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Title: Oklo reactors and implications for nuclear science
Author: E. D. Davis, C. R. Gould, E. I. Sharapov

We summarize the nuclear physics interests in the Oklo natural nuclear reactors, focusing particularly on developments over the past two decades. Modeling of the reactors has become increasingly sophisticated, employing Monte Carlo simulations with realistic geometries and materials that can generate both the thermal and epithermal fractions. The water content and the temperatures of the reactors have been uncertain parameters. We discuss recent work pointing to lower temperatures than earlier assumed. Nuclear cross sections are input to all Oklo modeling and we discuss a parameter, the175Lu ground state cross section for thermal neutron capture leading to the isomer176mLu, that warrants further investigation. Studies of the time dependence of dimensionless fundamental constants have been a driver for much of the recent work on Oklo. We critically review neutron resonance energy shifts and their dependence on the fine structure constantand the ratioXq=mq/\Lambda(wheremqis the average of theu anddcurrent quark masses and\Lambdais the mass scale of quantum chromodynamics). We suggest a formula for the combined sensitivity to\alpha andXqthat exhibits the dependence on proton numberZand mass numberA, potentially allowing quantum electrodynamic and quantum chromodynamic effects to be disentangled if a broader range of isotopic abundance data becomes available.

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Could Natural Nuclear Reactors Have Boosted Life on This and Other Planets?

Natural nuclear reactors occur when deposits of the radioactive element uranium build up in one spot, and eventually ignite a self-sustaining nuclear chain reaction where uranium divides, in a process called fission, producing other elements. The reaction releases a powerful punch of energy. This energy could prove beneficial and highly detrimental to developing life, depending on the circumstances.
The ionising radiation released by a nuclear reaction can damage DNA, the precious instruction code built into every cell of life. If organisms were living too close to the site of a reactor, they could have been wiped out completely. However, life hanging out on the outskirts of a nuclear reactor might have received a smaller dose of radiation - not enough to kill it, but enough to introduce mutations in its genetic code that could have boosted the diversity in the local population.

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The Two Billion Year Old Natural Fission Reactors in Gabon, Western Africa

Two billion years ago - eons before humans developed the first commercial nuclear power plants in the 1950s - seventeen natural nuclear fission reactors operated in what is today known as Gabon in Western Africa. The energy produced by these natural nuclear reactors was modest. The average power output of the Gabon reactors was about 100 kilowatts, which would power about 1,000 lightbulbs. As a comparison, commercial pressurized boiling water reactor nuclear power plants produce about 1,000 megawatts, which would power about one million lightbulbs.
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The Gabon Republic in Africa is rich in uranium. In 1972, A French factory imported uranium ore from Oklo, Gabon, and found to its surprise that the uranium had already been extracted.
Natural uranium contains 0.7 percent of uranium-235 (U-235), the fissionable isotope contained in nuclear fuel, but the uranium in Oklo contained less than 0.3 percent of uranium-235.
Scientists around the world gathered in Gabon to explore this phenomenon. They found that the site where the uranium was found is a highly technical underground nuclear reactor beyond the capabilities of our present scientific knowledge. This nuclear reactor came into being 1.8 billion years ago and was operational for about 500,000 years.

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The Speed Of Light
The speed of light,

zone15

Zone15

one of the most sacrosanct of the universal physical constants, may not be a `constant`, and may have been at a lower value as recently as two billion years ago. Re-analysis of OKLO natural fossil reactors data, that has long been used to argue for the constancy of the speed of light and other constants, reaches the opposite conclusion.
A varying speed of light contradicts Einstein's theory of relativity, and would undermine much of traditional physics. But some physicists believe it would elegantly explain puzzling cosmological phenomena such as the nearly uniform temperature of the universe.
It might also support string theories that predict extra spatial dimensions. The idea of an invariable speed of light comes from measurements of the parameter called the fine structure constant, or alpha, which dictates the strength of the electromagnetic force. The speed of light is inversely proportional to alpha, and though alpha also depends on two other constants, many physicists tend to interpret a change in alpha as a change in the speed of light.
The fine structure constant can be seen at many places. For example, the (squared) speed of electrons in the hydrogen atom is roughly 1/137 of the (squared) speed of light. As a consequence of this, the spectrum of the hydrogen atoms have the famous lines with energies 1/n2, but if you look at the lines with a better resolution, you find out that they are separated to several sublines; they form the so-called fine structure of the Hydrogen spectrum.
The distance between the main lines of the spectrum is 137 times bigger than the distance between the lines in the fine structure; therefore the name.
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Researchers

