Is Ganymede better than mars?

[KerikBalm]

I don't know what Neptune's atmospheric radiation levels are, but keep in mind it's still an "Ice Giant" (related to gas giants), so it is basically a failed star that didn't acquire enough mass to go fully nuclear.

It still has a good amount of fission going on in its interior and maybe some fusion too, so I can't imagine it's too safe that close to the core.

No, ice giants are nowhere close to being a failed star. Neither is Saturn. Jupiter is still far below being a brown dwarf. Jupiter would need to be ~13x as massive to be a brown dwarf - ie to fuse deuterium (which is a very very very tiny fraction of the total mass, unlike a normal star that fuses normal hydrogen "protium").

Bill Phill is correct that its a small fraction of the size of our sun, but red dwarves start at about only 7.5% the mass of the sun, so we should really be comparing Jupiter to red dwarves, not our sun.

A true star, and not a brown dwarf, is almost 2 orders of magnitude bigger than jupiter.

Fusion is out of the question... and fission is irrelevant - fission happens at earths core, and happened at high rates on Earth's surface ~2 billion years ago.

Nope, no appreciable amounts of fission will be going on in Neptune, you need fissile material and a moderator, neither of which it is going to have in large amounts.

Radioactive decay is technically fission. It is not a fission chain reaction as in a nuclear reactor.

Earth has had nuclear reactors before.

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

Clearly, having fission chain reactions does not indicate a similarity to a star.

Nor does it really affect the planetary radiation environment in most cases that can be envisioned (though I wonder what effect Oklo had on Earth's environment 2 billion years ago)

Lump of uranium the size of our Sun can't even exist. Hell, lump the size of a TV can't.

A lump the size of our sun wouldn't exist, true (mass, or volume, in terms of volume... that would end up being super massive, and would probably undergo gravitational collapse and go nova, but thats just my guess without doing any calculations).

A lump the size of a TV can exist just fine - provided that it is mostly U-238. Its a lump of pure U-235 where you start to get problems of reaching criticality.

This isn't an irrelevant point - large U-238 lumps can still generate heat, and are still very relevant to this discussion.

U-238 has a half life approximately the same as the current age of the Earth. U-238 decay is a very long lived source of internal and relatively consistent heating.

U-238 is generating half the heat it did at the planet's formation, whereas U-235 (excluding induced fission) was generating ~85x as much heat (as there was 85x more of it... due to the half life of 700 million years on a 4.5 billion year old earth).

The only threat comes from their magnetospheres catching and concentrating solar wind. You don't need a radioactive source to get ionizing radiation.

So to some extent, the proximity to the sun should also matter. A jupiter sized magnetic field at neptune's distance from the sun would be catching much more diffuse solar wind.

So the outer gas/ice giants must surely be much less hostile due to smaller magnetospheres, and more diffuse solar wind.

Edited March 23, 2015 by KerikBalm

[-Velocity-]

I don't know what Neptune's atmospheric radiation levels are, but keep in mind it's still an "Ice Giant" (related to gas giants), so it is basically a failed star that didn't acquire enough mass to go fully nuclear.

It still has a good amount of fission going on in its interior and maybe some fusion too, so I can't imagine it's too safe that close to the core.

You need at least 13 Jupiter masses to ignite the lowest temperature/pressure fusion reactions (with deuterium). An object has to have about 80 times the mass of Jupiter to ignite proton-proton chain fusion (which is the reaction that mostly powers the Sun). An object between 13 and 80 Jupiter masses burns dimly for some millions of years on deuterium (and, if bigger than like 60 Jupiter masses, lithium) fusion. Objects between 13 and 80 Jupiter masses are considered "brown dwarfs" (aka "failed stars") by many astronomers, though objects below 13 Jupiter masses might also be considered brown dwarfs if they were not formed around a star. (Personally, I just think we should call all large objects below 13 Jupiter masses planets regardless of where they formed.)

ANYWAY, Neptune is about one nineteenth the mass of Jupiter, so it would have to be about 247 times more massive to ignite any kind of nuclear fusion in its core (and be at least a brown dwarf). It would have to be 1500 times more massive to ignite proton-proton fusion and be a red dwarf. It is by NO means a "failed star". If you're gonna consider Neptune a failed star, you might as well consider Earth a failed star.

[lajoswinkler]

A lump the size of our sun wouldn't exist, true (mass, or volume, in terms of volume... that would end up being super massive, and would probably undergo gravitational collapse and go nova, but thats just my guess without doing any calculations).

A lump the size of a TV can exist just fine - provided that it is mostly U-238. Its a lump of pure U-235 where you start to get problems of reaching criticality.

This isn't an irrelevant point - large U-238 lumps can still generate heat, and are still very relevant to this discussion.

U-238 has a half life approximately the same as the current age of the Earth. U-238 decay is a very long lived source of internal and relatively consistent heating.

