London, May 16 : Two scientists have claimed that uranium could become sufficiently concentrated at the base of Earth's mantle to ignite self-sustained nuclear fission, as in a human-made nuclear reactor.
According to a report in Nature News, physicist Rob de Meijer of the University of the Western Cape in Cape Town, South Africa, and geochemist Wim van Westrenen of the Free University of Amsterdam in the Netherlands have come up with this idea.
Radioactive decay of unstable isotopes of heavy elements such as uranium happens all the time beneath Earth's surface. The energy released contributes significantly to the heat of Earth's mantle, which is also warmed by the planet's molten iron core.
This combined heating creates convection currents in the sluggish mantle rock that ultimately power the drift of tectonic plates at the surface, giving rise to mountain ranges and earthquakes.
Evidence of natural nuclear reactors can be seen in places like Crustal rocks at Oklo in Gabon, Africa, which bear unambiguous evidence of spontaneous ignition of uranium fission in mineral deposits 1.7 billion years ago.
De Meijer and van Westrenen's idea draws on recent discoveries about the distribution of an isotope of the rare element neodymium in rocks 2,3.
Those observations suggested that there is a 'reservoir' of material deep inside Earth, which formed soon after the birth of the planet, about 4.5 billion years ago, and has not mixed with the rest of the mantle.
The only place where such a reservoir could easily exist is at the very bottom of the mantle, at the boundary with the core.
The two researchers estimated how much uranium the reservoir could contain.
They noted that uranium and its decay product plutonium are more readily incorporated into calcium silicate perovskite, a mineral which makes up 5% of the lower mantle, than into the two other minerals that make up this part of the deep earth.
De Meijer and van Westrenen said that melting and other geological processes could quite conceivably concentrate up the fissile material further until it crosses the ignition threshold.
In fact, according to them, if there wasn't this initial shortfall, then the whole of the core-mantle boundary might conceivably have become a live nuclear reactor.
Such a reactor would probably function as a 'breeder' reactor, generating plutonium fuel as it burns the original uranium. This means that such reactors could still be running today.
As for proving the existence of these reactors, which are some 3,000 kilometres beneath our feet, De Meijer and Westrenen say that the reactions will generate very light subatomic particles called antineutrinos, which can mostly pass right through Earth and so could be detected by instruments at the surface.