Nuclear programs are constantly in the news, with reports that they’re enriching uranium.
Usually, the stories mention percentages and centrifuges, but to the uninitiated, it’s
not clear why these things are important.
What exactly does it mean to enrich uranium? And at what point should we be worried?
Now when I imagine enriching uranium, I picture teaching uranium to paint, or dance, or something,
but I looked it up... and that’s not what it is.
For nuclear power or weapons, the isotope of uranium we’re after is U-235, which has
92 protons and 143 neutrons.
But the vast majority of uranium we pull out of mines, 99.3% of it, is the isotope U-238
which has 3 more neutrons, and behaves differently.
Only about 0.7% of Uranium in the world is U-235.
U-235 is fissile, meaning all you need to do to split it is hit it with a low-energy
thermal neutron. The neutron doesn’t smash the nucleus apart, but instead is absorbed
by the atom, making it unstable.
When it splits, it releases energy and more neutrons that can then repeat the reaction
with other U-235 atoms.
You can still split U-238 atoms, but you’d need high-energy neutrons to do it, which
is less efficient.
So when it comes to producing energy, the higher concentration of U-235 relative to
U-238, the easier it will be.
Nuclear power plants that aim to produce energy steadily over a long period use pellets that
are between about three to five percent U235, while nuclear bombs that want to release a lot of
energy all at once have concentrations as high as 90% U235.
The challenge is, how do you take naturally-occurring Uranium that’s 0.7% U-235, and up its concentration
to those lofty numbers?
That’s where enrichment comes in. A couple of approaches have been used to enrich uranium.
The one used by the Manhattan Project to build the first atomic weapons was
gaseous diffusion.
In order to separate the uranium isotopes, the first step was to turn it into a gas by
adding six fluorine atoms.
Once gaseous, the uranium hexafluoride was pushed through membranes whose holes were
just big enough to let the molecules through. The lighter U-235 moved slightly more quickly
through the barriers, so after several hundred membranes, the gas at the other end had the
required amount of U-235.
The more modern method is using centrifuges, which are cylinders that spin at very high
speeds, enough to separate the lighter U-235 from U-238.
The first step is the same: turn all the uranium into gaseous uranium hexafluoride. Then, like
a hipster with a vinyl collection, get spinning.
As the gas is spun around, the heavier U-238 will move towards the outside of the cylinder,
while the U-235 will drift towards the center. The slightly enriched gas there will be piped
out, and fed into another centrifuge.
And on and on it goes, using thousands of centrifuges connected in series and parallel
formations, steadily upping the amount of U-235 in the mix.
Once uranium is 20% enriched, it’s a simple matter of rearranging the centrifuges to create
weapons-grade uranium very quickly. That’s why a vital part of ensuring a country is
only using their nuclear program for peaceful pursuits is inspecting how their centrifuges
are arranged.
The proliferation of nuclear weapons remains one of the most serious threats to the world.
But nuclear power’s potential to provide energy without adding greenhouse gasses to
our atmosphere is a huge possible benefit for humanity.
If we’re going to use uranium for the good of us and the planet, then it just needs a
little enrichment.
While the centrifuges enrich uranium, they’re also feeding the more concentrated U-238 the
opposite direction, producing depleted uranium at the other end.
Nuclear power is still something that worries a lot of people, and recently Russia decided
to turn a boat into a floating power plant. To see if this is a bad idea, check out my
video on it here.
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