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The World Nuclear Association(WNA) states:

Weapons-grade uranium is highly enriched, to over 90% U-235 (the fissile isotope).Weapons-grade plutonium has over 93% Pu-239

However, what WNA or any other nuclear weapon watchdog do not state is why Uranium or Plutonium is enriched in the first case before they can be used in nuclear warheads?

Is it because the amount of space available in the warhead is limited and therefore only a small amount of Uranium or Plutonium can be used & so they have to be enriched first?

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    $\begingroup$ Of the top of my head, $U_{235}$ is fissured by neutrons, and then emits more neutrons when fissured. I believe $U_{238}$ absorbs the emitted neutrons, without fissuring. To ensure we have a constant supply of neutrons, we have to ensure that most of our neutrons are not absorbed by $U_{238}$, so we have to enrich our uranium to have more $U_{235}$. $\endgroup$
    – Tweej
    Apr 10 '16 at 10:57
  • $\begingroup$ People are listening... $\endgroup$
    – ProfRob
    Apr 10 '16 at 18:48
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    $\begingroup$ Very closely related: Why does it take so long to make a nuclear bomb? $\endgroup$
    – user10851
    Apr 10 '16 at 18:58
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Weapons grade $U$ has to be highly enriched to boost its levels of $U^{235}$ from the natural level (about 0.7 w%) to > 90 w%. That's because $U^{235}$ is the fissionable isotope and its concentration in the bomb core has to be sufficiently high for an explosive nuclear chain reaction to be able to occur.

Weapons grade $Pu$ is not obtained by enrichment. It is obtained by irradiating $U^{238}$ with neutrons in purpose-built (non-civilian) nuclear reactors. There, neutron capture then converts $U^{238}$ to $Pu^{239}$ which is the fissionable isotope of $Pu$. Irradiation conditions and times are carefully chosen so that $Pu^{239}$ is almost the only $Pu$ isotope formed in the process.

After the irradiation process (aka 'breeding process') the $Pu$ is then chemically separated from the $U$ (and other isotopes). $Pu$ obtained in this way is typically higher than 93 w% $Pu^{239}$ and does not require enrichment.

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  • $\begingroup$ Thank You! So, when you say That's because U235 is the fissionable isotope and its concentration in the bomb core has to be sufficiently high for an explosive nuclear chain reaction to be able to occur what's the quantity of U235 that is needed in the bomb core? $\endgroup$
    – DSarkar
    Apr 12 '16 at 10:28
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    $\begingroup$ @D_S: it's not clear. Literature numbers vary from 80 w% to 95 w%. In theory, the higher the better but enrichment is very expensive, so I guess there's a trade-off there. $\endgroup$
    – Gert
    Apr 12 '16 at 13:06
  • $\begingroup$ The amount of critical mass required to ensure a chain reaction depends on a number of factors. The shape of the critical assembly is one. If the assembly is composed of two sub-critical masses which are combined rapidly using implosion, there may be a reduction in the total amount of fissile material used versus say a gun-type assembly method used in the first A-bomb. $\endgroup$
    – user16622
    May 20 '16 at 1:15

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