Number 706, October 27, 2004
by Phil Schewe and Ben Stein
A Natural Nuclear Reactor In Gabon
Since uranium-235 undergoes self-sustaining fission in commercial reactors
and since uranium lies in the Earth in great quantities, Paul Kuroda
predicted that naturally operating reactors are possible under special
conditions. Not nowadays, when the ratio of uranium-235 to uranium-238
is only about 0.7%, but in the past, when the ratio was much higher
owing to the fact that U-235 has a shorter half life than U-238.
The conditions necessary for self-sustained fission would be as follows:
a uranium deposit where U-235 was present at the 3% level (the level
at which modern reactors operate); the presence of material (such as
water, carbon, and most organic compounds) that could moderate, or slow
down, the neutrons issuing from fission reactions; and the absence of
material (such as Fe, K, Be, Gd) that would absorb the neutrons outright.
In 1972, such a natural reactor was found at the Oklo mine in Gabon,
in West Africa. There a 2-billion-year-old uranium deposit some 5-10
meters thick and 600-900 meters wide was bathed by an ancient river.
This “reactor” is reckoned to have released 15 gigawatt-years of energy
and operated at an average power of 100 kilowatts.
Now physicists at Washington University in St. Louis have defined a
likely mode of operation for this ancient reactor and confirmed one
of the proposed mechanisms of its self regulation. According to Alex
Meshik (am@wustl.edu), the reactor cycled on (producing heat that boiled
the nearby water) typically for 30 minutes and then off (when the now-scarce
water failed to moderate the nuclear fission process) typically for
2.5 hours.
This cycling saga is deduced from microscopic mass-spectrometric examination
of the rock samples from the area. Meshik says that tiny alumophosphate
grains found in the material of ancient reactor preserve a signature
of the reactor's operational mode. "It is fascinating that xenon isotopic
composition measured today provides us with such pristine timing records
for a natural reactor operated 2 billion years ago." (Meshik
et al., Physical Review Letters, 29 October 2004.)
Swimming In Newtonian Space
Michael Longo, a physicist at the University of Michigan, suggests
that a satellite going around the Earth can change its orbit by proper
gymnastic alteration of its shape, without the need for any external
force other than gravity. This proposition has been previously made
by invoking the properties of curved spacetime at the heart of general
relativity which predicted a very tiny effect (Science,
28 February and 21 March 2003), but Longo shows that Newtonian physics
predicts a much larger effect.
Consider, he says, a dumbbell in Earth orbit. The outer mass feels slightly
less gravity than the inner mass. If the masses are pulled together
the differential force on the separate masses will cause the center-of-mass
of the dumbbell to move inward slightly. How slightly? For a 100-meter-sized
dumbbell in a highly elliptical orbit, the object's perigee (the point
of closest approach to Earth) can move inward 1 mm on each pass. In
effect, the satellite could use solar energy or other stored energy
to change its orbit, without the need for propellant. (Longo,
American Journal of Physics, October 2004.)
Supernova Debris On Earth
Supernova debris on earth,in the form of deposits of iron-60, a
radioactive isotope of iron occurring on our planet at much smaller
levels, has been studied by German physicists. The same team of scientists
reported first signs of the deposits five years ago (Update
437). Back then they analyzed three layers of South Pacific sediment,
each over 2 million years thick in geologic time.
The new measurements, acquired at a site some 3000 km away, are much
more robust: 28 layers (rather than 3), from deeper depths (4830 m rather
than 1300 m), with a better dating method (beryllium-10 dating) and
a more accurate estimate of the layers' age (in some cases to within
a few 100,000 years). On the basis of their measurement, the researchers
deduce that the samples represent the remains of a star that exploded
2.8 million years ago (with an uncertainty of 0.3 million years) at
a distance from Earth of some tens of parsecs.
What, if any, were the implications of this splash of foreign matter
at the time? Gunther Korschinek at the Technische Universitat Muenchen
(gunther.korschinek@ph.tum.de) says that depending on exactly how far
away the supernova was, it might have had caused an increase in cosmic
ray flux for about 300,000 years. (Knie
et al., Physical Review Letters, 22 October 2004; accelerator-analysis
website at http://www.bl.physik.uni-muenchen.de/gams/index.html)
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