Energy and Human Ambitions on a Finite Planet, 2021a

15 Nuclear <strong>Energy</strong> 261
10 6
10 5
Te132
Ba140
radioactive power per kg 235 U (W)
10 4
10 3
10 2
10 1
10 0
10 -1
10 -2
10 -3
10 -4
Zr95
Ce144
Sr90
Cs137
Actinides
Tc99
Cs135
total
10 -5
0.001 0.01 0.1 1 10 100 1,000 10k 100k 1M
time (years)
Figure 15.19: Decay activity of fragments
from 1 kg of fissioned 235U
over time, on a
log–log plot. The vertical axis is the power
of radioactive emission, in W, for a variety
of relevant isotopes—each having their own
characteristic half life. The black line at the
top is the total activity (sum of all contributions),
<strong>and</strong> some of the key individuals are
separated out. The dashed line for actinides
is an approximate representative indicator
of the role played by heavy nuclides formed
in the reactor by uranium absorption of neutrons.
Minor tick marks are at multipliers
of 2, 4, 6, <strong>and</strong> 8 for each axis. As a matter
of possible interest, the exponential decays
of each element on this log–log plot have
the functional form of exponential curves
drawn upside-down.
Figure 15.19 shows how the fission decays play out over time. For the first
month or so out of the reactor, the spent fuel is really “hot” radioactively,
95 144
but falls quickly as Zr <strong>and</strong> then Ce dominate around one year out.
90 137
At about 5 years, the pair of Sr <strong>and</strong> Cs begin to dominate the output
for the next few-hundred years. Some of the products survive for millions
of years, albeit at low levels of radioactive power. In addition to the
daughter fragments, uranium in the presence of neutrons transmutes into
neptunium, plutonium, americium, <strong>and</strong> curium via neutron absorption
<strong>and</strong> subsequent β − decays, represented approximately <strong>and</strong> collectively
in Figure 15.19 by a dashed curve labeled Actinides. 47
The bottom line is that fission leaves a trash heap of radioactive waste
that remains at problematic levels for many thous<strong>and</strong>s of years. When
nuclear reactors were first built, they were provisioned with holding
tanks—deep pools of water—in which to place the waste fuel until a
more permanent arrangement could be sorted out (Figure 15.20). We are
still waiting for an adequate permanent solution for waste storage, <strong>and</strong>
the “temporary” pools are just accumulating spent fuel. Transporting
the spent fuel is hazardous—in part because it could fall into the wrong
h<strong>and</strong>s <strong>and</strong> be used to make “dirty” bombs—<strong>and</strong> no one wants a nuclear
waste facility in their backyard, making the problem politically thorny.
On the technical side, it is difficult to identify sites that are geologically
stable enough <strong>and</strong> have little chance of groundwater contamination.
Underground salt domes offer an interesting possibility, but political
challenges remain daunting.
47: Breeder reactors can “burn” the actinides,
reducing some of the long-term
waste threat, but will unavoidably still be
left with all the radioactive fission products.
Figure 15.20: A spent fuel rod being lowered
into a storage grid in a pool of water
at a nuclear power plant. Source: U.S. DoE.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.