**THIS STORY APPEARED IN DR KARL'S 26th BOOK PLEASE EXPLAIN**

Mellow Yellow

Australian scientist, Sir Gus Nossal, has said that because the problem of radioactive waste already exists, we must address the issue properly and find a permanent solution to disposing of it safely. Permanent solutions are better than fanciful ones, e.g. allowing tectonic plates to carry our nuclear waste deep under continents, shooting it into the Sun or sinking it into the polar ice.
Nuclear waste is not a relatively recent development - it's been around for billions of years. In fact, there were natural nuclear reactors on Earth nearly two billion years ago, and their radioactive wastes have remained safely buried until now.

Meet Uranium

Uranium (symbol U) is a soft, white metal, about 19 times more dense than water. It was first discovered in 1789 by the German chemist, M.H. Klaproth, who named it in honour of the planet, Uranus, discovered only eight years earlier.

Uranium burns easily at 170°C, but if you grind it up very finely, it will burst into flames as soon as it is exposed to air. Natural uranium is a mixture of three radioactive isotopes or varieties of uranium. They are almost identical chemically but each has a different weight.

No matter where you dig it out of the ground, the ratio of these different isotopes is the same everywhere on Earth. The ratio is approximately 99.28% uranium 238 (U-238), 0.71% uranium 235 (U-235) and 0.006% uranium 234 (U-234). Tinkering with these percentages gives you such terms as ‘enriched uranium', ‘depleted uranium' and ‘weapons-grade uranium'.

In other words, if you processed some uranium ore from the ground and extracted 100 g of pure, 100% uranium from it, 99.28 g will be the other two natural isotopes of uranium. The remaining 0.72 g will be U-235. This is what is meant by the ‘level' or
‘concentration' of U-235.

In technical terms U-235 is the only isotope of uranium that goes ‘bang'. In an enriched concentration it is the only isotope that can be used in nuclear reactors and in nuclear bombs. Natural nuclear reactors cannot exist today, because the present, natural levels of U-235 are much too low. This is why it took the combined brain power of a bunch of Nobel Prize-winning physicists, led by Enrico Fermi, to design, build and run the first artificial nuclear reactor in what had been a squash court at the University of Chicago on 2 December 1942.

Two Billion Years Ago

But two billion years ago, when the Universe was a lot younger, the level of U-235 was about 3% - easily high enough to be used in today's nuclear reactors. And it was then that the story of the natural nuclear reactor began.
A special set of climatic and geological conditions brought together many thousands of tonnes of uranium in an underground bed. The uranium, in seams 5-10 m thick and 600-900 m wide, was located in what is now Oklo, in Gabon in West Africa.

1 - U into River Beds

Originally, the uranium was deposited in rocks over a very large area of approximately 35 000 km2. Over hundreds of millions of years, rain eroded the rocks. The water washed the uranium down from the hills dumping it into the rivers, in the same way that other heavy elements like gold end up in rivers today. In this first stage, the uranium was concentrated into the river beds. At this time, there was very little oxygen in the atmosphere.

(Illustration by Adam Yazxhi)

Luck

It took quite a few coincidences for the natural nuclear reactors at Oklo to exist.
First, you need the appropriate geological and biological accidents to bring the uranium together in one spot, and then concentrate it even further.

Second, according to physicists, you need the percentage of U-235 to be at least 1%. This means that natural nuclear reactors could have operated at any time until about 400 million years ago. In fact when the Earth formed about 17% of natural uranium was U-235. Two billion years ago, it would have been ridiculously easy for a scientist to build a nuclear reactor. But it was harder for nature to build one.

Third, you need the concentrated ore to be in seams at least two-thirds of a metre thick. If the seams are any thinner, too many neutrons would escape and the nuclear reaction would not be self sustaining.

Fourth, you need something like water around the uranium. As uranium atoms split (which they do all the time) they give off neutrons. The neutrons have to be slowed down. Normally, they fly away with a very high energy, and end up becoming absorbed by U-238. However, if you slow
them down with hydrogen (which is part of water, H2O), they are much more likely to be absorbed by U-235.

Finally, a nuclear reactor cannot work if there are large quantities of elements that absorb neutrons. Nuclear physicists call these elements (e.g. lithium and boron) ‘poisons'. Luckily, there were no ‘poisons' in the earth at Oklo.

