Energy and Human Ambitions on a Finite Planet, 2021a

15 Nuclear <strong>Energy</strong> 262
15.4.6 Nuclear Weapons <strong>and</strong> Proliferation
Nuclear bombs are the most destructive weapons we have managed
to create. The first bombs from the 1940s were based on either highly
235 239
enriched U or on Pu. For uranium bombs, the idea is shockingly
simple. Two separate lumps of the bomb material are held apart until
detonation is desired, at which point they are slammed together. 48 It’s not
the collision that creates the explosion, but a runaway process based on
having a high concentration of fissile material <strong>and</strong> no neutron absorbers
present to control the resulting chain reaction. The concept is critical
mass. The combined lump exceeds the critical mass, <strong>and</strong> explodes. 49
As simple as nuclear weapons are to build, the bottleneck becomes
obtaining fissile material. Plutonium does not exist in nature, since its
24,100 yr half-life means nothing is left over from the astrophysical
processes that gave us uranium <strong>and</strong> thorium (Box 15.4). We only still
have the latter two thanks to their long half lives. So fissile material has to
start with uranium. But as we have seen, natural uranium is only 0.72%
235
fissile ( U). In order to be explosive, the uranium must be enriched
235
to at least 20% U, <strong>and</strong> generally much higher (85%). Reactor fuel, at
235
3–5% U will experience meltdown if the critical mass is exceeded,
but will not explode. Enrichment is technically difficult, <strong>and</strong> attempts
to acquire <strong>and</strong> enrich uranium are monitored closely. Often we hear of
countries pursuing uranium enrichment, claiming that they are only
interested in domestic energy production—a peaceful purpose. And it
is true that the first step in nuclear power generation is also enrichment.
So it is very difficult to ascertain true intentions. Once a country has
the ability to enrich uranium enough for a nuclear plant, they can in
principle keep the process running longer to arrive at weapons-grade
235
U.
48: For plutonium, this process is fouled
by the presence of 240 Pu, forcing a different
approach in which a sphere below critical
mass is imploded to create high density.
49: Never stack lumps of fissile material
together on a shelf, or a nasty surprise may
be in store.
235
While we worry about U falling into the wrong h<strong>and</strong>s, perhaps
239 235
more disturbing is Pu. Having a much shorter half-life than U
(24 kyr vs. 704 Myr), it is more dangerous to h<strong>and</strong>le. 50 But plutonium is 50: . . . much higher rate of radioactive decay
otherwise easy to deal with, since it requires no enrichment <strong>and</strong> can be
chemically separated to achieve purity. It is the material of choice for
nuclear weapons.
Serious pursuit of breeder reactors effectively means manufacturing lots
of plutonium, leading to proliferation of nuclear materials: it becomes
harder to track <strong>and</strong> keep away from mal-intentioned groups. The world
becomes more dangerous under a breeder program. Thorium breeding
233
(Box 15.5) is less risky in this regard because the U prize is mixed with
232
a ridiculously dangerous U isotope that puts plutonium to shame, so
working with it is pretty deadly, which may deter would-be pursuit of
this material by rogue groups.
A related concern involves proliferation of the abundant radioactive
waste from fission plants, which could be mixed into conventional explosives
51 to radioactively contaminate a city or local region—poisoning
51: . . . called a “dirty bomb”
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.