons. He will then find weird mid-sized atoms in the sample and realize that nucleii were split in half. Someone will then notice the energy source potential of the process and formulate a letter to the government...

Edit2: If the civilization builds enough of them, they can produce the needed fuel by spallation sources. But there is a more efficient way to use the spalled neutrons:

https://en.wikipedia.org/wiki/Accelerator-driven_subcritical_reactor

Subcritical reactors are safe, and able to convert Th 232 into U 233 effectively. No such reactor was realised so far, but this is a well established design.

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    $\begingroup$ As I point out in a comment on MichaelK's answer below, non-reactor fission sources will never yield enough fissile materials to make a reactor. I calculated there that it would take 600 million years to make a kg of Plutonium from U-238 using the spallation method. You are right they will achieve fission...for a few atoms at a time. You are wrong to say they will deem it impractical; making more than a few atoms at a time fission will be impossible. $\endgroup$ – kingledion Mar 7 '17 at 20:32
  • $\begingroup$ Also there are orders of magnitude differences in the difficulty of enriching uranium and breeding fissile materials without a reactor. Uranium enrichment requires chemistry and centrifuges. Breeding fissile materials without a reactor requires a particle accelerator at least. I ran was able to build thousands of centrifuges, how many particle accelerators can they afford to build? $\endgroup$ – kingledion Mar 7 '17 at 20:34
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    $\begingroup$ @kingledion I think you have misunderstood. This paper makes sure that 25-30 neutron is not generated per pulse. Its per proton. oxfordneutronschool.org/2013/Lectures/… This says that some accelerators (and not the largest) have 4.4 mA proton beam current. This yields in 2.74*10**16 protons per second, 728181s for one mole of neutrons. You can use tons of uranium to capture all of them. This means 239 grams of plutonium in mere eight days. And these are scientific accelerators designed for very bright, short pulses, not for maximal total numbers. $\endgroup$ – b.Lorenz Mar 7 '17 at 21:45
  • $\begingroup$ accelconf.web.cern.ch/accelconf/e98/PAPERS/TUX03B.PDF another link I have referenced for the beam current, but had not fit into the first post. $\endgroup$ – b.Lorenz Mar 7 '17 at 21:48
  • $\begingroup$ Subcritical reactors would use spallation source to kepp thorium reactor going: en.wikipedia.org/wiki/Subcritical_reactor $\endgroup$ – b.Lorenz Mar 7 '17 at 21:50
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No

You do not need natural fission, nor fusion as a source for neutrons. A spallation source will easily provide a rich fauna of decaying elements, some of which will give off neutrons. The concept of nuclear spallation was first coined in 1937 by Glenn Seaborg.

Once you have slugs or rods of these neutron sources, or if you simply just alloy them with Uranium-238, then creating Plutonium-239 is the same as before: make a stack of U-238, and with graphite as a moderator.

I will also remind that the first Plutonium was created with a cyclotron.

So in summary: no, fusion is not needed to make nuclear fission weapons in a place where most natural radioactivity has decayed away. That can happen anyway.

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    $\begingroup$ Your link explains that with a spallation source, 20-30 neutrons are generated per pulse. If you can do a million pulses per second with your spallation source, you will make a 0.01 mol of neutrons (suitable for making ~1kg of plutonium, assuming 100% neutron absorption) in about 2e16 seconds, or 600 million years. This great for science experiments, but is not a reasonable industrial production method for breeding fissile materials. $\endgroup$ – kingledion Mar 7 '17 at 14:26
  • $\begingroup$ You did not read my post. I did not say the spallation source would be used to make neutrons, but to make isotopes. When you are using a spallation source you create lots and lots of different of isotopes. Some of those will be naturally fissile. But you are right in saying this may perhaps not be enough. I will look into the issue. $\endgroup$ – MichaelK Mar 7 '17 at 14:35
  • $\begingroup$ The isotopes in the Wikipedia article are 'aluminum, beryllium, chlorine, iodine, and neon.' None of those can make a fissile isotope. The second thing mentioned is using a 'mercury, tantalum, or lead' to create neutrons. This, I assume, is the only way you will be able to make a fissile material, either U-233 from Th-232 or Pu-239 from U-238. $\endgroup$ – kingledion Mar 7 '17 at 14:41
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    $\begingroup$ After checking into this... it is still possible. You need an initial amount to start breeding, but that is not at all impossible to do. And once you have gotten to breeding you will be up and running in mass production scale in fairly short amount of time. $\endgroup$ – MichaelK Mar 8 '17 at 8:50
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It is unlikely that any industrial fission source would be available

After reviewing some various possibilities, it seems unlikely that any fission source would be available or made available. In short, there are ways to build neutron sources, but the neutron flux inside a nuclear reactor is something like 1e9 times higher than achievable with methods mentioned by MichaelK.

