[meteorite-list] Earths core

From: Göran Axelsson <axelsson_at_meteoritecentral.com>
Date: Mon Aug 9 11:47:10 2004
Message-ID: <41179648.4070307_at_acc.umu.se>

I'm not a nuclear engineer but a physicist and an amateur geologist so I
add my 2 cents
of worth to the discussion. I'm a bit sceptic that there should be any
natural reactors
in the earth core. I'm not familiar with the magazine and I don't know
the quality of
their articles... but here I go...

You are forgetting that the deeper you go the less gravity you have and
the core is
solid too. The fissible material should be evenly spread through the
core and as
we know in the mantle and the crust too. You need an enrichment process to
get the fissible material dense enough to start a chain reaction.
In the Oklo natural reactor the ore were rich enough and the process were
moderated by water. When the heat became too high the water turned into
steam and the effect went down.
Read more at http://www.curtin.edu.au/curtin/centre/waisrc/OKLO/index.shtml

If there have been any major nuclear chainreactions during geological times
then we should detect slight changes in the isotopic composition of uranium,
that was how Oklo was discovered.

I thought the idea that the heat of the earth core came from gravitational
sources only were disputed in the late 19'th century and when radioactive
decay were discovered the natural decay of K U and Th was enough to make
up for the missing energy. No "fission reactor" were necessary to speed up
the process to create the heat needed to keep the Earth volcanic active.

The reason that the other planets mostly lacks volcanic activity is that the
earth is the biggest of the rock planets and the volume (fissionable matter)
is proportional to the cube of the radius while the surface area is
proportional to the square of the radius. With only a tenth of the mass
of the earth to heat a fourth of the area of the earth it isn't hard to see
why Mars have so much less volcanic activities. It just have a more
efficient cooling solution.

Btw, the current relationship between U235 and U238 isn't high enough
today that a natural reactor could form. That's why we need to raise the
grade of the uranium from 0.72% to 3.5% to 5% that is used in reactors.
That fact by it self would say that the scientists will not find any reactor
inside the core!
The half life of uranium is 4.5 billion years and the Oklo reactor worked
2 billion years ago when the ratio were higher.

I would like to know who the scientists are that said this :

"They point out that natural nuclear reactors exist on earth already, in
area's where uranium is sufficiently concentrated in the rock, it has
undergone fission."

As far as I know Oklo is the only known natural reactor and that shut
down a loooong time ago. Maybe it should read "existed"

And it's NOT common sense that heavy elements should go deep into the
earth. Many elements that occurs in low levels are concentrated by
geochemical processes (copper, gold, lead) or dispersed in vast
quantities of
silicates (rare earth metals). Copper is a lot rarer in the crust than the
rare earth elements but the latter is very rarely concentrated in ores.

Anyone wanna bet some money on the discovery of an active core
reactor? I'm taking your monetary bets now.

This is of course a gross simplification based mostly on my memory and
that's usually fatal... :-)


Marc D. Fries wrote:

> A quick two cents... It makes sense that the heaviest elements will
>sink to the center of a planetary body, and that far more than enough
>would accumulate to sustain a nuclear reaction. The problem that nags
>at me is - how to moderate the reactor? In a nuclear reactor,
>moderator rods (graphite, I think...) are used to slow down fission
>neutrons enough that they can be absorbed by the fuel to sustain the
>reaction. Recall that the first nuclear reactor, under the stadium in
>Chicago, was composed of fissile material encased in graphite moderator
>blocks. The "natural reactors" in uranium deposits in Africa show
>signs that they only operated during periods of heavy rainfall -
>groundwater perculating through the deposits would act as a moderator
>and allow the deposit to sustain a nuclear reaction.
> So - what would act as a moderator in the core of a planet? I'm going
>way out on a limb here, but I bet that groundwater is right out...!
> Is it possible that a reactor in the core would simply be large enough
>to self-moderate? Any nuclear engineers out there care to have a shot
>at this?
>>Just been reading an interesting article in 'New scientist' (this
>>It is about the centre of the Earth (i.e the core) , apparently there is
>>simply far too much heat to be explained by the conventional 'still
>>molten since it formed' theory (or from gravitation actions).
>>One theory that is being taken seriously is that fissile radioactive
>>elements (These being the heaviest elements) would sink into the core
>>mixture) and start a nuclear chain reaction, so the extra heat could be
>>generated from nuclear processes. Scientists are looking for the tell
>>tale anti neutrino's that could indicate nuclear reactions going on.
>>They point out that natural nuclear reactors exist on earth already, in
>>area's where uranium is sufficiently concentrated in the rock, it has
>>undergone fission.
>>If this where the case, there ought to be similar processes going on
>>other planetary bodies (indeed this might explain why mars still appears
>>to have volcanism when it shouldn't really have, for it's size?).
>>My question:
>>Would we not expect to find iron meteorites with nuclear reaction
>>by-products or even higher than normal un-reacted radioisotope
>>concentrations - if this were feasible?
>>Or is it a case of Asteroids being too small to differentiate enough for
>>the heavier elements to collect in sufficient quantities?
>>Maybe we just haven't had a sample of 'inner core' yet, and somewhere
>>out there are chunks of natural reactor!!
>>Mark Ford
>>Meteorite-list mailing list
Received on Mon 09 Aug 2004 11:20:40 AM PDT

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