U-238 is generating half the heat it did at the planet's formation, whereas U-235 (excluding induced fission) was generating ~85x as much heat (as there was 85x more of it... due to the half life of 700 million years on a 4.5 billion year old earth).

So to some extent, the proximity to the sun should also matter. A jupiter sized magnetic field at neptune's distance from the sun would be catching much more diffuse solar wind.

So the outer gas/ice giants must surely be much less hostile due to smaller magnetospheres, and more diffuse solar wind.

I was refering to the natural uranium. Even a TV sized lump would not have a nice time. Even depleted uranium will experience massive problems because of the heat.

[Voyager55]

I don't think its 1.4 (Or something like that) M/sec2 of gravity would be very comfortable.

[KerikBalm]

I think a TV sized lump of DU would be just fine....

Its half life is really really really long... it doesn't give off much heat.

A lump of natural uranium would be in some sort of oxide... not a pure metal... and I'm not sure how that would fare with 0.7% U235 in it...

[lajoswinkler]

I think a TV sized lump of DU would be just fine....

Its half life is really really really long... it doesn't give off much heat.

A lump of natural uranium would be in some sort of oxide... not a pure metal... and I'm not sure how that would fare with 0.7% U235 in it...

Natural uranium meaning the isotopic content, 0.72% of fissile radioisotope.

Uranium in nature appears as dioxide, uranyl compounds, vanadates, phosphates, silicates, all in combination with earth alkali elements.

Oklo reactors show what happens when you concentrate a bit the compounds. Natural uranium metal ball the size of a TV would get hot very fast and would not make a healthy addon for a living room.

[Kryten]

Natural uranium meaning the isotopic content, 0.72% of fissile radioisotope.

Uranium in nature appears as dioxide, uranyl compounds, vanadates, phosphates, silicates, all in combination with earth alkali elements.

Oklo reactors show what happens when you concentrate a bit the compounds. Natural uranium metal ball the size of a TV would get hot very fast and would not make a healthy addon for a living room.

Oklo shows what happens if you have a large concentration of the compounds, with water available as a moderator, two billion years ago-when natural uranium had about four times the -235 content it has now. Natural uranium in it's current state will not produce fission without a very efficient moderator.

[magnemoe]

Natural uranium meaning the isotopic content, 0.72% of fissile radioisotope.

Uranium in nature appears as dioxide, uranyl compounds, vanadates, phosphates, silicates, all in combination with earth alkali elements.

Oklo reactors show what happens when you concentrate a bit the compounds. Natural uranium metal ball the size of a TV would get hot very fast and would not make a healthy addon for a living room.

Is it any safety rules on storage of piles of uranium oxide or uranium metal? This should ansver the question well enough. DU is safe weapon grade or even highly enchanted is not.

[peadar1987]

Is it any safety rules on storage of piles of uranium oxide or uranium metal? This should ansver the question well enough. DU is safe weapon grade or even highly enchanted is not.

Haha, nice typo, "enchanted uranium". I've watched nuclear fuel elements being built, the whole process would have been much more entertaining if we'd had a wizard standing by when each stringer was sent off to the fuelling machine!

On the TV-sized lump of uranium, natural uranium generates about 0.1W/tonne of decay heat (source). Its density is 19100kg/m^3, let's say a TV is 1mX0.5mx0.5m, so not a flat screen, but still a decent size, that's 0.25m^3, your lump of uranium would have a mass of 5 tonnes, and generate 0.5W of decay heat, which is pretty negligible, it's going to dissipate most of that through convection at the surface and reach thermal equilibrium pretty quickly.

[KerikBalm]

Oklo shows what happens if you have a large concentration of the compounds, with water available as a moderator, two billion years ago-when natural uranium had about four times the -235 content it has now. Natural uranium in it's current state will not produce fission without a very efficient moderator.

Technically 2 billion years ago is 2.85 half lives, or 7.24x the current concentration. But then again, technically it was 1.7 billion years ago... or 2.42 half lives, or 5.3 x as much U 235 as now.

Call it about 5, not about 4

On the TV-sized lump of uranium, natural uranium generates about 0.1W/tonne of decay heat (source). Its density is 19100kg/m^3, let's say a TV is 1mX0.5mx0.5m, so not a flat screen, but still a decent size, that's 0.25m^3, your lump of uranium would have a mass of 5 tonnes, and generate 0.5W of decay heat, which is pretty negligible, it's going to dissipate most of that through convection at the surface and reach thermal equilibrium pretty quickly.

Thanks for the calculations confirming my intuition.

0.5W is less heat than a lightbulb gives off.... not even an old incandescent lightbulb, but a newer more efficient one. A TV sized lump of Uranium would be quite stable.

