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asnys
04-21-2013, 07:51 AM
So, I've got these people living deep underground. Real deep - I haven't settled for sure on the exact depth, but at least a kilometer. For reasons of Plot, they can't ever go to or interact directly with the surface. This, needless to say, makes survival quite difficult, even though their technology is quite a bit better than ours.

The problem I'm having is figuring out what they can do with the waste rock from mining and tunneling out new living spaces. What I'm wondering is, could they just dump it into the mantle? Either by drilling into the asthenosphere - they've got about a century's worth of technology on us in terms of materials science etc. - or by tunneling into a volcano and dumping it into the magma chamber? The pictures of volcano cross-sections make it look like the tube feeding the volcano is a straight drop down to the mantle, but I'm not sure if that's actually correct. Is that how they actually work?

Thanks!

Drachen Jager
04-21-2013, 08:58 AM
Drilling a hole directly into a volcano like that would create some grave risks. I suppose there's nothing wrong in theory with pushing junk in there, so long as there's something to prevent it all from blowing back on you.

Can't you just have them near a large cavern network? I'm not sure how porous the deeper crust is, but some cave networks have huge chambers very deep in the Earth. That would be the simplest way to dispose of rubble. Of course if that were the case it might just be easier to adapt the existing caves to suit their purposes.

Bufty
04-21-2013, 02:45 PM
Unless it's important I know what they do with waste diggings isn't it easier to use existing caverns - no? Telling me how they dig fresh areas makes me curious how and why they started to live underground and that's maybe not an issue you want to raise.

If it's fantasy it doesn't really matter and imagination is all that's needed. They are 'people' and perhaps could break or reconstitute the material down into other basic usable materials - breathing and eating seem more important than disposing of waste diggings.

asnys
04-21-2013, 05:26 PM
I'd like to figure it out just for my own sake even if it never comes up in the story - and, honestly, if I read my own story and it wasn't explained in a footnote or something, it would really bug me trying to figure it out.

I thought about using existing caverns, but they'll never be able to perfectly compactify the rock, so they'd be slowly losing volume over time. While volcano disposal might be dangerous, from a narrative perspective that's not necessarily a bad thing - and they're probably going to be drilling into magma pockets for geothermal energy anyway.

Sarpedon
04-21-2013, 05:55 PM
There isn't much space the deeper you go. The problem with sending it down to the mantle is that at the mantle, there's pressure pushing up, so you can't really force it down. Really the surface is the only place to go. You could have a massive robot elevator, that just dumps it all on the surface, like an enormous anthill.

If they can't do that, maybe they are limited to their original space, becoming ever more crowded and demented as the years wear on.

Also, you need to worry about the fact that the deeper you go, the warmer it gets.

asnys
04-21-2013, 06:30 PM
There isn't much space the deeper you go. The problem with sending it down to the mantle is that at the mantle, there's pressure pushing up, so you can't really force it down. Really the surface is the only place to go. You could have a massive robot elevator, that just dumps it all on the surface, like an enormous anthill.


As long as there's no outlet to the tunnel network, so the magma can't squirt out under pressure, I would think the pressure wouldn't matter? You take advantage of the higher density of solid rock compared to molten rock, and just let it sink into the mantle. That said, I'm not a geologist, hence why I'm asking.


Also, you need to worry about the fact that the deeper you go, the warmer it gets.

That one I'm already aware of, and have a plan for dealing with. I'm not sure yet if it'll work, but figuring out whether it will or not is a good math and coding exercise anyway. It involves active refrigeration and a massive network of somewhat-closer-to-the-surface tunnels through which waste heat is pumped, to use enormous amounts of surface area to make up for the poor thermal conductivity of rock.

