[konaice]

The energy generated by the nuclear reaction boiled the groundwater around the deposit. Water is a natural neutron moderator, so as it was converted into steam, it stopped absorbing neutrons and shut down the chain reaction. As the rocks cooled down, the steam condensed, and the presence of water once again slowed the neutrons down and restarted the chain reaction. Meschik calculates that the reactor operated for about half an hour at a time, then shut down for two and a half hours.

Does this sound backwards to you as it does to me?

If it stopped absorbing Neutrons as water was deminished that would SPEED UP the reaction.

[LibWhacker]

Yep, sounds backward to me, too. And I can't pretend to understand it. Thought it was fascinating, though, that there was a natural fission reactor on Earth that ran safely, unattended for 150 million years!

[WildTurkey]

Does this sound backwards to you as it does to me? If it stopped absorbing Neutrons as water was deminished that would SPEED UP the reaction.

You are absolutely correct. More later.

[Axenolith]

Neutrons need to be slowed for fission. Sounds counterintuitive but thems the breaks.

As for "and storing its own waste in a safe manner.", others I've read (ages ago) have hypothesized the site as one of the main drivers of the initial diversification of life through the halo of higher than normal radiation induced mutation around the structure...

[fuente]

Depends on the type of reactor the dynamics of the natural core. Funny thing is, natural cores act have a tendency to act the opposite of what was just described, i.e, + void coefficient of reactivity. Some have the opposite effect.

[WileyPost]

I'm not sure, but if the water absorbs an atoms neutrons, it destabilizes the atomic weight of the atom losing the neutron and that then creates the reaction with another atom. That's why as it steams out, the reaction settles don.

[Wonder Warthog]

"If it stopped absorbing Neutrons as water was deminished that would SPEED UP the reaction."

The water wasn't "absorbing" the neutrons, it was changing their speed. If neutrons are moving too fast, they cannot be "captured" by the uranium-235 muclei, and fission won't occur. The protons (hydrogen atoms) of the water are very similar in size to a neutron, so when a neutron hits a proton, it loses its kinetic energy to it (think one billard ball hitting another--the first ball slows down drastically or stops completely, and the second "zips away" from the collision site). If the water is converted to steam, its density goes down by a factor of >1000, and the possibilities of a neutron hitting a proton and slowing down also drop. Therefore there are fewer "slow" neutrons available to be captured, and the fission process slows down.

[Robert A Cook PE]

The other guys pretty much replied to this: Water DOES slow down neutrons so they fission, but the absence of water (in low density steam) allows the same neutrons to escape the immediate area of the reactor center.

By escaping the middle of the reactor, they DON'T cause secondary fissions,

In a perfectly regulated reactor, so the overall reactor rate goes down since these neutrons are lost, so heat production goes down, and so less water turns to steam, so more water is present, so more neutrons are slowed down close to the center of the reactor, and so more secondary fissions take place, and so more heat is produced ....

He's a little optimistic (let's face it = simple-minded) in his projection of this "natural reactor" since we've been using negative thermal feedback since the early 50's ....

A real power reactor couldn't waste time going on and off like a geyser either ... 8<)

[Robert A Cook PE]

See my comments on reactivity in Nbr 32 & 33.

[pickrell]

That's sharp thinking! You are correct about the discrepancy. Perhaps, what the author meant to say was that the water acted as a moderator, much like graphite did, when it was used in the crude reactors of the 1950's.

The graphite slowed the neutrons enough that their absorption was more likely by the uranium 232. Else the neutrons were simply lost to the reaction, and the reaction would diminish.

What the author didn't remark on was that Uranium 238 will also absorb neutrons, and after a microsecond or so as uranium 239, will emit an electron and become neptunium 239. The Neptunium 239 will then after a few microseconds emit yet another electron and become Plutonium 239, which, if my old memory doesn't fail me from so long ago then has a half-life of 24,000 years.

If in fact, this reaction has occurred as postulated, the presence of Plutonium, (a case of the world's first natural slow-breeder reactor), would be discovered.

At least, that's what they taught us back in the 70's.