I know that the critical size of natural uranium metal with graphite moderator is about 27 tons "oak ridge graphite reactor". for comparison highly enriched uranium have only 17 kg critical size with a thick steel reflector . the difference is huge .

it is often mentioned that the minimum enrichment for uranium to be considered weapon-usable is about 20% , but logically it seems even natural uranium can make a bomb too !! especially if it is fusion boosted . it seems that this enrichment is required only for pure fission bomb , but not for fusion boosted system by fast neutrons.

so, if it possible to make effective nuclear bomb by using heavy implosion system consisted of 100s of kilograms or even few tons of natural uranium with thick copper or steel reflector and few kilograms of lithium-6 deuteride as a fusion fast neutron booster ?

when we take Little boy bomb as example , they use 64 kg of 85% enriched uranium . this amount require about 7500 natural uranium , using 10% of this amount would be more economical and the resulted bomb would have a yield of 200 Kt assuming it will have the same efficiency of the little boy . so it will be more cheaper and 10 times stronger !!

fusion boosting allow using reactor grade plutonium in the weapons ,does this mean fusion boosting can allow too using natural uranium in the bombs ?

  • $\begingroup$ Why is it logical to believe something that hasn’t been demonstrated for a reason? $\endgroup$
    – Jon Custer
    Commented Apr 12, 2019 at 15:20
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    $\begingroup$ This idea was used in the design of the Tsar-bomba. This reduced costs to 60 cents per kiloton, see en.wikipedia.org/wiki/Tsar_Bomba $\endgroup$ Commented Apr 12, 2019 at 15:44
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    $\begingroup$ @JonCuster because it work with reactors , there is no reason to think it is impossible to work with bombs , especially when fusion boosting which produce very fast neutrons is introduced . $\endgroup$
    – Johnny
    Commented Apr 12, 2019 at 20:42
  • $\begingroup$ @AlexTrounev I didn’t understand your point . natural uranium is not even mentioned in your link at all !! $\endgroup$
    – Johnny
    Commented Apr 12, 2019 at 20:44
  • $\begingroup$ @Johnny natural uranium basically consists of uranium-238 (99.286% ). On wiki page there are a few words: "In theory, the bomb would have had a yield in excess of 100 megatons if it had included a uranium-238 tamper". $\endgroup$ Commented Apr 13, 2019 at 11:31

1 Answer 1


Natural uranium is about 0.711% U-235. As an implosion bomb's size increases, the minimum percentage U-235 decreases. When the bomb size is infinite, the percentage U-235 required is 5.4% so some enrichment is necessary.

The 5.4% figure is from the 1998 article "Definition of Weapons-Usable Uranium-233" by Forsberg, Hopper, Richter and Vantine. See the figure on page 14: "Infinite Mass U in U-233 Concentration Limit = 3.2 wt %", "Infinite Mass U in U-235 Concentration Limit = 5.4 wt %"

from https://web.archive.org/web/20131102011417/http://web.ornl.gov/info/reports/1998/3445606060721.pdf


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    $\begingroup$ If someone could copy in the figure on page 14 it would be an improvement. My computer seems to be having trouble capturing images right now. $\endgroup$ Commented Apr 13, 2019 at 12:11
  • $\begingroup$ then, how natural uranium reactors go critical ? I believe real-life experiments more than any calculation . $\endgroup$
    – Johnny
    Commented Apr 13, 2019 at 15:41
  • $\begingroup$ another point these calculations ignoring possibility of using fusion boosting which may change everything . $\endgroup$
    – Johnny
    Commented Apr 13, 2019 at 15:43
  • $\begingroup$ Nuclear weapons need "prompt criticality" which is more difficult than criticality. If you try to make a bomb with criticality it blows itself apart before it lights much of the urnium. Prompt criticality is about producing neutrons promptly rather than eventually. As far as fusion bombs go, I believe they begin with fission "primaries" so you need to make a fission bomb first. Then you can add as much plain jane uranium (or DU) as you like. $\endgroup$ Commented Apr 13, 2019 at 23:52

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