A cyclotron or particle accelerator/spallation source would be sufficient for nuclear physics experiments, and it is likely that the people of your world would have comparable level of understanding to us of how nuclear physics works.

However, with no industrial scale neutron sources, they simply won't be able to make fissile materials in quantities sufficient for making bombs or powerplants. In your world, there will be only two isotopes heavier than bismuth available to work with: Th-232 and U-238. Both of these have very low neutron absorption cross sections and will not generate very much neutron flux as their fission chain reactions are far from sustainable. If you put a big block of them together, they would generate significant $\alpha$-flux, but few neutrons. The $\alpha$-flux wouldn't help much either; the two isotopes, if they absorbed an alpha particle, would turn to U-236 and Pu-242 respectively, both of which have half-lives in the million year range, are not fissile, and then alpha decay back down whichever isotope they came from.

Conclusion

Without natural U-235, there does not exist a sufficiently large neutron source to breed viable amounts of fissile material for bombs or power plants. Furthermore, without a fission starter, it is unlikely that there would be any fusion bombs, at least not with current technology. Fusion bombs use a fission starter to generate the temperatures and pressures needed to start fusion. Better off spending all your Manhattan project money developing clean, safe fusion power. This world is starting to sound better than ours.

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No. Not really. On Earth, we did know the main principle behind atomic bomb many years earlier than developing the first atomic bomb test, but remember that here on Earth we also have had ample amounts (at least when nationwide resources were utilized) of uranium and plutonium available to us, which actually had significant effect in learning the mechanics of building an atomic bomb.

On a planet where neither uranium, not plutonium are available at all, I don't think those people (or whoever those intelligent creatures are) would get any idea about the principle behind a nuclear bomb.

The problem with building a hydrogen bomb/reaction (fusion, as compared to atomic bomb or fission) is that currently this can only be achieved under extremely high temperature and pressure. Basically you need a fission-grade explosion to trigger the fusion reaction. Without having sufficient amounts of fission fuels to play with, I don't think any civilization would be able to build a fusion reactor.

While there are some ideas about a cold fusion reactor (which wouldn't require the incredibly high temperatures to initiate the fusion reaction), nobody has been able to build any of these so far. So, practically speaking, no, it would be extremely improbable (the closest term to impossible in science) for any civilization to jump directly at fusion reactors/bombs without first having the materials required for fission reactors/bombs.

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    $\begingroup$ Fission bombs are used to boost a fusion bomb, that is, to create enough heat and pressure to ignite fusion in the tritium fuel. I am not aware of fission bombs being used to ignite fusion in fusion reactors: that would destroy the containment vessel, scientists, neighboring towns, etc. (your third paragraph appears to conflate the ideas of bombs and reactors, using both terms) $\endgroup$ – user1975 Mar 7 '17 at 21:30
  • $\begingroup$ "On Earth, we did know the main principle behind atomic bomb many years earlier than developing the first atomic bomb test" Many years? Several, sure, but many? $\endgroup$ – David Richerby Mar 8 '17 at 17:23
  • $\begingroup$ @Snowman That's not what boosting is. Boosting is putting fusion fuel in a fission bomb so that the fusion produces neutrons that enhance the fission. $\endgroup$ – David Richerby Mar 8 '17 at 17:26
  • $\begingroup$ @DavidRicherby eh, you're right, I had it backwards. Fusion is the boost. $\endgroup$ – user1975 Mar 8 '17 at 17:27
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No

To summarize other answers - fission material can be generated without fusion. However, this implies a very steep learning curve for their nuclear physics, and fission process in particular. So, they may develop a practical fusion reactor before mass-producing any heavy isotopes needed for fission.

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Once an initial critical mass is available, it will be possible to use that to breed more of it. Putting plutonium in a reactor and surrounding it with u238 will allow them to generate more plutonium than they are consuming. But all of this requires an initial critical mass of fissile material, which would be impractical to produce because the only techniques of neutron bombardment that can measure up to practical scale are either another chain fission reaction(which is a catch-22) or a fusion reaction.

Deuterium-Tritium is the easiest fusion reaction to achieve and provides neutrons abundantly. The problem even then is that tritium is about as impossible to get from nature as U-235 would be in your world. We produce it by neutron bombardment in the same way as Plutonium. Which means it will also be out of your civilization's reach.

The conclusion I reach is that no, a civilization would not be able to breed their initial critical mass until they can sustain fusion, and at that, they would even reach the technology at a later stage than humankind has, due to the absence of tritium. They would have to achieve practical deuterium-deuterium fusion before they can produce neutrons to the scale needed to breed plutonium.

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