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Those
who followed the citation Jaro provided would discover the PNAS paper,
Deep-Earth reactor: Nuclear fission, helium, and the geomagnetic field, by D.F.
Hollenback and J.M. Herndon, PNAS, v. 98, no. 20, 25 Sept 2001. The paper
answers many of the questions being asked. Herndon has been interested for
a number of years in what might be providing the energy for the Earth's magnetic
field, with the puzzle being the relatively frequent (in geological terms) flips
in the polarity of the field.
Herndon hypothesizes that the flips result from a nuclear reactor that
operates for a period of time, then shuts down because of accumulating fission
product neutron "poisons". Later, as the fission products diffuse out of
the uranium/plutonium/other_actinides core, the reactor starts up
again.
When I
first saw media reports of this hypothesis a few weeks ago, I thought. "Of
course, how obvious". The key is the large size of the
reactor. In general, the neutron production will be proportional to
volume = (4/3)*pi*r^3. Neutron leakage out of the reactor will be
proportional to surface area = 4*pi*r^2. Thus the leakage fraction will be
proportional to 4*pi*r^2/(4/3)*pi*r^3 = 3/(pi*r), which is inversely
proportional to the radius of the reactor. The hypothesized 5 mile
diameter ball of uranium is HUGE. Neutron leakage would be
negligible. Hollenbach and Herndon describe the reactor as a fast neutron
reactor, but, in fact, with the reactor that big, the only thing that would
prevent a neutron from slowing down to speeds that will cause fissions in U-235
or Pu-239 would be either causing a fast fission of one of the many actinides or
being captured by U-238, with the eventual creation of an atom of
Pu-239.
Hollenback and Herndon use one of the standard reactor code systems,
SCALE, to investigate the evolution of a roughly five mile ball of U-235 and
U-238 from 4.5 billion years ago to the present. The code calculates the
value of k-effective, the parameter that determines whether an assembly of
fissile material and other materials will be critical.. They treat two
cases: 1) at each time step, fission product poisons are simply removed from the
system, and 2) fission product poisons accumulate in the system. In the
first case, k-eff begins around 1.8, drops steadily, and settles into a steady
state around 1.02 about 2.5 billion years ago. In the second case, k-eff
drops below 1.0 at around 2.7 billion years ago. The two cases probably
bound the behavior of the real system. The authors hypothesize, but do not
yet support with calculation, that poisons will shut the reactor down, then
would diffuse/float out of the system over a relatively short time, after which
the reactor would start up again. They posit the on/off oscillations as
the source of the magnetic field flips.
Table
2 of the paper summarizes the quanties of actinides present at different
times. Beginning 4.5 billion years ago with approximately 40% U-235 and
60% U-238, by 3 billion years ago, there are a wide variety of actinides
present, dominated by Th-230, Th-232, U-233 (fissile), U-235 (fissile), U-236,
U-238, Np-237, and Pu-239 (fissile), with the quanties of Th-232, U-235, and
U-238 being 2-4 orders of magnitude greater than the others.
They
argue is some detail, that this hypothesis is consistent with the observed ratio
of He-3 to He-4 in the deep mantle (as evidenced by the ratio in magma from some
volcanoes).
The
paper is extremely interesting and is available online from www.nas.edu.
Best
regards.
Jim
Dukelow
Pacific Northwest National Laboratory
Richland, WA
These
comments are mine and have not been reviewed and/or approved by may management
or by the U.S. Department of Energy.
-----Original Message-----
From: High Plains Drifter [mailto:magna1@jps.net] Sent: Friday, July 19, 2002 6:51 AM To: Kim D. Merritt; radsafe@list.vanderbilt.edu Subject: Re: RE: Nuclear Planet in DISCOVER Kim, very good point. The fluid
dynamics of fission product movement in a natural homogenous
reactor at the center of the earth would be complicated to say the least.
As it heated up and melt was added the k-effective would go down for one
thing. Just my gut feelings.
H. Dean Chaney, CHP
URS Corp. Sacramento, CA (916) 679-2086 "In science there is only physics; everything else
is stamp
collecting."
--Ernest Rutherford
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