How to Enrich Uranium (in Case You're in the Mood)

It's impossible to watch the news lately (perhaps, at all) without seeing photos of bazillions of shiny centrifuges all in a row. But do you understand why they are there and what they're doing?

That's about to change, if you can stand getting through this article.

Uranium enrichment is a highly complex, labor-intensive process that spans mining, ore processing, purification, and a lengthy separation and purification sequence, involving a range of truly hideous chemicals. I'm not sure which is worse - doing it or learning about it. 

Nonetheless, here are the steps involved, and if you can get through this, you'll understand many of the terms seen on the evening news (interspersed with drug commercials with dancing idiots, which are arguably worse). 

"Yay!!! I'm so happy I have Type 2 diabetes and am handling it well!" Let's all dance! This hideous commercial for Jardiance features a deranged woman who is so deliriously happy with her type 2 diabetes that her colleagues cannot help but join in the dance! Or maybe they have the joyful disease as well! Imagine how tragic her life would be without diabetes to sing and dance to.

Step 1:  Mining 

Mined uranium ore is typically found in the ground in the form of uranium dioxide (UO₂), also known as uranite, with smaller amounts of uranium trioxide (UO₃). (Sorry, music fans, U2 is not found in the mines.) 

Step 2:  Baking the yellowcake​

During the milling process, the crude UO₂ is heated to about 500 °C. Atmospheric oxygen converts it into what is commonly referred to as yellowcake. (Contrary to popular belief, Betty Crocker does not do this.)

 6 UO₂ ​+ O₂ ​⟶ 2 U₃​O₈ (triuranium octoxide, aka yellowcake)​

Why? Yellowcake is more stable and easier to handle and ship than uranium oxide (UO₂).

Step 3: Going Backward – Why We Reduce What We Just Oxidized

In what may seem to be one step forward and one backward, yellowcake, having been formed by oxidation of UO₂ in the milling process, is reduced back to UO_2. While this may seem futile, the oxidation-reduction sequence removes many of the impurities (technical term: ickies) from the original ore.

Step 4: Line up the nasty chemicals. Making "Green Salt" (UF₄)

The purified uranium dioxide is then reacted with anhydrous hydrogen fluoride (HF), a lovely substance by any measure, at around 350°C. This produces uranium tetrafluoride (UF₄), a compound nicknamed "green salt" for its olive-green appearance. Do not put this on your popcorn.

UO₂ + 4 HF → UF₄ + 2 H₂O

UF₄, as if it's not hideous enough, it needs to become even more so. This is done by reacting it with another charmer.

Step 5: UF₆ – The Devil’s Gas

The final and most hazardous step is the reaction of UF₄ with fluorine gas (F₂), the most reactive of all the elements. 

UF₄ + F₂ --->  UF₆+ Fear

This produces uranium hexafluoride (UF₆), a white crystalline solid. But at 56°C (133°F) it turns into a gas. This shows how crazy chemistry can be. Uranium metal melts at 1,132°C (2,070°F) and doesn't boil until 3,818°C (6,904°F), but a fluorinated derivative vaporizes at a temperature not too much higher than the pickleball court I played on today. 

Uranium hexafluoride, the only volatile uranium compound, is the chemical that goes into the centrifuges, where it becomes enriched. Which isn't as easy as you'd think. UF₆ is unique among uranium compounds for its volatility, which is why all that horrible chemistry above it is required.

Step 6: It's a bitch to enrich

To understand why, you have to know a bit about isotopes. 

Hello? Anyone still there?

Isotopes are atoms of the same element with the same number of protons (which determine chemical behavior) but different numbers of neutrons (which don’t). As a result, isotopes behave identically in chemical reactions. (All elements have isotopes, but only a few are stable.) Uranium-235—so named because it has 235 total protons and neutrons—is the only uranium isotope suitable for making a bomb [1] (see Table 1). But since it’s chemically indistinguishable from the far more common U-238, it can't be separated by chemistry alone—just physics. It’s the same element, just a few "pounds" heavier.

Table 1. The four uranium isotopes. Note the relative abundance of U-238 and U-235. It is approximately 140 times more abundant in U-238, making it a pain in the ass to obtain purified U-235. The minute difference in weight is what makes it possible to obtain U-235 via high-speed rotation in gas centrifuges. Possible, but not easy.

Step 7: Now, the fun begins

UF₆ is not user-friendly. It’s highly toxic, corrosive, and reacts violently with moisture, forming hydrogen fluoride (itself deadly) and uranyl fluoride. It's also radioactive. 

However, because of research (part of the Manhattan Project), UF₆ was used in centrifuges to isolate enriched (about 90%) U-235 hexafluoride. Without going into too much detail, a 2005 training manual reveals that 7,896 centrifuges are used in 11 different parallel stages. Each time the mixture is spun, the heavier U-238 will "stick" to the sides of the centrifuge while the lighter 235 isotope stays in the center, where it can be physically removed. 

Talk about slow progress. After a "ride" on the centrifuge, there is a slight increase in the 235/238 ratio. How slight? A single trip through one centrifuge might increase the amount of U-235 from 0.70% to 0.71%. This is why the process of uranium enrichment is so long and arduous. 

Step 8: Finally - U-235 metal

Before it can be used in a bomb, the UF6 must be converted to uranium metal. This is done using a two-step reduction, first with hydrogen, then with calcium.

UF₆ + H₂ → UF₄ + 2 HF

UF₄ + 2 Ca → U (metal) + 2 CaF₂

Although required, this last phase seems almost trivial compared to the other steps. 

Bottom line

In short, enriching uranium is like trying to separate grains of sand by weight, while blindfolded, using a washing machine, and hoping it doesn’t kill you. It’s big-time science, high stakes, and just a little bit insane. But now, at least, you know what those shiny machines are doing. They're not making smoothies.

NOTE:

[1] U-238 won’t explode on its own, but it can be turned into plutonium, which very much will.