Sarpedon
04-21-2013, 07:07 PM
As long as there's no outlet to the tunnel network, so the magma can't squirt out under pressure, I would think the pressure wouldn't matter? You take advantage of the higher density of solid rock compared to molten rock, and just let it sink into the mantle. That said, I'm not a geologist, hence why I'm asking

Not correct. See, you can compress air a lot more than you can compress rock. So the air pressure in your caverns won't be able to keep the magma from squirting up, causing the air pressure and temperature to increase to lethal levels. Magma is a lot less friendly than water is.

asnys
04-21-2013, 07:12 PM
Not correct. See, you can compress air a lot more than you can compress rock. So the air pressure in your caverns won't be able to keep the magma from squirting up, causing the air pressure and temperature to increase to lethal levels. Magma is a lot less friendly than water is.

Why can't you isolate it behind airlocks? Use a very high-pressure gas in the chamber over the magma tube to keep it from squirting out, sealed behind steel access doors, with robots to give the rock the final push into the tube. Given the heat and other potential hazards, you don't want actual people getting that close to the tube anyway.

Sarpedon
04-21-2013, 08:10 PM
If you have magic technology, sure I guess. Its just that having all this equipment right next to the magma would excaberate your heat dissipation problem. The pressures would be enormous, and by doing this you'd be potentially weakening the continental plate your thing is in, providing a soft spot where magma can infiltrate up.

asnys
04-21-2013, 08:29 PM
If you have magic technology, sure I guess. Its just that having all this equipment right next to the magma would excaberate your heat dissipation problem.

I don't want to go into magic territory, but they're going to need to have significantly better tech than we do just to survive down there at all. As long as it's something we could plausibly be able to do in the next century, I think it's fair.

As for heat dissipation, we'll see what the model says. I'm really not sure my dissipation loop plan will work, and if it doesn't I don't see any option other than allowing them some limited surface access, in which case the question of rock disposal is moot. But I'd really prefer to avoid that for aesthetic reasons - I'm not 100% wedded to the "absolutely no surface access" thing, but I'd really prefer to avoid it if there's absolutely any other way.


The pressures would be enormous, and by doing this you'd be potentially weakening the continental plate your thing is in, providing a soft spot where magma can infiltrate up.

Hmm. That could be a problem. Is there any way to determine how big an issue that would be? They're probably going to need more than one of these boreholes, after all, potentially quite a few depending on how fast the rock sinks and what the hole diameter is.

King Neptune
04-21-2013, 09:32 PM
I'd like to figure it out just for my own sake even if it never comes up in the story - and, honestly, if I read my own story and it wasn't explained in a footnote or something, it would really bug me trying to figure it out.

Very understandable, but there are ways around that problem. There are other problems of waste disposal/recycling. I woould hope that there is a source of water that can be tapped, and tht was running off to some other place. If such a source exists, then dropping what waste they need to get rid of into that stream would work.


I thought about using existing caverns, but they'll never be able to perfectly compactify the rock, so they'd be slowly losing volume over time.

Most caves become larger longer before they get smaller. Theere are huge, and I mean huge, cave complexes that appear to have existed for hundreds of thousands of years. You might want to look up some information on tunnelling.


While volcano disposal might be dangerous, from a narrative perspective that's not necessarily a bad thing - and they're probably going to be drilling into magma pockets for geothermal energy anyway.

How much energy will tyhey need? a little electricity for lights and cooking. They might also be able to recycle heat from the cave for that. The ground temperature starts rising when one goes more than a few hundred feet down. In any case, they will not want to drill into magma pockets, because that would start a volcano.

They will have to have an airshaft, even if they don't interact with surface dwellers.

asnys
04-21-2013, 10:09 PM
Very understandable, but there are ways around that problem. There are other problems of waste disposal/recycling. I woould hope that there is a source of water that can be tapped, and tht was running off to some other place. If such a source exists, then dropping what waste they need to get rid of into that stream would work.

I haven't yet decided if they're going to have an external water source or try to swing a completely closed water cycle. In either case, that might work for liquid wastes, but if you try that with waste rock, sooner or later you'll clog the stream.


