a Brief history of uranium
From Stardust to the Uranium Mine
The NUI was proud to kick off our Isaac Newton Lecture series with a wonderful presentation by Professor Gustav Obermeier on the life of a Uranium atom. This two part lecture was a retrospective of the life of the universe, the solar system, humanity and, of course, Uranium deposits. It was a fascinating explanation suitable for scientists of all disciplines, but not so technical as to be out of reach to the interested layman. Below is a brief synopsis of the talk. For a full synopsis, click here.
The current scientific consensus is that 13.7 years ago the Universe erupted out of nothingness in the event now referred to as the Big Bang. Recent studies have shown on computer models that it is not only possible for something to come from nothing, but that it happens all the time, everywhere, even in the space inside a proton. However, to be clear on this point, we have no idea what the universe erupted from because there is no information on the state of the universe before the Big Bang.
In fact, we have no idea about the state of the majority of the universe, because the majority of the universe is made of dark matter and dark energy. However, we do know quite a bit about the visible universe. The visible universe, the bits we can see and feel and breath, are made of matter, and matter is made of atoms. Atoms, in turn are made of protons and neutrons and electrons.
In the natural state of an atom, protons and neutrons form the core, or nucleus, of the atom, and electrons occupy a virtual cloud around the nucleus. However, the early Universe was so hot that atoms could not yet coalesce into their natural forms. Protons, neutrons and electrons are swimming around in an indistinct plasma. Unions of protons and neutrons are swiftly torn apart by other particles, and all light from these particles is immediately reabsorbed.
After about 380 thousand years, the Universe cooled enough for atoms to take their natural shapes and began to emit radiation. When this cosmic background light was released, it was as hot and bright as the surface of a star. But with the expansion of the universe, these light waves have stretched and cooled down. This explosion of light is now detectable all around is in the form of radio waves. This event is responsible for the cosmic microwave background that won a couple of unwitting lab guys with a radiotelescope at Bell telephone a Nobel prize, and is responsible for a small percentage of the static you hear on your radio when not tuned into a particular station.
The universe at this stage was mainly Hydrogen stage with a bit of Helium and Lithium and not much else. Gravity brings these atoms together in great clouds, or protogalaxies, likely attracted by primordial black holes. These clouds have heavier condensations of matter within them which form protostars, which eventually condense into stars. Gravitational condensation heats them up and causes them to ionize (lose the attraction of their electrons) again.
This condensation continues until Hydrogen atoms are compressed together so closely that they unite to form Helium atoms, and a star is born! The same compression occurs between Helium atoms, often three at a time, creating Carbon, which is the basis of organic life on Earth. Since the light explosion which caused the Cosmic Microwave Background, there has been little new light in the universe. About 800 million years after the big bang enough of the first stars were created to claim an end to the universal “dark age,” and finally there was light in the Universe!
In a normal star like ours, and up to three times the size of our sun, this process of condensing smaller atoms into larger ones (larger meaning more protons at the nucleus) continues until it starts creating iron, which has 26 protons. The pressure is not enough to condense the nuclei any further, and iron sinks to the core of the star. Eventually the gravity of the iron accumulating at the core and the lack of outward energy as the star runs out of fissionable materials combine to collapse the entire star. The outer layers explode outward, leaving behind a dwarf star mainly composed of molten iron, which lasts for a very long time.
To condense atoms into larger elements than iron takes even larger stars. Stars with masses of 5 to 50 times the size of the sun get much higher core temperatures and fuse smaller elements into larger ones much faster. An average star lasts about ten billion years, but these larger star can burn up in mere millions of years.
When the last light elements are used up, there is not enough energy from the fission process to keep the star alight. It collapses within days, and the core is compressed to nuclear density, like a reverse big bang, and a supernova is ignited. This is the greatest show in the universe! About one happens per galaxy every hundred years. Within a few days a supernova will produce more energy than our sun produces in millions of years. The explosion is so big that sometimes it emits more light than the galaxy that contains it for a few days. Some of the “stars” you see in the sky are actually galaxies, and some are actually supernovae!
This crazy, unstable, super dense, super hot supernova is where iron atoms fuse together with other atoms and then the new atoms fuse with other atoms to create the heaviest atoms in the universe, including the hero of our story, Uranium.
Many of these heavier particles are blown into space, and many are condensed with the rest of the star material into a new, enormously dense star known as a neutron star. It is called a neutron star because the particles in its core condense proton and electrons together to create what is essentially a giant neutron the size of Manhattan spinning at rates sometimes near the speed of light, so dense a sugar cube sized portion of it would weigh more than a billion tons.
The residue from these supernovae is cast into the universe and sometimes finds a home in another gas cloud, which condenses into new stars and solar systems. Our sun is generally considered a third generation star, not born from the dust of the same two stars, but likely from several or possibly scores of different massive stars which lived hard and fast lives. These left behind supernovae, neutron stars and maybe even black holes now drifting around in the galaxy. Remnants from all of these explosions, including Uranium, eventually coalesced into the huge gas cloud that gave birth to our star and its solar system about five billion years ago.
Over 99.9% of the matter in our solar system ended up sinking into the sun. The rotation of the matter before it became a star, called a protostar, spun the other material in the solar system into a disc shape. Heavier elements that form minerals rocks and metals tended to gather closer to the sun, creating the inner planets, while lighter elements like Hydrogen and Helium lingered toward the outer edges of the solar system, creating the gas planets.
The solid crust of the earth contains on average 4 parts per million of Uranium: This means for every million atoms roughly 4 are Uranium. By comparison, gold is 5 parts per billion, or 800 times rarer and 800 times more expensive than Uranium. It was recently discovered that over 99% of gold in the universe, including the gold on Earth, was formed in the collision of two neutron stars. Therefore it is likely that all of the gold on earth’s crust was deposited by meteorites cast off by these collisions and striking the earth after the its crust was fairly well formed. This is why gold is usually found in veins or clumps.
Uranium, however, was deposited fairly evenly over the earth’s crust in its early formation. Profitable Uranium ores contain about 400 parts per million, in “clumps” much more dense than the average distribution. Unlike gold, however, which were deposited in clumps, Uranium gathered itself into clumps over many millions of years. This is because of its slight electromagnetic charge. These charges attract each other, and, literally atom by atom, through natural channels in the atomic structures of minerals. This process may lead atoms toward each other at a rate of perhaps 1 millimeter per year. In this sense these atoms “seek each other out” and make clumps on their own.
At the rate of 1 mm per year, it takes a long time for atoms to find their way to each other. This is why Uranium ores are found in places that have been relatively undisturbed for a long time – a really long time – at least 100 million years. At 800 degrees Celsius, temperatures found deeper in the crust of the earth, they move about 100 times faster. This is helpful for not-yet-evolved-human Uranium miners only if the geology is left undisturbed long after this accelerated process.
In Namibia it has been about 130 million years since the separation the great continent of Gondwana. Human ancestors at this time were rodent like primates scurrying out from under the feet of giant dinosaurs. The ancestors of you, me and all humanity on earth was about the size of a rhesus monkey and probably not half as smart. Since then the earth has had two mass extinctions, humans have evolved into respectably intelligent life forms (a matter of opinion, granted), and enough uranium atoms have gathered together in the relatively undisturbed geography of Namibia to produce several of the most profitable Uranium ores in the world.