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.'

[Robert357]

The thing seems to have stopped seriously radiating waaaayy before humans evolved.

But the question is what was the amount of radiation in the area, was it sufficiently above normal background levels to have induced much in the way of mutations? Probably not a lot if it was "way way" before human life evolved, but a thought none-the-less.

[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.

[JoeSixPack1]

This is outstanding. Not only have they discovered a natural process to learn from, this also bolsters the hypothosis that nuclear power is possibly a galactic standard.

Controllable, renewable <--- we knew,
and now, at times, natural.

Time to rewrite the books and search the stars again.

[DrDavid]

"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!!

Who ever said that nature was safe?

[Flightdeck]

"natural uranium will not undergo nuclear reactions, because it contains too little of the fissionable isotope, uranium-235 (U235)."

U-238 is also fissionable. U-235 is fissile, however, which means a neutron with no additional velocity will cause it to fission.

[Bernard Marx]

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.

I recall hearing about that as well. I don't know whether it was the "original" but last I heard it was still operating IIRC.

[WildTurkey]

Nuclear waste is radioactive in terms of being very harmful to human health for 100,000 years. It doesn't return to the background radiation that natural uranium has for upwards of 1,000,000 years.

Are you anti-nuclear or just ignorant?

[WildTurkey]

U-238 is also fissionable. U-235 is fissile, however, which means a neutron with no additional velocity will cause it to fission.

Close. Fissile materials will undergo fission with any velocity neutron.

[NicknamedBob]

Question for Robert A. Cook, P.E. = Since, “hydrogen-containing 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.”, a rocket ship with a nuclear engine could use solid Carbon Dioxide very efficiently as a reaction fuel, by extruding a cylinder of dry ice into a nuclear chamber where it would incandesce like a candle to produce an exhaust gas at high speed. Theoretically a simple design, but would it be better to use dry ice, or simply pressurized CO₂?

Question for Sionnsar = Was this the “Valley of the Shadow of Death”? (For this accretion of material, it had to be a valley.)

Question for Wonder Warthog = Does this mean I have to use Heavy Metals for my basement cold fusion plant? I really hate that kind of music.

[fuzzthatwuz]

"Nothing that wasn't studied in beginning Nuclear Engineering classes decades ago."

Darn, I must have missed those classes! Probably the reason the article impressed me. :>)

[Robert A Cook PE]

Hmmmmmmmmmmn.

One-way pass-through CO2 as moderator/throw-away coolant for space thrust?

Hmmmmmmmmmmmmmmmnnnnnnnnnn.

[Robert A Cook PE]

Yeah, but absorption probablity goes up considerably as nuetron energy decreases into the thermal range.

[Robert A Cook PE]

I'd go with solid CO2 since there's no weight lost for tanks, coolant required and refrigerators for the liquid CO2, and the handling of solid CO2 would seem to be easier.

Little bit of thermal stress as cold solid CO2 at one end of the reactor changes into hot gasses in the middle and far end of the reactor. That change would make calculations interesting, shall we say.

[NicknamedBob]

Solid storage would be good for long duration shuttle tugs, asteroid miners (and rock-chuckers), but I think for the atmosphere ships I'd want a tank to pressurize.

I wonder what the trade-off would be for a Star-Launch Tug using an expendable Nuke engine with massive dry ice propellant?

[Flightdeck]

"U-238 is also fissionable. U-235 is fissile, however, which means a neutron with no additional velocity will cause it to fission.

Close. Fissile materials will undergo fission with any velocity neutron."

Isn't that what I wrote?

[WildTurkey]

Yeah, but absorption probablity goes up considerably as nuetron energy decreases into the thermal range.

Of course. Hence the reason for the moderation of the neutrons.

[WildTurkey]

You wrote "a neutron with no additional velocity will cause it to fission". While this is technically true, it is also true that "a neutron with additional velocity will cause it to fission".