8 Things About Uranium

As is fairly obvious, the recently inaugurated President of the USA, Donald Trump, has no idea what uranium is. To quote him directly:

“You know what uranium is, right? S’thing called nuclear weapons and other things and lots of… things are done with uranium including some BAD things.”
Donald Trump

Very… astute. And eloquently put. Reminded me of something I’d seen before. Needless to say, “s’thing called nuclear weapons and other things and lots of… things” is not a particularly good description of uranium. Despite its understandably bad reputation, it’s quite an interesting element, for a number of reasons. So for anyone else who’s a little hazy on what exactly uranium is, here are 8 things about it.

1. Uranium makes excellent fuel

Fission energy is the thing everyone will know about uranium. Being unstable and consequently radioactive, uranium’s atomic nuclei tend to fall apart when bombarded with neutrons, releasing prodigous amounts of energy. The actual physics behind this were first understood in 1939 by Lise Meitner, who also realised the dangerous potential behind it.

xkcd-energy-densityWhen discussing fuels, many scientists talk about energy density – which just means how much energy any given fuel contains, per kilogram. It’s often measured in megajoules per kilogram (MJ/kg). Atomic nuclei contain a massive amount of energy, and nuclei which can be split are the densest source of usable energy of any fuel, by quite a long way.

The petrol which you put in your car has an energy density of 46.4 MJ/kg. Uranium, on the other hand, contains 79,390,000 MJ/kg. That’s 1.7 million times as much energy! This xkcd comic explains that rather nicely.

2. Uranium has several isotopes

Isotopes are different forms of any chemical element. An element’s identity is defined by it’s protons but the number of neutrons it has can vary, so any atomic nucleus with 92 protons will always be uranium, with the rest of the mass being made up by neutrons. Most uranium atoms have 146 neutrons, making uranium 238 (or U-238 for short).

U-238 is relatively stable, with a half life of 4.468 billion years (I’ll come back to that later) and accounts for over 99% of the uranium on our planet. The other most common isotopes are U-234 and U-235. The one to be aware of here is U-235 which is “fissile”, meaning that it can sustain a chain reaction and can therefore be used in nuclear reactors to generate energy. In fact, U-235 is the only fissile atomic nucleus to be found naturally in any significant amount.

3. Uranium contains stored energy from a supernova

crab-nebula-sliceThis one’s pretty cool. Uranium is the heaviest element to occur naturally in the universe (unless you count the stuff that happens in neutron stars, but they’re weird). Atomic nuclei have what we call a valley of stability. Elements of a certain mass range are the most stable, and natural nuclear processes occurring in unstable nuclei are constantly trying to move towards that mass range. At the bottom of this valley is Iron-56.

Much heavier than this, and elements are radioactive. The nuclei shed fragments as alpha particles to try and get back to a more stable place on the periodic table. The heaviest naturally occurring elements can only be created in supernovae.

Any star is in a constant battle between gravity pushing everything inwards and the energy created by nuclear fusion pushing everything outwards. In the most massive stars, fusion in the star’s core creates heavier and heavier elements. The process keeps going because fusion gives out more energy than it takes in. That is, until iron is formed

Iron is the first element which needs more energy to fuse than it gives back. Once the star has no choice but to fuse iron, gravity wins the fight. Rapidly. The star’s entire mass starts to fall inwards, compacting the material in the star’s core. The outer layers, full of unreacted material, continue to rush in and a gargantuan flurry of fusion creates an immense explosion. The supernova that results is bright enough to be seen halfway across the universe.

The mindbending temperatures and pressures in supernovae create a huge array of heavy but unstable elements. Some fall apart nearly instantly, existing for millionths of a second. But the more stable ones persist. The energy which forged them was the energy released by an exploding star, which they’ll slowly release over millions, or even billions of years.

4. Uranium can help us work out the age of minerals

Uranium dating might be a good name for a website where you can meet singles, but it’s actually the name of a technique used by geologists in a study known as geochronology.

ancient-zirconAs I said before, U-238 has a half life of 4.468 billion years. When it does decay, it fragments into a chain of other isotopes, one of which is Thorium-230. Essentially, the older a rock is, the more uranium will have transmuted into thorium, and the ratio of the two can be used to find the age of the rock you’re looking at. This is known as uranium-thorium dating and can determine ages up to 500,000 years.