2 - Dissolve U with Oxygen

The second stage began about two billion years ago with the appearance of blue-green algae - the first creatures able to carry out photosynthesis, i.e. they are able to get energy from sunlight.

This gave them a tremendous biological advantage. While every other creature had to find and eat food to get energy, all the bluegreen algae had to do was float around and soak up the sunlight. This algae multiplied and began to release huge amounts of a very corrosive by-product into the atmosphere - oxygen.

This oxygen dissolved in the waters, oxidising the uranium - and, as it turns out, oxidised uranium is quite soluble. Therefore, the uranium that had been lying in little pockets on the bottom of the creeks and rivers dissolved into the water, the natural flow of the rivers carrying it downstream.

Half-life
The half-life of a radioactive element is the time needed for half of the original atoms to spontaneously decay.

For example, if you start with two million atoms of U-234, it will take 247 000 years for them to decay to just one million atoms of U-234. The half-life for U-235 is 713 million years, while the half-life for U-238 is longer again at 4.51 billion years.
So two billion years ago, when the Earth was 60% of its current age and the Universe was 85% of its current age, there was a lot more U-235 in the Universe because it hadn't yet decayed. Natural uranium would have had about 3% U-235 - enough to fuel a nuclear reactor.

3 - Concentrate U

The uranium floated in solution until it reached the delta of the river system. Because the organic ooze at the bottom of the river delta was very low in oxygen, the waters immediately above the ooze were also very low in oxygen.
The uranium deoxidised, became less soluble and fell out of solution into the ooze at the delta mouth. The uranium that had originally covered 35 000 km2 was now concentrated at Oklo into area of a few square kilometres.

4 - Cover, Add Water

Over millions more years, the river carried down more rocks and sand, dumping them on top of the uranium. The uranium ore was squashed by the weight of these upper layers to make a layer of radioactive sandstone. At this stage, the uranium ore was now 50-500 m underground. Then the earth moved and the sandstone cracked. As it cracked, the uranium ore body fractured and water began to trickle from above through 50-500 m of ‘overburden' down to the ore.

The water did two very important things. First, the water dissolved some of the uranium and carried it further underground. As the water dried out, it left behind small pockets of very concentrated ore. These pockets were about the size of a back-yard swimming pool.

5 - Natural Nuclear Reactor

Second, the water reacted with the neutrons coming off the U-238 as it decayed. It modified these neutrons so that they were able to split the atoms of the U-235. It turned them from ‘fast' neutrons to ‘slow' neutrons.
The so-called ‘fast' neutrons cannot make U-235 go bang. Only so-called ‘slow' neutrons can do this.

When Uranium Splits ...

When an atom of U-235 absorbs a neutron, the nucleus gets excited, changing its shape from round to eggshaped.
It will return to its original shape 15% of the time. But 85% of the time, it will continue to change shape until it looks like a peanut shell. The nucleus will then give off either 2 or 3 neutrons and split into two unequal fragments. These fragments are radioactive and, in time, will themselves decay further. Eventually, the single atom of U-235 will give rise to about 30 different stable daughter elements.
At Oklo, more than half of these daughter elements are still present in the ore body. The only ones missing are those that dissolve in water, and the gases.

Lo and behold, almost two billion years ago, the first nuclear reactor on Earth fired up, breaking the U-235 into smaller atoms and giving off heat. At the same time, it also created some radioactive waste. About 30 minutes later, the reactor was so hot that the water turned into steam and escaped, pushing its way through the soil. No water meant no slow neutrons and the reactor switched off. After a few hours, the reactor had cooled down enough to allow more liquid water in, this providing more slow neutrons. And so it started up again.

The reactor ran like a geyser in a volcanic park - on for half an hour and off for 2-3 hours. It stuttered along like this for a few hundred thousand years, until the U-235 was ‘burnt' to make waste, about 50-500 m underground. Until it was mined, the uranium had moved fewer than 4 m from where it was made nearly two billion years ago.
So far, we have discovered about 18 of these burnt-out natural nuclear reactors in Oklo. They were not particularly efficient, each one generating an average power of less than 100 kW - enough to run a few dozen toasters.

