PHYSICS H190

Natural Nuclear Fission Reactor

Fission

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 phenomenon

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 U-235

Natural Uranium contains .7202 percent U-235

The process of measurement, mass spectroscopy, is very accurate when applied to Uranium Hexafluoride

www.euronuclear.org

Possible Explanations

Maybe it had been contaminated with some depleted Uranium?

Tests of U-236 available in reactor showed no signs of contamination

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

ocrwm.doe.gov

The Byproducts of Nuclear Fission

Recall how Nuclear Fission works.

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 radioactive

After years of decay the fission products consist of multiple isotopes of 30 different elements

By Products of Nuclear Fission and Detainment at Oklo

ocrwm.doe.gov

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 fission

Number of Fissions that have occurred isX=τσN0

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 moderator

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 meter.

A Neutron “moderator,” to increase cross section.

Lack of neutron absorbers (boron, lithium, etc.)http://

electricalandelectronics.org

A Tale of Two Half-Lives

Todays Uranium has only .72% U-235It 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 yearsU-238 has half-life of about 4.5 Billion yearsTo 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 ore

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 zones2. Sandstone3. Uranium ore layer4. 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 reactions.

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 megawatt-years

Average output at about 100 kW a day, enough to run about a few dozen toasters

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 years

Long-term storage of radioactive and hazardous material.

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

Implications

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 formulated

Implications

Models of the universe would be transformed

Room to include theories that break conservation of energy

"It's like opening a gateway," says Dmitry Budker

Sources

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