What does it mean to "enrich" Uranium?

A must-read entry for all future dictators. Take a look at the meaning of "enriching uranium," why it's necessary, and some of the processes used to do it.

Let's say that, as an up-and-coming president-for-life of some unfortunate swath of land, you order some of that yellow cake uranium that you've been hearing so much about. After the initial shock of realizing that you can't frost it and eat it in order to gain superpowers, you acquire a few engineers, have them set up a bomb and set it off.

Nothing happens.


This is because the uranium is not enriched. A nuclear bomb fueled by uranium releases energy through fission, the breaking apart of an atom. When a neutron hits the nucleus of a uranium atom, the atom breaks apart. The resulting pieces weigh slightly less than the original whole; that weight having been turned into huge amounts of energy.

Here's the key – each split nucleus has to release at least two neutrons of its own. They hit other nuclei and start two more reactions, and those atoms release more neutrons and start more reactions. The result is either a steady hum of energy or a big boom, depending on how you work it.

Let's face it. If it were that simple, nuclear bombs would have been invented long ago. If there's one thing the internet proves, it's that people will never stop looking for flashy ways to hurt themselves and others. The reason why everything, from the pyramids to the Eiffel Tower aren't reduced to little particles of dust wafting around the upper atmosphere is only very, very rare isotopes of uranium break apart easily enough and give off enough neutrons to sustain a nuclear reaction. The most common uranium isotope is uranium-238, with ninety-two protons and 146 neutrons. It doesn't sustain a reaction. Uranium-235, with 143 neutrons, is the money isotope.

First, start with about 64.1 kilograms of uranium. Don't worry. Bombs can be made with only about 15 kilograms, so you'll have wiggle room.

There are three main ways of doing this. The first is running the uranium through a strainer. Or, more accurately, running the uranium next to a strainer. The uranium is made into gas and rushed past a membrane with allows the smaller isotopes to pass through more easily than the large ones. Then it is run past the membrane again. And again. A few hundred passes later, and the precious, active lump of uranium-235 is separated out from the glorified doorstop lump of uranium-238.

The second method of uranium enrichment is to put the uranium on a carnival ride. You know the one. The one that doesn't take you anywhere, and doesn't let you win prizes, it just chains you against the wall of a sealed drum and spins you around and around until you pray a seatbelt gives and releases you from your torment.

What does it mean to "enrich" Uranium?

That one.

Uranium gas is pumped into a centrifuge, which spins it mercilessly. The heavier uranium-238 head for the sides of the tube, elbowing aside the uranium-235, which stays toward the center. Once that process is repeated enough times, the uranium-235 is rich enough to explode something, which after a hundred spins in a centrifuge it must be aching to do.

The last, and coolest, way to get enough uranium-235 is using lasers. I'm not even sure anyone needed another process, or that it was more efficient. I just think that someone decided that if there is going to be an explosion, there should be lasers involved at some point. And they were right. In this case, the lasers are pointed at the uranium until it photoionizes. Ions are atoms that have an unequal number of protons and electrons, giving them an net charge. Photoionization is simply shining a light on something until the electrons in it amscray. Ionized particles are much easier to separate from non-ionized particles than slightly heavier particles are from slightly lighter ones.

Assuming you started with the 64.1 kilograms of uranium to make a decent bomb, and spent millions of dollars and several years enriching it, you will now have approximately 450 gram paperweight. Uranium-235 is only about .7 percent of the mass of any given lump of uranium. Atomic bombs require somewhere in the neighborhood of 15-25 kg of enriched uranium. You will die a defeated, despised social outcast, probably crushed under one of your own statues in a town square. Sorry.

[Via Cosmos and the United States Nuclear Regulatory Commission.]