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About two billion years ago, uranium ore at Oklo, in today's Gabon, started a self-sustaining chain reaction of splitting atoms, as a nuclear power station does, with no human involved.

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In May 1972, routine tests at a French uranium enrichment plant at Pierrelatte found that ore from the Oklo mine held only 0.60 per cent of uranium-235, the form that splits easily, instead of the 0.72 per cent found everywhere else. Some of it had been used up long ago. Uranium-235 decays faster than uranium-238, so in the distant past natural uranium held about 3 per cent of it, close to the fuel of some modern reactors. Groundwater soaking the rich ore slowed the neutrons enough to keep a chain reaction going. Traces of xenon gas show a rhythm of about 30 minutes of activity, then two and a half hours of pause while the water boiled off and seeped back. Sixteen reactor zones are known at Oklo, still the only place on Earth where this has been found.

Türkçe oku
Yaklaşık iki milyar yıl önce, bugünkü Gabon'daki Oklo'da uranyum cevheri kendiliğinden bir nükleer reaktör gibi çalışmaya başladı: atom çekirdekleri, bir nükleer santraldeki gibi zincirleme bir tepkimeyle bölünüyordu.

Mayıs 1972'de Fransa'nın Pierrelatte kentindeki bir uranyum zenginleştirme tesisinde yapılan olağan ölçümler, Oklo madeninden gelen cevherde kolayca bölünebilen uranyum-235'in oranının, her yerde görülen yüzde 0,72 yerine yalnızca yüzde 0,60 olduğunu ortaya koydu. Eksik uranyum çok uzun zaman önce tüketilmişti. Uranyum-235, uranyum-238'den daha hızlı bozunur; bu yüzden uzak geçmişte doğal uranyumun yaklaşık yüzde 3'ü uranyum-235'ti. Bu oran, bazı günümüz reaktörlerinin yakıtına yakındır. Zengin cevheri ıslatan yeraltı suyu nötronları yavaşlattı ve zincirleme tepkimenin sürmesini sağladı. Kayaçlarda kalan ksenon gazı izleri bir ritim olduğunu gösteriyor: yaklaşık 30 dakika çalışma, ardından su kaynayıp gidene ve yeniden sızana kadar iki buçuk saat duraklama. Oklo'da bilinen on altı reaktör bölgesi var; burası, bunun yaşandığı bilinen tek yer olmayı sürdürüyor.

Sources and further reading

Background reading: Natural nuclear fission reactor
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