Natural nuclear reactors?

[TJ McCaustland]

Hey, just had a question for the physics community. Are natural nuclear reactors possible? would it be possible for a natural uranium-235 deposit to be introduced to a neutron emission somehow, and it to begin a fission reaction?

Edited October 8, 2014 by TJ McCaustland

[Greg H.]

Ten seconds with google would have answered this question for you.

 

Google search: natural nuclear reactors

 

First result: http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

[TJ McCaustland]

Ok well thank you I knew that already but I wanted to leave the topic open for discussion as well as the application of different circumstances

Ok well thank you I knew that already but I wanted to leave the topic open for discussion as well as the application of different circumstances

You see, sometimes the essence of science is not merely cold hard fact but discussing that fact with others.

[swansont]

Today the concentration of U-235 is too low for a natural reactor to happen; even for a commercial plant the only way to go critical with unenriched uranium is by using heavy water.

 

(also: moving this to physics because it's a physics question)

[Danijel Gorupec]

Just thinking.... would unenriched uranium go critical if squeezed enough? At 350GPa, maybe?

[swansont]

Just thinking.... would unenriched uranium go critical if squeezed enough? At 350GPa, maybe?

 

How much compression would you get? (and where would you get such pressures?) I don't know how much would be needed for criticality.

[Danijel Gorupec]

 

How much compression would you get? (and where would you get such pressures?) I don't know how much would be needed for criticality.

Center of the Earth... I have no idea how much compression that makes nor if this would be enough for uranium to go critical. I also guess that at temperaures down there, it would not be possible for sedimentation process to generate pure-enough uranium deposits... Still I had this sci-fi thought that maybe some self-regulating uranium reactor could be possible at the center of the Earth (say, uranium sinks due to its wieght, concentrates and criticality is created. Increased temperature then mixes the concentrated uranium with surrounding iron reducing its concentration and keeping criticality under control. Maybe in a pulse regime...one criticality event every several xy millions of yeras killing dinosaurs... oops, I entered the speculation forum...).

[swansont]

One of the features of a natural reactor on the surface is that water is available for neutron moderation. Presumably the reactor went critical when water was available and "turned off" when the water boiled away. Without that you are relying on fast neutrons, which I think is a less efficient process

[Enthalpy]

Natural reactors can be big. Then density plays no role. Density only serves to intercept the neutrons before they escape the core. Have a core so big that all neutrons are absorbed, and it can have any density.

 

With slow neutrons - this demands human intervention - a core can be critical with the present concentration of ²³⁵U; besides heavy water, one design that has worked uses graphite and carbon dioxide.

 

At the center of Earth, provided uranium accumulates (I have still to make my opinion about it; the standard answer is "uranium goes to the crust", I doubt it), you would have all dense elements (W, Ta) as well, which precludes a chain reaction.

http://www.webelements.com/tungsten/geology.html

 

I wish we had soon a directional observatory for neutrinos of the proper energy: it would tell us where the radioactive elements are in our Earth, and what proportion of the internal heat they provide.

[swansont]

Natural reactors can be big. Then density plays no role. Density only serves to intercept the neutrons before they escape the core. Have a core so big that all neutrons are absorbed, and it can have any density.

If the neutrons are absorbed in non-fissionable material, then no, you can't have any density.

 

With slow neutrons - this demands human intervention - a core can be critical with the present concentration of ²³⁵U; besides heavy water, one design that has worked uses graphite and carbon dioxide.

Regular water, which slows neutrons, does not require human intervention.

[Danijel Gorupec]

Natural reactors can be big. Then density plays no role. Density only serves to intercept the neutrons before they escape the core. Have a core so big that all neutrons are absorbed, and it can have any density.

Now I am confused... I would agree with your point if there are no other atoms between U235 atoms. However if there are many neutron-eating atom nuclei around, would not they disable the chain reaction even in the case of a big reactor? (BTW, my knowledge in this field is basic so I have no idea if iron atoms would absorb neutrons.)...

 

What confuses me is that if I think further in the same direction, I am not sure any more that a natural uranium sample could become critical if squeezed enough (while I always thought that squeezing helps). Squeezing will increase also U238 density and this, I suppose, might act negatively on the chain reaction.

 

I agree with you that it is probably impossible to have pure-enough uranium deposits near the center of the Earth - except for the high temperature (to prevent crystalization or some similar process) there is also near-weightlessness to prevent sedimentation.

[Enthalpy]

 

If the neutrons are absorbed in non-fissionable material, then no, you can't have any density.

 

But if the core is big enough, only its composition matters, not its density, which was my point.

Regular water, which slows neutrons, does not require human intervention.

 

With present-day natural uranium, regular water won't let a core diverge. It takes a moderator that absorbs neutrons less, ilke heavy water, graphite, helium, carbon dioxide, and few others.

[swansont]

 

But if the core is big enough, only its composition matters, not its density, which was my point.

 

You didn't mention composition before, so if that was your point it wasn't particularly clear.

[Duda Jarek]

Radiogenic heat is significant in Earth's internal heat budget ( http://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget ) and its effect can be observed e.g. as high He3/He4 ratio from volcanoes and geysers:

http://www.nature.com/nature/journal/v506/n7488/full/nature12992.html

http://www.wired.com/2014/04/what-helium-can-tell-us-about-volcanoes/

[Enthalpy]

Maybe... These are indirect observations. The big amount of ⁴He results supposedly from alpha rdioactivity, but where and when? There are big uncertainties about how much heat results from radioactivity, accumulated early impacts, present solidification. I really wish we had a directional neutrino observatory for neutrinos or radioactivity energy - it would give the first direct measure.

[swansont]

Radiogenic heat is significant in Earth's internal heat budget

 

Radiogenic does not imply fission. If it's measured by He isotope ratios, you are certainly NOT talking about fission.