Older and more refined is uranium-lead dating. Lead-206 is the ultimate product of uranium decay, and looking at the ratio of the two in any mineral, geologists can tell you the age of that mineral, if anywhere from 1 million up to over 4.5 billion years, with precision of around 0.1-1%.  Given that Earth itself is only 4.543 billion years old, this lets us identify some of the oldest rocks our planet has. Like the blue crystal fragment in that picture there which, at 4.4 billion years old, is perhaps the oldest mineral ever found.

5. Nuclear reactors aren’t all created by humans

oklo-fossil-reactorSince our species discovered nuclear chain reactions, nuclear power has been a significant, albeit controversial, source of energy. It’s typically much cleaner than using coal, but if catastrophic circumstances occur, the end result is dramatically worse.

The interesting thing is that nuclear power is not new. In the small country of Gabon, on the west coast of Africa, are the Oklo fossil reactors. Mineral deposits showing what used to be a total of 16 nuclear reactors which occurred entirely naturally (one of which is shown here in this photograph). U-235 has a half life of roughly 704 million years. There’s not much around today, but 1.7 billion years ago about 3% of uranium on Earth was U-235. With the right conditions, this is enough U-235 to sustain a nuclear reaction to generate energy – which is exactly what happened in the Oklo fossil reactors.

They didn’t generate much energy, but the nuclear reactions in them were self-sustaining for a few hundred thousand years.

6. We’ve known about uranium for over 200 years

uraniteUranium was first discovered in 1789 in a mineral called pitchblende. Now more commonly known as uranite, this mineral is actually uranium oxide – mostly UO₂, but with some amounts of U₃O₈.

It’s a black mineral, as its name might imply, and given uranium’s tendency to decay and spontaneously transmute into other substances, it contains a host of other elements too, including thorium, radium, promethium, and the curious and ultra-rare technetium. You’ll also find lead, the end product of uranium decay, and helium from alpha particles. In fact, after helium was discovered in the spectrum of the Sun, it was first observed here on Earth in uranite.

7. It’s been used in glassware

Uranium glass is glass made with uranium oxides. It has an unusual lime green colour and used to be quite popular. It goes by a number of different names, including vaseline glass, Burmese glass, and custard glass.

While typically made with about 2% uranium, there were some pieces of uranium glass made in the 20th century containing up to 25% uranium! Uranium glass only fell out of fashion due to the cold war, when uranium became significantly more difficult to buy.

 In fact, using uranium in glassware dates back as far as the Ancient Romans. Nearly 2000 years ago, it was used in ceramic glazes to give a yellow colour, and glass containing 1% uranium was discovered in a Roman villa in Naples.

Another fun thing is that uranium actually fluoresces. Shine a UV light on some uranium glass and it’ll glow with an eerie green colour not unlike comic book depictions of radioactive materials.

uranium-glassware

8. Uranium has found a few other surprising uses too

uranium-denturesOne interesting use which people have found for uranium was actually in early photographic chemicals. Uranium nitrate in particular, was used as a toner to adjust the final colour of photographic prints.

Other odd uses for uranium in the past have included lamp filaments for stage lighting, as stains and dyes for wood and leather,  and in mordants for fixing dyes in wool and silk. It was also once used in dentures, to improve their appearance! I promise I’m not making this up.

In more modern times, the organo-uranium compounds uranyl acetate and uranyl formate have been used as stains in electron microscopy, where they can increase the contrast in ultrathin sections being studied, and for small, isolated subjects like viruses and organelles.

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About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
This entry was posted in astronomy, chemistry and tagged , . Bookmark the permalink.

4 Responses to 8 Things About Uranium

  1. Pingback: An Uncomfortable Truth About Spacecraft | Supernova Condensate

  2. Pingback: [BLOG] Some Tuesday links | A Bit More Detail

  3. A most informative post, but, in the last line of the second paragraph of section 4; was that a misplaced comma, or a missing zero?

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