Energy and Human Ambitions on a Finite Planet, 2021a

15 Nuclear <strong>Energy</strong> 258
Element Abund. Element Abund. Element Abund.
silicon 282,000 carbon 200 thorium 9.6
aluminum 82,300 copper 60 uranium 2.7
iron 56,300 lithium 20 silver 0.075
calcium 41,500 lead 14 235U
0.02
titanium 5,650 boron 10 gold 0.004
Table 15.9: Example material abundances
in the earth’s crust, in parts per million by
mass.
approach. First, we take 0.72% of the 7.6 million tons available to
235
represent the portion of uranium in the form of U. Enrichment (next
235
section) will not separate all of the U, <strong>and</strong> the reactor can’t burn all of it
away before the fuel rod is essentially useless. So optimistically, we burn
235
half of the mined U in the reactor. Multiplying the resulting 27,300
235
tons of usable U by the 17 million kcal/g we derived earlier yields
a total of 2 × 10 21 J. Table 15.10 puts this in context against fossil fuel
proven reserves from page 127. We see from this that proven uranium
reserves give us only 20% as much energy as our proven oil reserves,
<strong>and</strong> about 5% of our total remaining fossil fuel supply. If we tried to get
all 18 TW from this uranium supply, it would last less than 4 years! This
does not sound like a salvation.
Table 15.10: Proven reserves, in energy
terms.
Fuel
10 21 J
Coal 20
Oil 10
Gas 8
235U
2
Proven uranium reserves would last 90 years at the current rate of use,
so really it is in a category fairly similar to that of fossil fuels in terms
of finite supply. To be fair, proven reserves are always a conservative
lower limit on estimated total resource availability. And since fuel cost
is not the limiting factor for nuclear plants, higher uranium prices can
make more available, from more difficult deposits. Still, even a factor of
two more does not transform the story into one of an ample, worry-free
resource.
15.4.4.2 Breeder Reactors
238
But what if we could use the bulk uranium, U, in reactors <strong>and</strong> not
only save ourselves the hassle of enrichment, but also gain access to
140 times more material, in effect? Doing so would turn the proven
reserves of uranium into about 7 times more energy supply than all of
our remaining fossil fuels. Well, it turns out that despite its not being
235
In its native form, U is too dilute in natural uranium—overwhelmingly
238
dominated by U—to even work in a nuclear reactor. It must be enriched
to 3–5% concentration to become viable. 40 40: Uranium bombs need at least 20% 235
Enrichment is difficult to
U
235 238
achieve. Chemically, U <strong>and</strong> U behave identically. The masses are concentration, but typically aim for 85% to
be considered weapons grade.
so close—just 1% different—that mechanical processes have a difficult
time differentiating. Centrifuges are commonly used to allow heavier
238 U to sink faster41 235
than U. But it’s inefficient <strong>and</strong> usually requires 41: ...ingaseous form
many iterations to work up higher concentrations. The process is also
235
lossy, in that not all of the U finds its way to the enriched pile. 42 42: Depleted uranium is defined as containing
0.3% or less in the form of 235 U, which
is not a huge reduction from the 0.72% starting
point.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.