﻿What Keeps Nuclear Weapons from Proliferating: The hardest step in making a nuclear bomb
I have here a scale-model of the first atomic bomb ever used.
This bomb, which destroyed Hiroshima
contains about sixty kilograms of uranium-235
of which only about six-hundred grams underwent fission.
Enough though to generate an explosion equal
to more than thirteen kilotons of TNT.
The bomb’s designers divided the amount needed into two piece.
At the tip they placed about 40% of the necessary uranium.
They loaded the remaining 60% at the other end.
A conventional explosion drove the projectile
into the target initiating the nuclear explosion.
Now the exact details of this bomb remain classified
because they could still be used.
Although this design involved some brilliant
innovation by engineers in the twentith century
the really difficult part is preparing the uranium.
This lies at the heart of all efforts to
stop the spread of nuclear weapons.
The key problem.
Separating two nearly identical variants of the element uranium.
Natural uranium occurs as a metal ore
and it contains primarily two isotopes.
Most uranium is U-238.
U-235 however, can easily sustain a chain reaction
that releases tremendous energy, whereas the more common U-238 will not.
Most elements are stable so that when bombarded
with neutrons they simply absorb them
and decay later, or they require very high energy neutrons
but bombarding U235 with low energy neutrons causes its nucleus to split.
The emission of these extra neutrons allows
the initial fission to generate a chain reaction.
So how do we go about enriching the U-235 in natural uranium.
When we separate two items we make use of their differences.
The two major uranium isotopes have identical
magnetic and chemical properties
no magnets will tug on one more than the other
no solvent will wash away only one isotope
and neither will boil before the other.
So, to separate them engineers exploit the
one small difference between them
U235 weighs slightly less than U238.
Less than a two percent difference
just enough to make separation possible, but it’s not easy.
That tiny weight difference means that the two isotopes
will move at slightly different speeds when exposed to an equal force.
To enrich uranium for the first atomic bomb engineers
built immense gaseous diffusion plants that
capitalized on the differing speeds.
A gas containing uranium flows through miles of piping
in a kind of race, where the lighter U-235 wins out.
The gas flows through a tube encased in a chamber.
A pressure difference between the chamber
and the tube causes more of the U-235 to pass
through perforations in the tube’s wall.
To increase the separation the slightly enriched
stream in the chamber is passed through many more stages like this.
To enrich 3% U-235 to 90% takes nearly 4,000 stages.
Enriching uranium for the first atomic bomb
required a diffusion plant that covered over 40 acres.
It housed a maze of 100 miles of piping.
These diffusion plants use great amounts of energy to run
Compressors generating the pressures needed
and the energy to heat gas flowing throughout the miles of tubing.
Another method of separation exploits the small mass
difference by using a centrifuge.
A typical device consists of a stationary outer cylinder
and an inner rotor that spins.
A gaseous mixture of the two isotopes flows up a tube
along the central axis, filling the rotor.
As it spins rapidly more of the U238 is thrown out to the
edge than the lighter U235, which stays closer to the middle.
The enriched stream can be removed from the rotor
and sent to another centrifuge to be separated even more.
The amount of separation is exaggerated here
In an actual centrifuge the amount of enrichment
is a fraction of a percent so a typical plant might have
60,000 centrifuges to enrich natural uranium to 30% U235.
Such a plant uses four percent the energy of a gaseous diffusion plant.
Even though this is a much more efficient process
the precision that the rotors need to be manufactured with
makes them very difficult to engineer.
The smallest defect and the rotor spins itself to pieces.
That’s lucky for us
otherwise we might have nuclear devices
right next to our microwave ovens.
I’m Bill Hammack, the Engineerguy.
This video is based on a chapter
in the book Eight Amazing Engineering Stories.
The chapters features more information about this subject.
Learn more about the book at the address below.