과학 [MP3 for IELTS] Mellow Yellow (ABC - Great moment)

[초롱이]

Mellow Yellow
 
 

The Australian scientist, Sir Gus Nossal, has said that the problem of radioactive waste already exists, and so we have to find a permanent solution. Fanciful solutions include letting tectonic plates carry this waste deep under continents, shooting it into the Sun, or sinking it into the Polar ice. What most people don’t know is that there were nuclear reactors on our planet two billion years ago, and that their radioactive wastes stayed safely buried from then till now. Two billion years (that’s 2,000 million years) is an immense period of time – animal life left the oceans to invade the land only 400 million years ago, and humans have been around for no more than 3-5 million years.

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

Uranium is a soft white metal, about nineteen times more dense than water. It was first recognised as a separate element back in 1789. Uranium salts were used to tint pottery glazes a yellow colour 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. Uranium burns easily at 170 o C, but if you grind it up very finely, it will burst into flame as soon as you expose it to air. Uranium is also the fuel of nuclear reactors.

Natural uranium is a mixture of the three isotopes of uranium, but only one of them, U-235, can be used in reactors or bombs. In technical terms, U-235 is the only isotope of uranium that goes "bang". U-235 makes up only a very small percentage of natural uranium – about 0.72%. But two billion years ago, the universe was a lot younger, and the level of U-235 was about 3% - high enough to be used in today’s nuclear reactors. And that’s when the story of the natural nuclear reactor begins - about two billion years ago. Tinkering with these percentages gives you terms such as "enriched uranium", "depleted uranium", "weapons-grade uranium", and so on.

But it was a special set of climatic and geological conditions that brought together many thousands of tonnes of uranium in an underground bed. It ended up being 5-10 metres thick and 600-900 metres wide – and was located in what is now Oklo, in Gabon in West Africa.

The uranium in this bed was decaying and giving off so-called "fast neutrons", which cannot make U-235 go bang. Only so-called "slow neutrons" can do that. But the land above shifted, and this subterranean bed cracked, allowing water to trickle through the bed of uranium. The water turned the fast neutrons into slow neutrons, and, lo and behold, the first nuclear reactor on the planet fired up, breaking the U-235 into two smaller atoms, and giving off heat. About 30 minutes later, the reactor was so hot that the water turned into steam and escaped. 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, which provided more slow neutrons.

And so the reactor ran like a geyser in a volcanic park, on for half-an-hour, and off for two-to-three hours. It stuttered along like this for a few hundred thousand years, until the U-235 was burnt up. So far, we have discovered some 18 of these burnt-out natural nuclear reactors in Oklo. They were not particularly efficient, and generated an average power of less than 100 kW – enough to run a few dozen toasters.

The reactors were discovered when alert technicians noticed that the percentage of U-235 was lower than normal. This implied that some of the U-235 had been already burnt up, as if it had been in some kind of nuclear reactor – which it had.

Today, we are stuck with several hundred thousand tonnes of radioactive wastes, and well-meaning political action has stopped the plans for long-term safe disposal. But 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...

© Karl S. Kruszelnicki Pty Ltd 2007.