map of Oklo in Gabon

Map of Oklo in Gabon

claimed in 2001 that alpha had increased by a few parts in 105 in the past 12 billion years, by analysis of light from distant quasars, absorbed by intervening gas clouds.
However German researchers studying photons emitted by caesium and hydrogen atoms reported in 2004 that they had seen no change in alpha to within a few parts in 1015 over the period from 1999 to 2003 - though the result does not rule out that alpha was changing billions of years ago.
Throughout the debate, physicists who argued against any change in alpha have had one set of data to fall back on. It comes from the worlds only known natural nuclear reactor, found at Oklo in Gabon.
Natural fossil reactors have (so far) only been found in the country of GABON in equatorial Africa. All but one of the reactors are located at a place known as the OKLO uranium deposit located in the south eastern corner of the country.
Another fossil reactor has also been discovered in Gabon at another U deposit at Bangombe, some 35 km south east of the OKLO mine.
Radioactive clocks play important roles in deciphering the Oklo phenomenon. To date the age of the U deposit, uranium-lead, rubidium-strontium and samarium-neodymium clocks have been used.
The fact that sufficient 235>U relative to 238>U were present in the reactors is related to the difference in the decay rates of these two isotopes.
During the reactions a vast array of radioactive clocks were initiated by the fission reactions. Virtually every fission product isotopes produced was a radioactive clock. The majority of these isotopes are far too short to be of any real benefit in studying the Oklo phenomenon but there are still several dozen with half-lives of several years to millions of years or more which have been used to determine many reactor parameters including
* The time period over which each of the individual reactors operated
* How much 235U each the individual reactors "bred" from 238U
* How long it took for some of the more mobile fission products to move out from the reactor zones and
* How much of each of the fission products was retained inside the reactors.
Reactor Zone 15: Of the seventeen known fossil reactors, 9 have been completely mined out. Reactor zone 15 is the only reactor which is accessible underground through a tunnel bored into the existing mine pit. The remains of fossil reactor 15 are clearly visible as light grey/yellow coloured rock that is mostly Uranium oxide.
The light coloured streaks in the rocks above the reactor is quartz which has been crystallized from the (hot) underground waters circulating around during and after the reactor's operating lifetime.
The Oklo reactor started up nearly two billion years ago when groundwater filtered through crevices in the rocks and mixed with uranium ore to trigger a fission reaction that was sustained for hundreds of thousands of years.
Several studies that have analysed the relative concentrations of radioactive isotopes left behind at Oklo have concluded that nuclear reactions then were much the same as they are today, which implies alpha was the same too.
That's because alpha directly influences the ratio of these isotopes. In a nuclear chain reaction like the one that occurred at Oklo, the fission of each uranium-235 nucleus produces neutrons, and nearby nuclei can capture these neutrons. For example, samarium-149 captures a neutron to become samarium-150, and since the rate of neutron capture depends on the value of alpha, the ratio of the two samarium isotopes in samples collected from Oklo can be used to calculate alpha. A number of studies done since Oklo was discovered have found no change in alpha over time.
"People started quoting the reactor [data] as firm evidence that the constants hadn't changed,"
Recent re-analysis the Oklo data using more `realistic figures` for the energy spectrum of the neutrons present in the reactor have lead to a startling conclusion.
Alpha, it seems, has decreased by more than 4.5 parts in 108since Oklo was live .
That translates into a very small increase in the speed of light (assuming no change in the other constants that alpha depends on), and the new analysis is so precise that we can rule out the possibility of zero change in the speed of light.
"However, the claim is so revolutionary there should be many independent confirmations."

While Victor Flambaum of the University of New South Wales in Sydney found that alpha was different 12 billion years ago, the new Oklo result claims that alpha was changing as late as two billion years ago. If other methods confirm the Oklo finding, it will leave physicists scrambling for new theories.
For example, if it had been lower in the past, meaning a higher speed of light, it would solve the "horizon problem". Cosmologists have struggled to explain why far-flung regions of the universe are at roughly the same temperature.
It implies that these regions were once close enough to exchange energy and even out the temperature, yet current models of the early universe prevent this from happening, unless they assume an ultra-fast expansion right after the big bang. However, a higher speed of light early in the history of the universe would allow energy to pass between these areas in the form of light.
Variable "constants" would also open the door to theories that used to be off limits, such as those that break the laws of conservation of energy. And it would be a boost to versions of string theory in which extra dimensions change the constants of nature at some places in space-time.

There are a few difficulties however;
the exact conditions at Oklo aren't known.
Nuclear reactions run at different rates depending on the temperature of the reactor, And at Oklo was assumed was between 227 and 527C. the temperature could have varied far more than this.
"You need to reconstruct the temperature two billion years ago deep down in the ground"
Also the relative concentrations of samarium isotopes may not be as well determined as has been assumed, which would make it impossible to rule out an unchanging alpha.
Another unknown is whether other physical constants might have varied along with, or instead of, alpha. Samarium-149's ability to capture a neutron also depends on another constant, alpha(s), which governs the strength of the strong nuclear attraction between the nucleus and the neutron.
It has been claimed that the ratio of different elements left over from just after the big bang suggests that alpha(s) must have been different then compared with its value today.

