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A natural nuclear reactor

 
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Hello everyone,

Brace yourselves, a long post ahead.  I had wanted to add this to the Permaculture Acceptable Nuclear Reactor thread located HEREMolten Salt Reactor, but I decided it deserved its own thread.  If some of the concepts (half-life, fission, decay, what does a nuclear moderator do, etc.) are unfamiliar, the original post is a great place to brush up.

So in an earlier post I mentioned that the first nuclear reactor is generally considered to be the Chicago Pile #1 reactor.  While that is the generally accepted first reactor, believe it or not, there was a previous reactor that started up some time earlier—as in 2 billion years earlier!

That’s right, there is solid geological evidence that there existed a NATURAL nuclear reactor!  But first a bit of background.  In the late 40s and early 50s, The United States along with Great Britain, France, Belgium, and basically the rest of NATO tried to corner the market on Uranium as it was (incredibly wrongly) assumed that the Soviet Union had no supply of Uranium.  With that in mind, all of the then-known sources or suspected sources were explored, mapped and documented.  Among the findings was that in all of these areas, the ratio of U235 (the useful fuel part) to U238 (the “inert” part) was always right around 0.7% U235 to 99.3% U 238 (with a very small variance hardly worth documenting among them).  This in fact is exactly what is expected given the creation of all Uranium from a supernova about 5 billion years ago.  However, there was a region in what is today Gabon, West Africa that had a supply of Uranium ore, but it was considered to be unusually low quality, meaning that instead of being 0.7% U235, it was more like around 0.3-0.4% U235 with there being a strangely large variance between samples located fairly close to each other.  No other source of Uranium ore showed either the low levels of U235, nor the weirdly large differential in concentration.  At first, this was a real mystery as to why this one spot was so different.

The reason has to do with the moon.  It is established today that the moon, due to tidal interactions and orbital mechanics, recedes away from the earth at the rate of about one inch per year.  Backtrack 2 billion years and the moon was a LOT closer to the earth (actually, exponentially close.  That 1 inch per year figure is SLOWLY reducing).  The effects of a much closer moon upon the earth were dramatic.  For example, tides were enormous 2 billion years ago.  Today’s tidal bulge is about 2 feet or so (based on the rise of water on the ocean as it interacts with shoreline).  2 billion years ago it was more like 100-200 feet!  At high tide, water crashed inland for miles, utterly inundating coast lands (today’s record high-tide zone is the Bay of Fundy which gets something like a 45’ high tide).
In the area that would eventually become the Gabon region, there was Uranium ore that apparently had freshly emerged from the earth through volcanic vents and froze into porous rock.  It was close enough to the coast that at high tides, water would flood the ore.  Water, being an excellent moderator, enabled a nuclear reaction that took place right in the rocks.  Based on calculations, the ore-bearing rocks heated up enough that they boiled the water, and after high tide receded, the rocks were hot enough that they boiled the remaining water right out of the rocks and dried themselves out, thus snuffing out the nuclear reaction!  Based on decay rates and daughter products, the Gabon region of Africa actually has taught us quite a bit about the nature of radioactive decay and the spread of radioactive waste.   These natural reactors likely ran for about 100,000 years before shutting down, and it is not entirely clear if they ran out of fuel or if the tides stopped moderating the neutrons or if neutron poisons built up and stopped the reaction.  

These areas can’t operate today, and it is thought that no place on earth today could sustain a natural nuclear reactor because of the natural decay of U235.  2 billion years ago, there was something like around 10 times as much U235 in natural uranium ore than the 0.7% that exists today thanks to radioactive decay of U235 with its roughly 700 million year half life.  The rough estimate is that natural uranium ore 2 billion years ago was closer to 95% U238 (not a fuel) and 5% U235 (a very useful fuel).  Today, the thinking is that ALL Uranium ore deposits have undergone enough decay that there is simply not enough U235 concentration in the ore to sustain a chain reaction unless incredibly heavily moderated (the Chicago Pile #1 reactor used “natural” uranium and it just barely produced power).

I realize that this is a somewhat unusual post on a gardening site.  I am not trying to steer this into the politics of nuclear, I am only trying to point out that this seemingly unlikely event actually once took place.  For those who found the Molten Salt Reactor concept interesting, even if only for the physics behind the reactor, I thought this was a nice addendum to that older post.  If you find this interesting, I would love to hear your thoughts

Eric
 
Eric Hanson
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Just to reassure that this phenomenon won’t happen today, I just looked up specs from the Chicago Pile 1 when it first operated in November 1942, the whole thing weighed a bit over 400 tons and was fueled by 45 tons of uranium oxide and 5 tons of pure uranium metal.  The rest was graphite which is an excellent moderator.  The uranium was pure (no other chemicals around it) but not enriched (meaning it was 0.7% U235 fuel to 99.3% U238 inert).

Upon initial operation when the whole thing was turned on, the output was less than spectacular—1/2 watt!

It is hard to imagine how these circumstances could possibly happen accidentally in nature today.

Eric
 
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