Physical Insights

An independent scientist’s observations on society, technology, energy, science and the environment. “Modern science has been a voyage into the unknown, with a lesson in humility waiting at every stop. Many passengers would rather have stayed home.” – Carl Sagan

Archive for the ‘neutrinos’ Category

Detecting a nuclear fission reactor at the centre of the Earth.

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Some months ago I wrote a post discussing Marvin Herndon’s controversial theory regarding a nuclear fission reactor, a “georeactor”, at the molten core of the Earth. One point I noted is that it should be entirely practical to falsify such a theory, to test it, to prove the existence of and to study the characteristics of such a nuclear reactor, by simply studying the flux of neutrinos (electron antineutrinos, specifically) from inside the Earth.

Here’s an interesting paper I found discussing just that.

Of course, the usual cautions regarding ArXiv preprint material apply – it is not peer-reviewed, and should be treated with skeptical scrutiny and caution.

Ultimately, the best possible site for a geo-reactor search is Hawaii (panel d) in Fig. 1). This option requires however, construction of a new excavated laboratory. In Hawaii, situated entirely on the oceanic crust with very low geo-U/Th. only the small signal from U/Th deep in the Mantle is visible. The remoteness from populated continents on either side reduces the power reactor signals to a comfortably low level.

With these considerations in mind, I wonder if the IceCube experiment at the South Pole wouldn’t be a much more useful detector site, with perhaps the best possible isolation from manmade reactors as well as geological U/Th radioactivity as you could possibly get? Not to mention the fact that the detector itself already exists, and doesn’t need to be constructed.

Written by Luke Weston

October 14, 2008 at 5:04 pm

The “Georeactor” Hypothesis.

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This post was inspired, in part at least, by Rod Adams’ post on NEI Nuclear Notes recently, asking about the georeactor theory. I hope you find it useful, Rod.

The “georeactor” hypothesis is a proposal by J. Marvin Herndon that a fissioning critical mass of uranium may exist at the Earth’s core and indeed that it serves as the energy source for the Earth’s magnetic field. You can read all about Herndon’s ideas at his website.

Herndon’s georeactor hypothesis is not widely accepted at all by the scientific community, outside of Herndon himself and a very small number of defenders.

Herndon’s georeactor hypothesis is sometimes confused with the existence of natural nuclear fission reactors in the Earth’s crust in rich uranium deposits at Oklo in Western Africa – however, it must be stressed that these are not the same thing – there is absolutely no doubt at all, scientifically, as to the occurrence of nuclear fission and the formation of natural nuclear “reactors” at Oklo approximately two billion years ago.

However, Rob de Meijer and associates at the Nuclear Physics Institute in Groningen, the Netherlands, are amicable towards Herndon’s theory, and have indeed proposed an experiment by which it should be somewhat falsifiable – involving measurement of the antineutrino flux from the Earth’s core which they believe will validate the georeactor hypothesis.

Fission reactors generate huge numbers of electron antineutrinos – about 10^26 per day from a typical manmade power reactor. Several thousand of these can be measured per day in a detector of modest size, outside the reactor, outside the containment, tens of meters away.

The antineutrinos resulting from each fission event from uranium and plutonium have different total count rates and energy spectra – the antineutrinos are not actually produced by nuclear fission itself, but rather by the beta decay of fission products. The antineutrinos therefore carry with them information about the amount and type of fissile material in the reactor core, and the rate at which it is being fissioned.

Because of this, incidentally, the use of neutrino detectors has raised considerable interest in recent times in the context of providing a real-time online and very simple measurement of the fuel burnup, operating status, power level, plutonium production and such characteristics of operating nuclear reactors, which is of considerable utility in enforcing non-proliferation safeguards.

(There’s more information on this application here if you’re interested.)

Personally, I don’t see why existing underground neutrino observatories, such as Super-Kamiokonde, the Sudbury Neutrino Observatory, and the IceCube experiment in Antarctica shouldn’t be sufficient to provide significant insights into the presence – or absence – of georeactor antineutrinos. Clearly all neutrinos from a “georeactor” come exclusively from exactly the centre of the earth as observed at every detector, and they should be detectable at all neutrino observatories worldwide with a similar flux everywhere.

Combining these simple pieces of information with the expected energy spectra of neutrinos from uranium fission, it seems extremely plausible that the georeactor hypothesis can well and truly be put to the test, using existing experiments, and probably even with existing collections of raw data from these experiments.

As one of Herndon’s recent papers puts it:

Uranium, being incompatible in an iron-based alloy, is expected to precipitate at a high temperature, perhaps as the compound US. As density at Earth-core pressures is a function almost exclusively of atomic number and atomic mass, uranium, or a compound thereof, would be the core’s most dense precipitate and would tend to settle, either directly or through a series of steps, by gravity to the center of the Earth, where it would quickly form a critical mass and become capable of self-sustained nuclear fission chain reactions.

Of course, there is what seems like one significant problem with this theory – whilst several billion years ago, the portion of uranium-235 in natural uranium was much higher than it is today – equivalent to that of manmade enriched uranium, because U-235 decays faster than U-238, although a much larger ratio of U-235 was originally formed when the uranium was formed inside supernovae than is seen in the Earth today. That is why fission occurred at Oklo two billion years ago, but does not occur today – there is not enough of a concentration of U-235 in nature. Therefore, how can a “georeactor” exist?

Herndon explains away this question by postulating that the georeactor is something like a fast breeder reactor, started up aeons ago when the U-235 was more abundant, and today burning the abundant U-238 into plutonium-239.

However, if this is the case, couldn’t it be likely that we could observe plutonium-fissioning “breeder reactors” in rich uranium deposits in the Earth’s crust, like at Oklo, today?