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.cfm?id=ancient-nuclear-reactor