Most caves become larger longer before they get smaller. Theere are huge, and I mean huge, cave complexes that appear to have existed for hundreds of thousands of years. You might want to look up some information on tunnelling.

I plan to. I'll look into that. :)


How much energy will tyhey need? a little electricity for lights and cooking. They might also be able to recycle heat from the cave for that. The ground temperature starts rising when one goes more than a few hundred feet down. In any case, they will not want to drill into magma pockets, because that would start a volcano.

They're going to want a lot more juice than that if they're going to run even a small industrial economy. And they'll need to have some amount of industry to survive down there for very long, completely cut off from the surface.

Tapping magma for energy, by the way, is not my idea; Sandia Lab studied it in the 70s. I haven't done any in-depth reading on it yet, but I've got a stack of pdfs on my harddrive from the Department of Energy. (I'm prioritizing the problems by difficulty of solution: waste rock seems hardest, followed by waste heat, then the air/water/food cycle, then energy. There's lots of ways to get energy; if geothermal won't work for some reason, there's always breeder reactors.)


They will have to have an airshaft, even if they don't interact with surface dwellers.

The plan at the moment is to have a mostly closed oxygen cycle based on algaeculture, with supplements from cracking oxide rocks to replace leaks.

Thanks, everyone, for the help! :)

King Neptune
04-21-2013, 11:52 PM
I haven't yet decided if they're going to have an external water source or try to swing a completely closed water cycle. In either case, that might work for liquid wastes, but if you try that with waste rock, sooner or later you'll clog the stream.

The problem of clogging would be far off in the future, if the waste dust were put in in small doses.


They're going to want a lot more juice than that if they're going to run even a small industrial economy. And they'll need to have some amount of industry to survive down there for very long, completely cut off from the surface.

Tapping magma for energy, by the way, is not my idea; Sandia Lab studied it in the 70s. I haven't done any in-depth reading on it yet, but I've got a stack of pdfs on my harddrive from the Department of Energy. (I'm prioritizing the problems by difficulty of solution: waste rock seems hardest, followed by waste heat, then the air/water/food cycle, then energy. There's lots of ways to get energy; if geothermal won't work for some reason, there's always breeder reactors.)

There are a number of geothermal power plants around the world. They don't get near magma; they drilled into rocks that are deep enough to be adequately hot. It is variable, but the rock is pretty warm a mile down, and the temperature rises at about 25C per km of depth, so you can work out how deep your setting will be and how deep they will have to drill to get enough energy.

[/QUOTE]The plan at the moment is to have a mostly closed oxygen cycle based on algaeculture, with supplements from cracking oxide rocks to replace leaks.

Thanks, everyone, for the help! :)[/QUOTE]

asnys
04-22-2013, 12:07 AM
There are a number of geothermal power plants around the world. They don't get near magma; they drilled into rocks that are deep enough to be adequately hot. It is variable, but the rock is pretty warm a mile down, and the temperature rises at about 25C per km of depth, so you can work out how deep your setting will be and how deep they will have to drill to get enough energy.

I know. Sandia was looking at something a little more exotic: immersing a heat exchanger in actual magma. This is kind of crazy, but may be worth it in this specific circumstance. As I understand it, conventional geothermal tends to produce relatively low-temperature steam. Combining that with the fact we're dumping heat to rock, not air or water, and our thermal efficiency would be absolutely terrible. That means you produce a lot of waste heat for every joule of electricity produced - even conventional power plants produce 1 - 2 joule of waste heat for every joule of electricity, and this will probably be significantly worse. Tapping magma means you have high-grade steam or gas, which should give a much higher thermal efficiency, meaning less waste heat over all. It also means you can use geothermal energy directly for process heat, which can improve your industrial energy efficiency quite a bit. And if we want to be able to tap the magma anyway for waste rock disposal...