In nuclear plants

(
update: 1st nov, 2004)

the reaction is kept under control by using 'moderators', that either slow down the chain reaction by absorbing some of the fission neutrons or encourage it by adjusting the neutron energies.
It seems that the Oklo reactors didn't plunge straight into a runaway chain reaction, or meltdown of the veins because of the presence of water in the rocks. When a uranium nucleus undergoes nuclear fission, the ejected neutrons are travelling too fast to be absorbed by other nuclei and trigger fission. So there is no chain reaction. But water slows the neutrons down. In the Oklo reactors, water allowed the chain reaction to be sustained.
New research indicate that active periods of about 30 minutes seem to have been followed by dormant spells of around two and a half hours., over a period of about 150,000 years.
"This similarity [of a geyser] suggests that a half an hour after the onset of the chain reaction, unbounded water was converted to steam, decreasing the thermal neutron flux and making the reactor sub-critical It took at least two-and-a-half hours for the reactor to cool down until fission xenon (Xe) began to retain. Then the water returned to the reactor zone, providing neutron moderation and once again establishing a self-sustaining chain."
Before this new research, it was known that the natural nuclear reactor operated two billion years ago for 150 million years at an average power of 100 kilowatts.


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New observations by the DEEP2 survey team,

19 April 2005


DEEP2 Survey

DEEP2 Survey

a collaboration led by the University of California, Berkeley, and UC Santa Cruz, have that the fine structure constant has not changed in more than 7 billion years, and show no change within one part in 30,000.
"The fine structure constant sets the strength of the electromagnetic force, which affects how atoms hold together and the energy levels within an atom. At some level, it is helping set the scale of all ordinary matter made up of atoms. This null result means theorists don't need to find an explanation for why it would change so much." - Jeffrey Newman.
DEEP2 was actually a five-year survey of galaxies more than 7- 8 billion-light years distant whose light has been stretched out or red shifted to nearly double its original wavelength by the expansion of the universe, but a subset of 300 individual galaxies from the 40,000 galaxies surveyed, and their oxygen emission lines, were used to see any variation in the fine structure constant.

fine-structure constant

Fine-structure constant

The DEEP2 data allowed the measurement of the wavelength of emission lines of ionized oxygen (OIII, oxygen that has lost two electrons) to a precision of better than 0.01 Angstroms out of 5,000 Angstroms. (An Angstrom is 10 nanometres, or about the width of a hydrogen atom).
The DEEP2 team compared the wavelengths of two OIII emission lines galaxies at various distances, ranging from a red shift of about 0.4 (approximately 4 billion years ago) to 0.8 (about 7 billion years ago). The measured fine structure constant was no different from today's value, which is approximately 1/137. There also was no upward or downward trend in the value of alpha over this 4-billion-year time period.
By using on a pair of emission lines from ionised oxygen in the actual galaxies, rather than using absorbsion lines generated by intervening clouds, the new study gets around the problems of misidentification of the lines and the introduction of theoretical assumptions which may not be correct. The absorbsion spectral lines are faint and can overlap, making it difficult to tell the lines' source, and that it must be assumed that all of the intervening clouds share the same basic composition.
With emission lines, astronomers can choose sources that have a very strong, characteristic appearance and are isolated so that there is no confusion from differing source galaxies. And because the average wavelength of the light emitted by the oxygen ions is a direct measurement of alpha, "the fine-structure constant pops out without any interpretation".

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The speed of light in a vacuum is exactly equal to 299,792,458 metres per second (1,079,252,848.8 km/h).
According to standard modern physical theory, all electromagnetic radiation, including visible light, propagates (moves) at a constant speed in a vacuum, commonly known as the speed of light, which is a physical constant denoted as c. This speed c is also the speed of propagation of gravity in the theory of general relativity.


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Title: Did natural reactors form as a consequence of the emergence of oxygenic photosynthesis during the Archean?
Authors: Laurence A. Coogan, Jay T. Cullen

The advent of oxygenic photosynthesis changed Earths surface environment in numerous ways, perhaps most notably by making possible the evolution of large and complex life-forms. Current models suggest that organisms that can perform oxygenic photosynthesis first took hold in isolated marine and freshwater basins, producing local oxygen oases. Here we present calculations that suggest that uranium deposits could have formed at the margins of these basins due to the strong local reduction-oxidation gradients. Because of the high abundance of 235U at this time, these uranium deposits could have formed widespread, near-surface, critical natural fission reactors. These natural reactors would have represented point sources of heat, ionising radiation, and free radicals. Additionally, they would have far-field effects through the production of mobile short- and long-lived radioactive daughter isotopes and toxic byproducts. It is possible that these fission products provided a negative feedback, helping to limit the proliferation of the cyanobacteria in the Archean environment. Secular decreases in the abundance of 235U in turn decreased the probability of such deposits forming critical fission reactors during the early Proterozoic.

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In May 1972 a worker at a nuclear fuelprocessing plant in France noticed something suspicious. He had been conducting a routine analysis of uranium derived from a seemingly ordinary source of ore. As is the case with all natural uranium, the material under study contained three isotopes - that is to say, three forms with differing atomic masses: uranium 238, the most abundant variety; uranium 234, the rarest; and uranium 235, the isotope that is coveted because it can sustain a nuclear chain reaction. Elsewhere in the earths crust, on the moon and even in meteorites, uranium 235 atoms make up 0.720 percent of the total. But in these samples, which came from the Oklo deposit in Gabon (a former French colony in west equatorial Africa), uranium 235 constituted just 0.717 percent. That tiny discrepancy was enough to alert French scientists that something strange had happened.

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