Steam secret of natural fission

e4engineering.com ^ | 11/22/04 | Stuart Nathan

[LibWhacker]

The world's only known natural nuclear reactor, which decommissioned itself over two billion years ago, could provide insights into how modern nuclear plants can operate more safely.

The site, in Gabon, West Africa, ran for 150million years without blowing up, and storing its own waste in a safe manner.

The reactor was a natural deposit of uranium. Today, and for the last two billion years, natural uranium will not undergo nuclear reactions, because it contains too little of the fissionable isotope, uranium-235 (U235).

But in the distant past, U235 was more abundant, comprising 3% of the total amount - the approximate concentration of enriched uranium used in nuclear fuel today. The Gabon deposit also contained, by a quirk of geology, a mixture of minerals which acted as a neutron moderator, slowing the neutron flux enough to allow the fission process to take place.

In a nuclear reactor, it takes large numbers of specialists and serious application of high technology to prevent reactions from running away. 'The big question we addressed was: when the uranium reached criticality, why didn't it blow up?' says Alexander Meschik of Washington University in St Louis, Missouri.

The answer, it appears, is that the site functioned like a geyser.

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.

Meschik deduced this by analysing the other neutron moderator in the deposit, a 'mineral assembly' containing lanthanum, cerium, strontium and calcium and known as alumophosphate. This also acted as a waste storage medium, the researchers found; it absorbed the isotopes of xenon which were formed by the fission of the U235.

Xenon is extremely rare on Earth and is a characteristic marker of a fission process. It occurs in nine isotopes, and it was the analysis of the relative abundances of these which gave the researchers the clue to the way the reactor operated.

The find could provide insight into how to operate industrial reactors more safely. 'This is very impressive, to think that this natural system not only went critical, it also safely stored the waste,' Meschik says. 'Just using the fact that the water boiled at the reactor site might give contemporary nuclear reactor researchers ideas on how to operate more safely and efficiently.'

[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.

[nightdriver]

There was an article about this in Scientific American about 20 years ago.

[LibWhacker]

I knew I should've put that in quotes! :-)

I'm guessing by "safely" they mean it did not blow up.

[LibWhacker]

My thoughts exactly, thanks! He always shows us the way.

[D Rider]

Wow!! The Creator is amazing. Theres nothing new under the sun. Everything man does God has already done.

Pull the pin, toss it in.

[sionnsar]

The world's only known natural nuclear reactor, which decommissioned itself over two billion years ago,

Guess this one qualifies for the geezer geeks... it is old technology!

Geezer Geek ping.
FReepmail me if you want on or off this list.

[sionnsar]

Wow!! The Creator is amazing. Theres nothing new under the sun. Everything man does God has already done.

Hadn't thought we'd done percolating reactors yet. +1 in God's column. *\;-)

[wizr]

Bump fer later. Just headin' over to Diablo Canyon ta roast some marshmellas 'n make s'mores.

Thanx

[sionnsar]

Glow-in-the-dark s'mores! The perfect thing for a camping trip...

[Robert A Cook PE]

Hmmmn.

I thought the original "natural" fission reactor happened in some "very highly concentrated" natural U238/235 rock in the Belgium Congo a little before WWII.

[sionnsar]

Hello, Robert! Is it still going, or did it shut down (or get excavated)?

[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]

Nah. It's even simpler than that.

Think of a single neutron, atomic weight 1, moving very fast as it leaves a fission reaction. The other two atomic nuclei that come from splitting the U235 are also moving (heat energy) and they also crash into adjacent atoms and cause THEM to heat up.

But we're gonna ignore those two and look at only the fast neutron for the time being.

If the neutron "hits" nothing, it escapes the core and eventually hits shielding or steel vessel walls and slows down, is absorbed, and is lost. A fraction do this.

If the neutron hits a nearby U238 nuclei it has a finite chance of being absorbed by the U238, turning it into U239 -> Np239 -> etc. and eventually into Pu239. That Pu239 can react later as new fuel. (Presto! Instant permanent fuel supply that Carter destroyed.)

If the neutron hits a material with a large cross section for absorption (such as Cadmium or boron or hafnium) then it is almost certain to be absorbed and lost to the reactor. These types of material are inserted into the core as control rods to stop the reaction, or removed to increase the reaction rate.

If the neutron its a heavy atom, but is NOT absorbed, then it merely bounces off (like a billiard ball hitting the edge of the table) and doesn't loose much energy. It stays in the reator, and continues to hit things. Or is lost when it eventually bounces out of the reactor. So low-absorbing but strong and corrosion resistance material is wrapped around the U235/U238/Pu239 to contain the material, prevent erosion and corrosion, and to contain the fission products.

If the neutron hits a "lightweight" atom, then the collisions energy is more evenly shared and BOTH the neutron and the lightweight target bounce away. The neutron bounces away with MUCH LESS kinetic energy (less heat energy), and the target atom is bounced away with much more kinetic energy (it become hotter.)

So, hydrogen-contianing material like water, or light metals like carbon, Al, polyethylene, plastic, and other light metals and liquids literally "slow down" the neutron better than heavier material.

These light-weight but non-absorbing material are called moderators in honor of our admin moderators who can't absorb anything either, but who control our reactions by bouncing ideas off of each other. 8<)

A perfect moderator also absorbs heat and transfer it out of the reactor. Again, like our admin moderator.

Remove the lightweight material, and the reaction slows down and eventually stops because no neutrons are slowed down by collisions..

One way of removing the light material is to boil off the water.

[Robert A Cook PE]

Dunno. Good question.

[Robert A Cook PE]

See my comments on reactivity in Nbr 32 & 33.

[Robert A Cook PE]

See my comments on reactivity in Nbr 32 & 33.

[Robert A Cook PE]

See my comments on reactivity in Nbr 32 & 33.

True: Navy boats use the same principle: except Rickover classified our tech manuals so God couldn't sell the idea to the Russians.

[DrDavid]

There was an article about this in Scientific American about 20 years ago.

I remember that article too!

This clearly demonstrates that nuclear fission reactors are more natural than internal combustion engines!

[nightdriver]

"This clearly demonstrates that nuclear fission reactors are more natural than internal combustion engines!"

I wouldn't have wanted to be near it when it was reacting, however!!

The authors of the article postulated that the critical mass was obtained when uranium salts were washed down this ancient stream bed and concentrated in a whirlpool or something.

It attracted the attention of the nuclear community when it was discovered that the U²³⁵ content at this location was only 0.4% instead of the more universal 0.7% found in uranium ores worldwide.

They called it evidence that nature had indeed formed a nuclear reactor.

[nightdriver]

"The site, in Gabon, West Africa, ran for 150million years without blowing up, and storing its own waste in a safe manner."

So, in only 150 million years, the waste of our modern reactors should be safe too, so noone should be concerned. (/sarc.)