Also, it's awesome. That's a not inconsiderable factor. :tongue

I'm not sure magma tap is actually practical - the atomic labs came up with some unusual ideas back in the day - or even if it is, that the math will work out to make it a superior option to conventional geothermal. But like I said, energy is at the bottom of the list of problems to solve; there are at least three viable approaches (magma tap, conventional geothermal, fission breeders), whereas I'm not sure any of my current approaches to waste rock or waste heat will work.

King Neptune
04-22-2013, 01:12 AM
I know. Sandia was looking at something a little more exotic: immersing a heat exchanger in actual magma. This is kind of crazy, but may be worth it in this specific circumstance. As I understand it, conventional geothermal tends to produce relatively low-temperature steam. Combining that with the fact we're dumping heat to rock, not air or water, and our thermal efficiency would be absolutely terrible. That means you produce a lot of waste heat for every joule of electricity produced - even conventional power plants produce 1 - 2 joule of waste heat for every joule of electricity, and this will probably be significantly worse. Tapping magma means you have high-grade steam or gas, which should give a much higher thermal efficiency, meaning less waste heat over all. It also means you can use geothermal energy directly for process heat, which can improve your industrial energy efficiency quite a bit. And if we want to be able to tap the magma anyway for waste rock disposal...

Also, it's awesome. That's a not inconsiderable factor. :tongue

I'm not sure magma tap is actually practical - the atomic labs came up with some unusual ideas back in the day - or even if it is, that the math will work out to make it a superior option to conventional geothermal. But like I said, energy is at the bottom of the list of problems to solve; there are at least three viable approaches (magma tap, conventional geothermal, fission breeders), whereas I'm not sure any of my current approaches to waste rock or waste heat will work.

There are problems with tapping directly into magma, but it could be done using tungsten, which has a melting temperature of 6,191F or 3422 C. Magma temperatures vary, but you'd have to go really deep before you'd find any that hot.

The problem with breeder reactors is that they are breeders; they create additional radioactive material, and that can be a problem if you have to hold onto it. Or would you be pumping the radioactive waste into the magma?

asnys
04-22-2013, 01:27 AM
There are problems with tapping directly into magma, but it could be done using tungsten, which has a melting temperature of 6,191F or 3422 C. Magma temperatures vary, but you'd have to go really deep before you'd find any that hot.

I haven't read through the pdfs I've got yet, but Sandia apparently built a prototype and tested it using lava produced by a thermal induction heater. The abstracts I've skimmed said the main anticipated problems were corrosion and finding suitable geological formations. I'd imagine cost would be an issue too.


The problem with breeder reactors is that they are breeders; they create additional radioactive material, and that can be a problem if you have to hold onto it. Or would you be pumping the radioactive waste into the magma?

Presumably they'd be disposing of the fission products however they dispose of the rest of their waste rock, either into the mantle or immobilized in some suitable material and dumped in a cavern or on the surface. (To be honest, I see these guys as living close enough to the edge that an elevated risk of cancer isn't going to be a big concern for them.) Excess plutonium could be used for a variety of purposes - maybe even Plowshare nuclear excavation charges, if the site is far enough from the main habitat to avoid risking ground shock damage.

I'd rather go with magma tap or conventional geothermal than breeders, however. Most of my previous setting ideas have been heavily atompunk, and I should break out of that rut. And if they're doing mantle rock disposal than they've got most of the infrastructure to do magma tap anyway.

King Neptune
04-22-2013, 02:49 AM
I haven't read through the pdfs I've got yet, but Sandia apparently built a prototype and tested it using lava produced by a thermal induction heater. The abstracts I've skimmed said the main anticipated problems were corrosion and finding suitable geological formations. I'd imagine cost would be an issue too.

Corrosion isn't much of a problem with tungsten, but finding as suitable location would be a new experience.


