Energy and Human Ambitions on a Finite Planet, 2021a

15 Nuclear <strong>Energy</strong> 257
Watt <strong>and</strong> installed utility-scale systems are $5–10 per Watt. In short,
nuclear power is not an economic slam dunk.
15.4.4.1 Uranium
So far, we have ignored a crucial fact. Only 0.72% of natural uranium on
235
Earth is the fissile U flavor. The vast majority, 99.2745%, is the benign
235
U The ratio is about 140:1, so for every U atom pulled out of the
ground, 140 times this number of uranium atoms must be extracted. The
origin of the disparity is a story of astrophysics <strong>and</strong> eons, covered in Box
15.4.
238 . 37 37: A trace amount, 0.0055%, is in 234 U .
Box 15.4: Origin of Uranium
The Big Bang that formed the universe produced only the lightest
nuclei. By-<strong>and</strong>-large, the result was 75% hydrogen ( 1 H) <strong>and</strong> 25%
4
helium ( He). Deuterium ( 2 3
H) <strong>and</strong> He were produced at the 0.003%
<strong>and</strong> 0.001% levels, respectively, <strong>and</strong> then the tiniest trace of lithium.
No carbon or oxygen emerged, which must be “cooked up” via fusion
in stars.
Fusion in stars does not “climb over” the peak of the binding-energy
curve in Figure 15.10, so stops in the vicinity 38 of iron. From where,
then, did all of the heavier elements on the periodic table derive?
Exploding stars called supernovae <strong>and</strong> merging neutron stars appear
to be the origin of elements beyond zinc.
235 238
The relative abundance of U <strong>and</strong> U on Earth can be explained
by their different half-lives of 0.704 Gyr <strong>and</strong> 4.47 Gyr, respectively.
235
Even if starting at comparable amounts, most of the U will have
decayed away by now. Solving backwards 39 to when they would
have been present in equal amounts yields about 6 Gyr, which is
older than the age of the solar system (4.5 Gyr) <strong>and</strong> younger than
the universe (13.8 Gyr). This is a reasonable result for how old the
astrophysical origin might be—allowing a billion years or so for the
material to coalesce in our forming solar system.
38: Iron has Z 26; stars tend not to produce
elements beyond zinc (Z 30) by
fusion.
39: This follows almost the exact same logic
<strong>and</strong> process as carbon-14 radioactive dating,
but using much longer half life nuclei to date
Earth’s building blocks!
Uranium is not particularly abundant. Table 15.9 provides a sense of
how prevalent various elements are in the earth’s crust. Uranium is
235
more abundant than silver, but the useful U isotope is four times rarer
than silver, <strong>and</strong> only about 5 times as abundant as gold. Proven reserves
of uranium [103] amount to 7.6 million (metric) tons available, <strong>and</strong> we
have used 2.8 million metric tons to date. The implication is that we
could continue about 3 times longer than we have gone so far on proven
reserves. But nuclear energy has played a much smaller role than fossil
fuels, so maybe this isn’t so much.
[103]: (2020), List of Countries by Uranium
Reserves
Evaluating the uranium reserves in energy terms is the most revealing
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.