How Do We Know?

The detective story began in the early 1970s when a nuclear fuel processing plant in France was sent uranium from a mine in Oklo. Strangely, the uranium ore had slightly lower than normal levels of U-235. By itself this was very unusual but only a very careful
technician would have noticed. Later, however, technicians found some samples that had much less than the normal level - 0.44% U-235 (instead of the standard 0.72%)!

In a nuclear reactor, U-235 ‘burns' and turns into two other elements. So the technicians went looking for some of these elements that are the by-products of a nuclear fission, U-235 reaction. They found them in the unprocessed uranium ore. The conclusion was obvious, but unbelievable. There had been a nuclear power plant at Oklo, in Gabon, nearly two billion years ago.

Two billion years (that's 2000 million years) is an immense period of time. Animal life left the oceans to invade the land only 400 million years ago and human beings have been around for no more than 3-5 million years.

Problem

Since the 1960s, approximately 400 nuclear reactors in about 30 countries have produced around 200 000 tonnes of radioactive wastes. Each year, another 10 000 tonnes are added. The wastes are usually stored above ground on the site where they are generated, definitely not a permanent solution.

Today we are stuck with several hundred thousand tonnes of radioactive wastes. For better or for worse, well-meaning political action has prevented plans for their long-term safe disposal from occurring.

From Curiosity to Nuclear Reactor

Not regarded as dangerous, uranium was merely a colouring agent and a laboratory curiosity until the 1920s. Uranium salts were used to give glass and pottery glazes (including household crockery) a yellow colour that was still being done until the 1960s. If you have a pair of World War II binoculars with yellow-tinted lenses, the yellow colour is probably due to uranium.

Then came the discovery of radium, a highly radioactive element extracted from uranium ore. It was assumed to have health benefits and was even added to tonics! Radium also became the miracle cure for cancers. So radium was extracted from the uranium and the tailings simply thrown away or stockpiled. But during World War II, these old tailings suddenly became very valuable. All three isotopes of uranium are radioactive, but the military were after just one of them - U-235. It was used in the first nuclear bomb detonated over Hiroshima in Japan on 6 August 1945, killing more than 75 000 people. U-235 is also the principal fuel used for the generation of electricity by nuclear reactors.

Solution?

The 18 natural nuclear reactor sites of Oklo have given us a good example of how nature itself kept radioactive wastes safely buried for two billion years.
Unfortunately, digging a hole a few kilometres into the ground to bury nuclear wastes is too expensive for most governments. It is cheaper to leave nuclear waste in steel drums in a shallow, plasticlined ditch. However, in Switzerland, where 40% of their electricity requirements comes from five nuclear power plants, they have ‘bitten the bullet'. They are taking 40 years and spending $3 billion to bury their nuclear wastes 1.2 km underground - 5% of the cost of generating the electricity.

It sounds easy. Just dig a hole a few kilometres deep in a geologically stable area, far away from any water table or aquifer.
Then, at the bottom of the hole, dig out a large number of horizontal side-tunnels - all radiating from the central hole like the spokes of a bicycle wheel -and fill these tunnels with radioactive wastes. Then come up 20 m and fill the central tunnel with 20 m of concrete. Keep repeating the entire process until you get to within a kilometre of the surface, then stop digging side-tunnels and fill the hole with 1 km of concrete. Yes, this would be very expensive and, at the same time, very safe.

© Karl S. Kruszelnicki Pty Ltd 2007

References

Chapman, Neil and McKinley, Ian, ‘Radioactive waste: back to the future?',
New Scientist, 5 May 1990, pp 36-40.
Cowan, George A., ‘A natural fission reactor', Scientific American, July 1976,
Vol 235, No 1, pp 36-47.
de Laeter, J.R., ‘The Oklo reactors: natural analogues to nuclear waste
repositories' Search, Vol 16, No 7-8, August/September 1985, pp 193-196.
Meshik, Alex, ‘The workings of an ancient nuclear reactor', Scientific American,
November 2005, pp 82-91.
Weiss, Peter, ‘Primordial nukes: the 2-billion-year-old tale of Earth's natural
nuclear reactors', Science News, 12 March 2005, pp 170-172.