Astronomy Picture of the Day 10-16-02

NASA ^ | 10-16-02 | Robert Nemiroff and Jerry Bonnell

[petuniasevan]

Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2002 October 16

Oklo: Ancient African Nuclear Reactors
Credit & Copyright: Robert D. Loss, WAISRC

Explanation: The remnants of nuclear reactors nearly two billion years old were found in the 1970s in Africa. These reactors are thought to have occurred naturally. No natural reactors exist today, as the relative density of fissionable uranium has now decayed below that needed for a sustainable reaction. Pictured above is Fossil Reactor 15, located in Oklo, Gabon. Uranium oxide remains are visible as the yellowish rock. Oklo by-products are being used today to probe the stability of the fundamental constants over cosmological time-scales and to develop more effective means for disposing of human-manufactured nuclear waste.

[petuniasevan]

I just know that someone will visit this thread and complain that the subject matter is off-topic.
Hey, I only mirror the site; I don't choose the APOD.

Astronomy Fun Fact:

Actually it's not really off-topic. We know that very heavy elements such as uranium are probably only synthesized in supernovae. No scientific discipline stands alone.

Oklo was a BREEDER reactor - it made more fissionables than it consumed. Very odd; only its long "run" made this possible. Plutonium it had "bred" decayed back to U-235!

Did you recently hear news of a theory that says Earth's core itself may be a reactor? Interesting to say the least! And the Oklo natural reactor played a part in the concept! Here's a link: Giant Nuclear Reactor May Run Earth's Magnetic Field

[petuniasevan]

A real treat for the physics/geology crowd today...

[Graewoulf]

If "no natural reactors exist today," then why is the dominant source of heat energy flow to the Earth's surface STILL considered to be from radioactive (fission) decay within the Mantle?

[petuniasevan]

Heat from residual fission and self-perpetuating reactors are two different things. Yes, there are radioactive isotopes of various elements in the mantle. They do decay, and from that energy some heat is created. There will be no sustained reaction.

The missing ingredient here is critical mass. You have to put so much of a single radioactive isotope such as U₂₃₅ into such and such a volume to set up a nuclear chain reaction. Careful measurement of both concentration of isotope and amount used are essential to success in controlling the reaction.

[Graewoulf]

Thank you. The key phrase then is "sustained reaction." Thanks again.

[RightWhale]

Earth's core itself may be a reactor?

It's fun and educational to consider such possibilities. Yesterday the possibility was announced that the earth's core may have an inner core about 350 miles in diameter. This is apparently suggested by a discontinuity of seismic waves at the boundary.