The "Natural" Nuclear Reactor

Have you heard of any natural nuclear reactors, other than stars? In Oxygen, Nick Lane talks about the one in Gabon (Africa), and how that came to be. But first, remember how fission bomb works? Uranium’s isotope, U-235 decays and emits a few neutrons. Slow down those neutrons using hard water, and you’ve increased the odds of those neutrons hitting the next U-235 atom. The process starts to repeat itself. If this process can continue long enough, you have a nuclear explosion. The key, of course, is to get enough concentration of that U-235 in the first place. So how did all this happen naturally on earth?

 

The solubility of uranium in water depends on the oxygen level. As the oxygen level started rising about 2 billion years back, “oxidized uranium salts leached out of uraninite ores in the rocks and washed away in streams”. Of course, it was very dilute, fewer than a few parts per million.

 

Many such streams converged in Gabon’s shallow lakes. As it turned out, those lakes had a certain special kind of bacterial mats:

“(The bacteria) had a penchant for soluble uranium salts as an energy source. They converted the soluble uranium back into insoluble salts… Over the next 200 million years or so, the bacterial mats deposited thousands of tons of black uranium ore in their lakes.”

Today, most of the naturally occurring uranium-235 on earth has already decayed. Two billion years back, however, much less of it had decayed. Which means there was more of it still lying around. Thus it came to be that:

“The uranium-loving bacteria in Gabon therefore stockpiled enough ore enriched in uranium-235 to start a nuclear fission chain reaction.”

 

Did enough of it accumulate to set off a nuclear explosion?

 

It turns out that the U-235 had indeed fissioned away, not decayed. Fortunately that fissioning had happened across multiple locations. The water feeding the lakes had played a key role in what happened and what didn’t happen:

1)      Water had moved the U-235 around over millions of years ensuring it was never too concentrated in any one place;

2)     Water slows down the neutrons emitted during a nuclear reaction, which in turn promotes the reaction to continue;

3)     The last point was fortunately countered by another aspect:

“Whenever the chain reactions approached danger levels, water boiled off,  allowing neutrons to escape. This scuttled the chain reactions and shut down the reactors until flow was re-established.”

 

Today, there is no evidence to suggest that a nuclear explosion ever occurred naturally on earth. In Gabon then lies the “testament to the ingenuity of bacteria 1.8 billion years before Enrico Fermi and his Chicago team”!