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gwern comments on Open thread, November 2011 - Less Wrong
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If we had had more fissionable material over the last 100 years how would that have made nuclear war more likely?
If life had evolved say 2 billion years earlier than there would be about 6 times as much U-235 on the planet, and most uranium ores would be around 3% U-235 rather than 0.7% U-235. This means that making nuclear weapons would be easier, since obtaining enough uranium would be a lot easier and the amount of enriching needed would go down as well. For similar reasons it would also then be easier to make plutonium in large quantities. However, the fact that one would still need some amount of enrichment means that this would still be technically difficult, just easier. However, fusion bombs are much more effective for civilizations destroying themselves, and even with cheap fissiles, fusion bombs are still comparatively tough.
There's another reason that this filter may not be that big a filtration event: Having more U-235 around means that one can more easily construct nuclear reactors. Fermi's original pile used non-enriched uranium, so one can have a (not very efficient) uranium reactor simply from that without much work, and modern reactors can use non-enriched uranium (although that requires careful designs). But on a large scale, in such a setting, somewhat enriched uranium (compared to what we consider normal) would be much more common, and functional, useful reactors can be easily made with percentages as low as 2% of U-235, and in this setting most of the uranium would be closer to 3% U-235. So making nuclear reactors much easier means one has a much easier source of energy (in fact, on Earth, there's at least one documented case of such a reactor occurring naturally about 1.7 billion years ago ). Similar remarks apply to nuclear rockets which are one of the few plausible ways one can reasonably go about colonizing other planets.
So the two concerns are: a) how much more likely would it be for a civilization to actually wipe itself out in this sort of situation and b) how much is this balanced out by the presence of a cheap energy source and an easier way to leave the planet and go around one's star system with high delta-V?
What, exactly, would the increased uranium level do?
It doesn't seem to me that it would speed up the development of an atomic bomb much because you have to have the idea in the first place; and in our timeline, the atomic bomb followed the idea very quickly (what was it, 6 years?); the lower concentration no doubt slowed things by a few months or perhaps less than 5 years, but the histories I read didn't point to concentrating as a bottleneck but more conceptual issues (how much do you need? how do the explosive lenses work? etc.)
Nor do I see how it might speed up the general development of physics and study of radioactivity; if Marie Curie was willing to go through tons of pitchblende to get a minute bit of radium, then uranium clearly was nowhere on her radar. Going from 0.6 to 3% won't suddenly make a Curie study uranium ore instead.
The one such path would be discovering a natural uranium reactor, but how big a window is there where scientists could discover a reactor and speed up development of nuclear physics? I mean, if a scientist in the 1700s had discovered a uranium reactor, would he be able to do anything about it? Or would it just remain a curiosity, something like the Greeks and magnets?
Well, one issue is that it becomes easier for countries to actually get nukes once the whole technology is known. One needs to start with less uranium and needs to refine it less.
Regarding the Curies, while that it is true, it might be that people would have noticed radioactivity earlier. And more U-235 around means more radium around also. But I agree that this probably wouldn't have a substantial impact on when things would be discovered. Given how long a gap there was between that initial discovery and the idea of an atomic bomb, even if it did speed things up it is unlikely to have impacted the development of nuclear weapons that mcuh.
Your points about profileration and effectiveness seems to both be strong. Overall, this conversation makes me move my view in the other direction. That is, this seems to be not just not a strong filtration candidate, the increased ease of energy access argument seems to if anything push things in the other direction. Overall, this suggests that as far as presence of U-235 is concerned, civilizations that arise on comparatively young planets should have less not more filtration. This is worrisome.
Yes, but how much does this help? There are multiple methods available of varying sophistication/engineering complexity (thermal easy, laser hard); a factor of 6 surely helps, but any of the methods works if you're just willing to run the ore or gas through enough times.
That's a good point. So the only advantage comes from not needing as much uranium ore to start with and since uranium ore is easy to get already that's not a major issue.