Its also worth noting that due to the high density, as far as radiation is concerned, you really only need to consider the surface of the lump, and I think the radioactivity would be pretty low too (I wouldn't want to live anywhere with elevated radiation, but I suspect the elevated radiation levels would not result in a very high increased in the risk of cancer)

[peadar1987]

Thanks for the calculations confirming my intuition.

0.5W is less heat than a lightbulb gives off.... not even an old incandescent lightbulb, but a newer more efficient one. A TV sized lump of Uranium would be quite stable.

Its also worth noting that due to the high density, as far as radiation is concerned, you really only need to consider the surface of the lump, and I think the radioactivity would be pretty low too (I wouldn't want to live anywhere with elevated radiation, but I suspect the elevated radiation levels would not result in a very high increased in the risk of cancer)

Oh, it would be very low. On the fuel build I mentioned, the radiation level was 6uSv an hour from standing right next to fuel enriched up to about 4% U-235. This was in the form of pellets with a diameter of 1.5cm, so a far higher surface area from which to emit radiation than a simple sphere. There are 8 fuel elements to a stringer, each with 44kg of uranium in them, so 352kg of uranium. U-235 is about 7 times as radioactive as U-238, and U-235 is about 1% of the mass of natural uranium. That means that the uranium I was working with was about 17% more radioactive than natural uranium, so as a back-of-the-envelope job, we can assume that natural uranium would give the same approximate dose rate.

That translates to an annual dose of 52mSv a year for my fuel build. The lowest annual dose linked to an increased risk of cancer is 100mSv a year, the average annual dose of radiation for someone going about their everyday lives is about 4mSv a year.

Working out the actual dose from the sphere is a little more tricky, but we can roughly do it by dividing the surface area by the mass. The surface area of uranium in the fuel build was 2*PI*R*H, radius of 1.5cm per pin, length of 900mm, 36 pins in an element, 8 fuel elements, so about 24.4m^2 of surface area for 350kg of Uranium. The 5-tonne TV-sized sphere would have a surface area of 2m^2 if it had a radius of 0.4m (giving our volume of 0.25m^3).

Essentially, there will be about 15x the radiation generated by the sphere, but it will have 10X less surface area to escape through, and therefore the dose will be about 1.5x as great. This is all very rough, but it means that having a sphere of uranium in your living room would give you a yearly radiation dose of about 75mSv a year, or just below the amount you would need to have any increased risk of cancer (and that's only if you're in your living room 24/7). Your main problem would probably be with fission products like radon building up in the air, where they can get inside the body and do far more damage.

[lajoswinkler]

Oklo shows what happens if you have a large concentration of the compounds, with water available as a moderator, two billion years ago-when natural uranium had about four times the -235 content it has now. Natural uranium in it's current state will not produce fission without a very efficient moderator.

Of course it won't produce a chain reaction today, but it will generate heat. TV-sized lump probably won't become dangerous unless shielded by asbestos or something similar.

Sun-sized... oh yeah. It would turn into a molten ball. It should even fission-explode because of the enormous pressure in the core. I'd say it would go nova.

Is it any safety rules on storage of piles of uranium oxide or uranium metal? This should ansver the question well enough. DU is safe weapon grade or even highly enchanted is not.

As a reagent, uranium is used in the depleted form. Such compounds are poisonous like any other heavy metal compounds (thallium, mercury, lead, silver, barium, ...) but don't require special shielding against radiation. They can just stay on your shelf in your lab in the reagent bottle.

Haha, nice typo, "enchanted uranium". I've watched nuclear fuel elements being built, the whole process would have been much more entertaining if we'd had a wizard standing by when each stringer was sent off to the fuelling machine!

On the TV-sized lump of uranium, natural uranium generates about 0.1W/tonne of decay heat (source). Its density is 19100kg/m^3, let's say a TV is 1mX0.5mx0.5m, so not a flat screen, but still a decent size, that's 0.25m^3, your lump of uranium would have a mass of 5 tonnes, and generate 0.5W of decay heat, which is pretty negligible, it's going to dissipate most of that through convection at the surface and reach thermal equilibrium pretty quickly.

I meant a ball or uranium, not a plate. Ball of uranium of 1 m diameter ("size of a TV") would have nearly 10 tonnes and its surface area is minimal. 0.1 W/t is a rough estimate for even depleted uranium, meaning our ball would produce approx. 1 watt of power. That is safe. Phew.

Oh, it would be very low. On the fuel build I mentioned, the radiation level was 6uSv an hour from standing right next to fuel enriched up to about 4% U-235. This was in the form of pellets with a diameter of 1.5cm, so a far higher surface area from which to emit radiation than a simple sphere. There are 8 fuel elements to a stringer, each with 44kg of uranium in them, so 352kg of uranium. U-235 is about 7 times as radioactive as U-238, and U-235 is about 1% of the mass of natural uranium. That means that the uranium I was working with was about 17% more radioactive than natural uranium, so as a back-of-the-envelope job, we can assume that natural uranium would give the same approximate dose rate.