Presumably they'd be disposing of the fission products however they dispose of the rest of their waste rock, either into the mantle or immobilized in some suitable material and dumped in a cavern or on the surface. (To be honest, I see these guys as living close enough to the edge that an elevated risk of cancer isn't going to be a big concern for them.) Excess plutonium could be used for a variety of purposes - maybe even Plowshare nuclear excavation charges, if the site is far enough from the main habitat to avoid risking ground shock damage.

I'd rather go with magma tap or conventional geothermal than breeders, however. Most of my previous setting ideas have been heavily atompunk, and I should break out of that rut. And if they're doing mantle rock disposal than they've got most of the infrastructure to do magma tap anyway.

So they'll just leave the excess radioactive material out with the rest of the recyclables in the bin.

I also think that the mantle tap would be an interesting idea, and it isn't just an interesting fantasy.

Flutterlight
04-22-2013, 09:36 AM
Here's an idea- they can't interact directly with the surface, but suppose they have a air-locked chamber connected to an underground steam vent? They dump the excess into the vent, the steam forces it to the surface. Steam would create less pressure than magma.

asnys
04-22-2013, 04:41 PM
So they'll just leave the excess radioactive material out with the rest of the recyclables in the bin.

Pretty much. Not much else you can do with the stuff, and after they extract the transuranics the remainder should only last about 300 years. Like I said, though, I'm leaning towards magma tap or conventional geothermal instead.


Here's an idea- they can't interact directly with the surface, but suppose they have a air-locked chamber connected to an underground steam vent? They dump the excess into the vent, the steam forces it to the surface. Steam would create less pressure than magma.

Would steam actually have enough pressure to transfer significant quantities of waste rock to the surface?

kellycoinguy
04-26-2013, 12:45 AM
So here's a thought for you...

When you drill for oil, you pump a slurry down the pipe for lubrication and so forth. If the subterranean dwellers drilled UP to the bottom of an ocean or river bed, they could pump a slurry of water containing sand from their mining operations. They would have to have something like this to get water from the surface in any case, as they would need that for many uses, not the least of which would be air conditioning. Do you have any idea of the heat level that far down?

Or maybe it's on a cooler planet? LOL

-Kelly

kellycoinguy
04-26-2013, 12:48 AM
If you want to use reactors, I'd suggest Thorium reactors. No waste products to deal with.

-Kelly

asnys
04-26-2013, 01:24 AM
So here's a thought for you...

When you drill for oil, you pump a slurry down the pipe for lubrication and so forth. If the subterranean dwellers drilled UP to the bottom of an ocean or river bed, they could pump a slurry of water containing sand from their mining operations. They would have to have something like this to get water from the surface in any case, as they would need that for many uses, not the least of which would be air conditioning.

Hm. That's a thought, and it would also be a good way to simultaneously dispose of waste heat. I'd still like to keep them completely sealed off from the surface if possible, though - have them use recycling and underground aquifers for their water supply.


Do you have any idea of the heat level that far down?

The temperature rises about 25 C per kilometer, possibly more since they're probably in an area of significant geological activity if they're doing magma tap.


If you want to use reactors, I'd suggest Thorium reactors. No waste products to deal with.

That's not really true. Don't get me wrong, I'm a total nuclear geek, and I wish Kirk Sorenson and his team all the best; if their machine actually works the way they claim it will they'll be doing the world an enormous favor. But thorium is not the magic bullet for waste it's been claimed to be. Thorium eliminates 99% of the transuranic waste, the waste formed when an atom of uranium absorbs a neutron instead of fissioning. Since the transuranics form 99% of the stuff that lasts thousands and thousands of years, that makes the waste problem much easier to handle. But it produces just as many fission products - the "ash" atoms left behind when a uranium atom splits - as conventional reactors. Almost all of the fission products die off after 300 years, so a thorium reactor's waste stream is a lot better than a conventional reactor's, but it's not zero. And there are ways to achieve the same result or better using uranium-based fuels in fast reactors, although currently available fast reactor technology has cost and safety issues.

