Missing Matter in a Uranium
Mine
The Oklo Phenomenon
Initial Problem: 1972
• A worker notices that the isotopic
composition of mined uranium from the Oklo
site is missing Uranium 235
Initial Problem: 1972
• The collection of uranium is regulated to
ensure that it is not being siphoned off into
weapons programs. This lead to further
investigations of the missing mass.
• Scientists were stumped for weeks. In one
section of the mine some 200kg of Uranium
235 was missing. The author notes this is
enough to make 6 nuclear weapons.
• What is the yield in megatons?
1 megaton is 4.184 petajoules (10^15)
An Earlier Prediction
• 19 years earlier George Wetherhill and Mark
Ingrahm postulated that a natural version of
the then popular fission reactors might have
occurred.
• Paul Kuroda then formalized this prediction
with calculations and came up with three
postulates
Requirements for A Naturally
Occurring Fission Reactor
1. The size of the deposit must exceed the
average path traveled by the neutrons, about
2/3 of a meter
2. Concentration of uranium 235 must be
greater than it is today, roughly 3%
3. Some type of natural moderation must be
present
Why does the size of the deposit
matter?
Remember our reproduction factor k. For the
chain reaction to be self sustaining at least one
of the 2.5 neutrons released in the fission must
be captured by another Uranium 235 atom(k=1).
If the deposit is smaller than average length
traveled to many of these will escape and the
reaction will not continue.
Requirements for A Naturally
Occurring Fission Reactor
1. The size of the deposit must exceed the
average path traveled by the neutrons, about
2/3 of a meter
2. Concentration must be greater than it is
today, roughly 3%
3. Some type of natural moderation must be
present
Why is the concentration important?
The reaction of interest occurs when Uranium
235 captures an incident neutron becoming
Uranium 236. Uranium 236 then undergoes
fission releasing daughter products and high
energy neutrons (1 Mev).
Requirements for A Naturally
Occurring Fission Reactor
1. The size of the deposit must exceed the
average path traveled by the neutrons, about
2/3 of a meter
2. Concentration must be greater than it is
today, roughly 3%
3. Some type of natural moderation must be
present
What is moderation and why is it
important?
• Neutron moderation is when something is
used to slow high energy neutrons down via
elastic scattering
• This is important because the cross section is
small for high energy electrons and relatively
large for low energy electrons making neutron
capture much more likely.
Confirmation of the Natural Reactor
• The problem was considered solved when it
much higher than naturally occurring
concentrations of daughter products were
found in the ore samples.
• Enrico Fermi was apparently not the father of
the fission reaction as evidence showed that
this self sustained reaction took place almost
2 billion years before the Pile Reactor
Further Investigation
• After this discovery many scientist spent time
studying the Oklo Phenomenon
• A conference (party) was held in 1975 to
summarize the findings.
• George Cowan summarized these findings in
an article for Scientific American in 1976
which included the following conclusions
Properties of Oklo Reactor
• Neutron capture by Uranium 238 lead to the
production of nearly two tons of plutonium
some of which itself underwent fission.
• The reactions occurred over hundreds of
thousands of years
• The total power released was around 15,000
megawatt years
• Average power output was around 100
kilowatts. Enough to run a few dozen toasters?
How many toasters?
Wait, how much does it cost to make
toast?
12c per Kwh
What costs more the electricity or the bread?
This lead to more questions
• Why did parts of the deposit not just explode
after the reaction started?
• What mechanism is responsible for the
regulation of the reaction?
• Was the operation continuous or did it start
and stop?
Noble Gas (xenon) Epiphanies
But why Xenon?
• There are 9 isotopes of xenon produced in
various nuclear processes.
• It is chemically inert making isolation for
isotopic analysis simple
• It is extremely rare
How does xenon help answer our
question?
• The location and isotopic abundance of the element
was dramatically different from expectations.
• This was used to help support the theoretical
descriptions of the natural reactors function.
A Side Note on Sampling
• Normally materials are melted and the released
xenon is captured
• After a detailed x-ray map was made of the
constituent elements of the sample, a method
called laser extraction was used to melt single
grains of material. This allowed the researchers to
keep track not only of concentration, but what
elements it was stored in.
• The collected xenon was then placed in a highly
accurate xenon mass spectrometer to determine
the isotopic composition
The Deviations
• The heavier isotopes were missing
– Why?
• Diffusion?
– The ratios of depletion would depend on the atomic mass and they didn’t
• Chemical Reactions?
– All isotopes are chemically identical
• The abundance of xenon, in higher concentrations than
found anywhere ever, was found in grains of aluminum
phosphate
– Normally it is found in higher concentration in the uranium rich
mineral grains
The theoretical function of the reactor must be considered to
understand these deviations
Theoretical Model of Reactor
• Natural ground water provided neutron
moderation and regulation of the reaction. As
the reaction proceeded the elastic scattering
off the water molecules heated the water until
it vaporized stopping the reaction. As ground
water flooded back in the reaction could
continue as the moderator was reintroduced.
Deviations Explained
• The heavier (missing) isotopes of xenon are
produced first. And decay from water soluble
elements.
• In the initial stage of function the heavier
isotopes of xenon were produced almost
immediately and driven off with the flux of
ground water
Deviations Explained
• The aluminum phosphate probably didn’t exist
before the reactor started working
• It was formed as the ground water cooled to
around 300 degrees Celsius
• The more abundant precursors for the lighter
isotopes were included in a higher
concentration as the heavier ones had
decayed or been washed away.
Mathematical Modeling of The
Reaction
Lessons To Learn From Oklo
Nuclear Waste Storage
• Scientists are studying in detail how the waste
products have migrated away from the site
over time. This may provide insights into how
we can manage our nuclear was deposits
• The trapped xenon suggests that it may be
possible to trap waste products in aluminum
phosphate which can retain them stabally for
billions of years.
Sources
• http://www.scientificamerican.com/article.cf
m?id=ancient-nuclear-reactor