That translates to an annual dose of 52mSv a year for my fuel build. The lowest annual dose linked to an increased risk of cancer is 100mSv a year, the average annual dose of radiation for someone going about their everyday lives is about 4mSv a year.

Working out the actual dose from the sphere is a little more tricky, but we can roughly do it by dividing the surface area by the mass. The surface area of uranium in the fuel build was 2*PI*R*H, radius of 1.5cm per pin, length of 900mm, 36 pins in an element, 8 fuel elements, so about 24.4m^2 of surface area for 350kg of Uranium. The 5-tonne TV-sized sphere would have a surface area of 2m^2 if it had a radius of 0.4m (giving our volume of 0.25m^3).

Essentially, there will be about 15x the radiation generated by the sphere, but it will have 10X less surface area to escape through, and therefore the dose will be about 1.5x as great. This is all very rough, but it means that having a sphere of uranium in your living room would give you a yearly radiation dose of about 75mSv a year, or just below the amount you would need to have any increased risk of cancer (and that's only if you're in your living room 24/7). Your main problem would probably be with fission products like radon building up in the air, where they can get inside the body and do far more damage.

Have you accounted for the fact that uranium is a very efficient radiation shield?

Also, the main problem with a 1 metre uranium ball in your room would be the fact its surface would become dusty with oxide. It's a chemically far more reactive metal than iron. That dust would soon get everywhere.

Edited March 24, 2015 by lajoswinkler

[-Velocity-]

I would think that, in general, the chemical toxicity of depleted uranium might be a bigger danger than the radiation. I could be wrong though.

[lajoswinkler]

No, you are absolutely right. Its radioactivity is negligible compared to the chemical toxicity. If you eat its salts you can say goodbye to your kidneys. And life. LOL

[magnemoe]

Haha, nice typo, "enchanted uranium". I've watched nuclear fuel elements being built, the whole process would have been much more entertaining if we'd had a wizard standing by when each stringer was sent off to the fuelling machine!

On the TV-sized lump of uranium, natural uranium generates about 0.1W/tonne of decay heat (source). Its density is 19100kg/m^3, let's say a TV is 1mX0.5mx0.5m, so not a flat screen, but still a decent size, that's 0.25m^3, your lump of uranium would have a mass of 5 tonnes, and generate 0.5W of decay heat, which is pretty negligible, it's going to dissipate most of that through convection at the surface and reach thermal equilibrium pretty quickly.

You could simply use an enchanting table like the one I have in my Skyrim house, they was not so expensive Lesson learned is that spell check has its limits.

And yes 0.5W can be ignored, its ability to act as an heat storage would be far more noticeable

Have you accounted for the fact that uranium is a very efficient radiation shield?

Also, the main problem with a 1 metre uranium ball in your room would be the fact its surface would become dusty with oxide. It's a chemically far more reactive metal than iron. That dust would soon get everywhere.

This is true, they use DU for radiation shielding sometimes.

Not sure how uranium oxidizes, most metals except iron generates an thin outer layer they don't rust. Uranium oxide is also pretty inert, yes breathing it as dust is not good.

[GreenWolf]

But would it be better to colonize a Ganymede-sized sphere of uranium or a Mars-sized sphere of uranium?

[Bill Phil]

But would it be better to colonize a Ganymede-sized sphere of uranium or a Mars-sized sphere of uranium?

Neither. Deep space.

So, Ganymede is NOT better than Mars?

[lajoswinkler]

This is true, they use DU for radiation shielding sometimes.

Not sure how uranium oxidizes, most metals except iron generates an thin outer layer they don't rust. Uranium oxide is also pretty inert, yes breathing it as dust is not good.

Whole cases for cobalt-60 cameras are made out of it. The heavier the nucleus is, the better it attenuates gamma and x-rays.

Uranium does not create enormous flakes as iron does, effectively exposing more of itself and ultimatively ruining itself, but it neither gains a stable passivated layer like aluminium. It's something in between.

Freshly cast is VERY heavy and if it wasn't so hard (6 by Mohs, more than steel!), you'd think it's an unusually dense lead by its surface.

Being a very similar to any typical rare earth element, it's subjectible to attack by moist air. This is how it looks after a while. You can see the dioxide layer.

It's best to keep it under argon or in vacuum, but kerosene or inert mineral oil work, too. Just like for sodium, except uranium is nowhere near reactive as any alkali metal.

DU is therefore never used pure. Those penetrating bullets are alloy of DU and usually tiny percentage of titanium.

Other than penetrators, nuclear weapons and radiation shields, I'm not immediatelly aware of any other use of elemental uranium. Power generation uses its dioxide as it's very inert.

Edited March 25, 2015 by lajoswinkler