The natural nuclear reactor at Oklo

Natural Nuclear Fission Reactor
 Discovery and implementation of fission is arguably
one of the more important and controversial
scientific and technological discoveries in physics in
the 20th century
 Surprisingly this may just been a naturally occurring
 Precambrian Scientists would have found making a
fission reactor remarkably simple
A Small Discrepancy
 In 1972 at a French-Fuel
processing plant, routine
analysis of a standard
sample of UF6 showed that
it consisted of .7171 percent
 Natural Uranium contains
.7202 percent U-235
 The process of
measurement, mass
spectroscopy, is very
accurate when applied to
Uranium Hexafluoride
Possible Explanations
 Maybe it had been
contaminated with some
depleted Uranium?
 Tests of U-236 available in
reactor showed no signs of
 Uranium was traced back
to mine in Oklo where
some samples hadU-235
at %.35
 Since the isotopic
composition of Uranium
is though to be a constant
of the solar system, the
isotope shift must have
occurred at the site
The Byproducts of Nuclear Fission
 Recall how Nuclear Fission
 Absorption of Neutron by
U-235 causes it to become
unstable with %85 chance
of it splitting into two and
releasing multiple neutrons
 The fission products of
nuclear fission are
 After years of decay the
fission products consist of
multiple isotopes of 30
different elements
By Products of Nuclear Fission and Detainment
at Oklo
Evidence: By Products
 Physicists came together to share their work on “the Oklo
phenomenon” at a special 1975 conference held in
Libreville, the capital of Gabon.
 The ore at the site at Oklo contained 15 of these elements
 The analysis of Neodymium help present an argument
for there being fission reactions taking place
Its chemical abundance gives the absolute number of fissions
Provides an age of the reaction
 Note: Corrections are made based off
Natural Neodynium present
Nd-143, 145 readily absorb neutrons
Evidence: Neodymium Byproduct
Source: GA Cowan - Scientific American, 1976
Evidence for Natural Fission: Age
 By measuring Nd/U ratios the total number of
neodymium atoms formed by fission can be obtained.
 Correct for Natural Nd-142 which does not occur in
 Number of Fissions that have occurred is
 Where τ is integrated neutron flux (measured by Nd-143
Nd-144 Isotope ratios), σ is absorption cross section of
U-235, N0 is average number exposed to flux
 Solve equation Nav=N0ave-λt for t
 t≈ 1.7-1.9 X 109
Four Conditions for Fission
 Uranium 235 concentrations
must be high enough to
sustain reactions with water
 The size of the uranium
deposit is large enough to
ensure that the neutrons
given off by one fissioning
nucleus are absorbed by
another, about two thirds of a
 A Neutron “moderator,” to
increase cross section.
 Lack of neutron absorbers
(boron, lithium, etc.)
A Tale of Two Half-Lives
 Todays Uranium has only .72% U-235
 It is possible to sustain a reaction using this density
but special non-natural conditions needed such as
deuterium as a moderator.
 U-235 has half-life of about 700 Million years
 U-238 has half-life of about 4.5 Billion years
 To sustain fission with water-moderator needs to be
about 1% U-235. 1.9 Billion years ago it was about
3% U-235
Isotopic Composition History
Source: GA Cowan - Scientific American, 1976
Natural Nuclear Moderators
 Recall Moderators are
used to slow down the
neutrons before they can
be efficiently absorbed by
the fuel.
 Change brought about by
blue-green algae 2 billion
years ago provided for the
oxidization, reduction and
compactification of
Uranium into sandstone
 Here geological
transformations fracture
the uranium ore layers
allowing for the flow of
water creating pockets of
rich ore
 Here the Fission occured
The Geological Formation of a Reactor
1. Nuclear
reactor zones
2. Sandstone
3. Uranium
ore layer
4. Granite
Geological Formation of a Reactor
 A petroleum formation eventually produced an oxygen-
free zone (chemical reactions between petrolium and
water remove oxygen)
 Though Uranium is highly soluble in oxygenated waters
it is insoluble in non oxygenated water. Hence in
petroleum zones it fell out of solution.
 Eventually they became large enough to sustain fission
 Studies of Xenon found in the ore show that these
reactions were cyclical- as soon as there was too much
steam and not enough water moderator the reaction
ceased. Therefore this was not a closed system!
Energy Production
 Total Energy released
about 15,000 megawattyears
 Average output at about
100 kW a day, enough to
run about a few dozen
 Ran for over a few
hundred thousand years
 Must have spontaneously
turned on and off
What Can We Learn From Oklo?
 A nuclear reactor with no
sign of meltdowns in
hundreds of thousands of
 Long-term storage of
radioactive and hazardous
 The storage of Xenon 135,
Krypton 85 by products of
nuclear reactions in
aluminum phosphate
materials which have may
have the ability to hold these
for thousands of years
Fundamental Constants as Functions of Time?
 α=(kee2)/(ћc)
 The coupling constant determining the strength of
the interaction between electrons and photons.
 Also found in electron magnetic moment, mass, and
atomic energy levels
 1/ α=137 (a-b) Berkeley QM Courses!
 Two possible tests of this question
The spectral lines of distant astronomical objects
The products of radioactive decay in the Oklo natural nuclear
fission reactor.
How To Use Oklo to Measure Change in Alpha
 Alpha directly influences the
ratio of isotopes found at Oklo
This is because rate of
neutron capture depends on
the value of alpha
Many tests have been done
that shown alpha as constant.
One recent test (Lamoreaux
Los Alamos) reworked the
problem with different values
for the energy spectrum of the
neutrons present
This study found the value of
alpha decreased 4.5 parts in
108 since Oklo
 Many scientists believe that
the analysis done by
Lamoreaux is valid and his
assumptions sound though
the exact conditions at Oklo
were not known.
 But it is a very large and
revolutionary claim with
many implications and needs
to be confirmed many
different places independently
 If it is found and accepted to
be scientific fact then many
new theories must be
 Models of the universe
would be transformed
 Room to include theories
that break conservation
of energy
 "It's like opening a
gateway," says Dmitry
The Discovery and Study of the Nuclear Reactor in Oklo
E. ROTH- Journal of RadioanalyticalChemistry, Pol. 37 (1977) 65- 78
A Natural Fission Reactor
GA COWAN -Scientific American, 1976
Speed of light may have changed recently
ES Reich- New Scientist, 2004
Mineralogy and Geochemistry of Natural Fission Reactors in Gabon
Janueczek, J.- Reviews in Mineralogy vol. 38, 1999

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