Oleg Zabluda's blog
Thursday, April 12, 2018
 
Enriched uranium - Wikipedia

Let's calculate the minimum amount of energy needed to completely separate U235 from U238.

Consider one mole of natural Uranium (238.03 grams). It's 6.022e23 atoms. When isotopes are completely separated, the entropy is 0. When mixed (0.711% U235, 99.284% U238 by weight), entropy (in nits) is

ΔS = ln(C(6.022e23, 6.022e23*0.00711)) = 2.544e22 nits

entropy in SI:

ΔS = k_B * 2.544e22 nits = 1.380e-23 J/K * 2.544e22 nits = 0.3511 J/K

At room temperature (300 K) the energy to reduce entropy by that much is

ΔE = TΔS = 0.3511 J/K * 300 K = 105 J (i.e. negligible)

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

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