If I do go with nuclear reactors, I'll go with fast breeders. From a narrative perspective, having them use dangerous technology is a positive, and fast breeders can make plutonium really easily, which means they can build stuff like Plowshare nuclear excavation charges. But I'm about 99% certain I'm going to go with magma tap instead of nuclear - it fits the atmosphere better.

Thanks again for the help, everyone!

jaksen
04-29-2013, 02:23 AM
I'd just ship it to the surface, which would be the worst job for anyone who had to do this. Yikes, pushing or lugging or transporting waste rock to the surface - the deadly surface? With the deadly light shining down and all that horrible waste gas the atmosphere is full of. And creatures, who are better adapted to the surface and sometimes just wait at the dumping hole entrances to - what? EAT US?

Yeah, that job would go to the peons, the slaves, the underlings, the untouchables, or criminals, the unwanted, etc.

kellycoinguy
04-29-2013, 05:16 AM
Hm. That's a thought, and it would also be a good way to simultaneously dispose of waste heat. I'd still like to keep them completely sealed off from the surface if possible, though - have them use recycling and underground aquifers for their water supply.


They would likely have to drill up even to find an underground aquifer.

Even using air conditioning, they have to have somewhere to dump the heat. Exchanging with an ocean bottom is undetectable by surface people because 1) the ocean is large 2) there are thermal vents under the ocean that are volcanic, so even if you know what to look for, you couldn't find it.



The temperature rises about 25 C per kilometer, possibly more since they're probably in an area of significant geological activity if they're doing magma tap.

Thanks again for the help, everyone!

Have you given much consideration to the fact that they would need a lot of plants down there for food and oxygen replenishment?

Sarpedon
04-29-2013, 05:37 PM
Don't forget that continents and oceans are two different kinds of plates, so you couldn't really have a underground civilization that is connected to both, unless it is connected to the continental shelf part of the continental plate, which would preclude it being connected to the 'deep' ocean.

asnys
04-29-2013, 09:34 PM
They would likely have to drill up even to find an underground aquifer.

Freshwater aquifers go down to 2 km, and salt water down to 10 km. They may not be available wherever they are, but sideways or down may be options as well. (If Dwarf Fortress has taught me anything, it's that you NEVER tap an aquifer from below. :tongue)


Even using air conditioning, they have to have somewhere to dump the heat. Exchanging with an ocean bottom is undetectable by surface people because 1) the ocean is large 2) there are thermal vents under the ocean that are volcanic, so even if you know what to look for, you couldn't find it.

That's my backup option at the moment, in case my preferred option doesn't work (waste rock into the mantle, waste heat via massive underground "radiators".) If I can make it work, I'd prefer to keep them totally cut off, for atmospheric reasons.


Have you given much consideration to the fact that they would need a lot of plants down there for food and oxygen replenishment?

Some. Some back-of-the-envelope calculations I did a long time ago suggest that the energy requirements for food crops will be positively titanic - about an order of magnitude greater than our current per capita energy consumption. Genetic engineering and using algae may be able to help with that, though - regular food crops are just ridiculously energy inefficient. Right now, though, I'm focusing on the waste rock and heat issues, since they seem harder to solve.


Don't forget that continents and oceans are two different kinds of plates, so you couldn't really have a underground civilization that is connected to both, unless it is connected to the continental shelf part of the continental plate, which would preclude it being connected to the 'deep' ocean.

Even if it weren't for the plate issue, it strikes me as economically implausible to try to maintain tunnels that long. At the moment I'm somewhat leaning in the direction of a sub-ocean habitat, because there's all that ocean for extra camouflage and shielding, it makes oceanic rock and heat disposal available if I can't make mantle disposal work, and the crust is a lot thinner in the deep ocean, making it easier to reach magma for power and waste disposal.

Sarpedon
04-29-2013, 10:02 PM
Sounds good, just remember that oceanic plates are basaltic, making metal deposits uncommon/nonexistent.