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Old September 15 2012, 05:09 AM   #136
Crazy Eddie
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Re: Envisioning the world of 2100

Mars wrote: View Post
So tell me why can't you run a nuclear reactor for 5000 years?
Because running at full power, a typical fuel rod will only last for 20 (25 if you're lucky) before it decays to the point of no longer producing useable heat and subsequently becoming a serious radiation hazard. And that's before you take into account neutron breakdown of shielding materials and the piping of the heat exchangers, which over time become brittle and have to be replaced considerably more often. You would essentially have to overhaul the entire reactor every ten years of continued operation, replacing the fuel rods every other overhaul. That is no easy task, even for a machine.

In other words, your robots will have to completely rebuild the entire reactor two hundred and fifty times before the end of the voyage. God help you if you've got multiple reactors on board.

Which goes to the overall point of this being a fundamentally impractical endeavor: that's a LOT of new capabilities being developed for a space craft that doesn't actually accomplish any concrete goal for anyone. Your stated goal is to ensure the survival of the human race 5000 years in the future, yet the sheer massive amount of resources that would be needed for a project this ambitious could be more efficiently used to eradicate world hunger, terraform Mars and tap the methane lakes of Titan to provide the world with an inexhaustible energy supply. It's a highly expensive and complicated solution to a problem that may or may not even exist.

Mars wrote: View Post
The AI can slow down his consciousness so he won't get bored
And you know this how?

Mars wrote: View Post
The problem is that it may not be left alone by the billions of other humans and sentient AIs that are also inhabiting the system...
Neither will your generation ship if the AIs decide to chase after it. Or, for that matter, if the pilot AI gets an email from Earth containing the Cyberdyne Manifesto and decides to turn around and head back.

There would be insufficient isolation if it stayed in the Solar System
Right, because PUTTING AN AI ON THE GENERATION SHIP is isolation enough.

How can you know that the human race will survive for the next 5000 years?
Because in the collected sum of mankind's knowledge about itself, its world, the solar system that contains the world and the immediate vicinity of our stellar neighborhood, there is no reason whatsoever to believe that it WON'T.

More importantly -- and more relevantly to this thread -- I, like most human beings, don't give a damn one way or the other what might happen five thousand years from now. This is a thread about the world of 2100, less than a century into the future, at a time when my son will be watching his grandchildren go on to take meaningful careers.

So if I'm to worry about the future at all, it'll be whether or not humanity is going to survive for the next FIFTY years. Is a generation ship on a 5000 year voyage a good way to insure that? No? Then why the hell would I want to spend money building one?


its harder to get billions of human beings and control them all so they don't build AIs
On the other hand, it's relatively easy to control the six or seven thousand people on the entire planet who are even remotely smart enough to attempt to build an AI. We ALREADY do this with nuclear non-proliferation.

On the other hand, you don't have a shred of evidence that the emergence of humanlike AI is even possible, let alone inevitable, let alone that any negative consequence would follow for humanity if it was.

Its not going to work, because the one who violates the treaty will always be at an immediate advantage.
Until the next biggest country bombs him back into the stone age for violating the treaty.

A lot more can go wrong with a community of billions of humans than with an isolated starship with frozen embryos traveling the void between the stars
Which is trivially true. The problem with this statement is that just about anything that can go seriously wrong on a space ship will usually result in the destruction of that space ship. With a population of 7 billion, a global-scale catastrophe could annihilate 99% of the human race and that would still leave more survivors than most countries have people.
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Last edited by Crazy Eddie; September 15 2012 at 05:42 AM.
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Old September 15 2012, 05:39 AM   #137
Crazy Eddie
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Re: Envisioning the world of 2100

sojourner wrote: View Post
newtype_alpha wrote: View Post
sojourner wrote: View Post
^Just to play devil's advocate on that last part, how do you prevent the Yellowstone supervolcano from erupting?
Assuming that theory has any validity, the simplest solution is to move the fuck out of yellowstone.
You do realize that if it blows you'd have to move out to the fucking moon to avoid the catastrophe it would cause. It's an ELE.
Which the Earth has experienced MANY times before, and yet is somehow still habitable. I'm not saying it wouldn't thoroughly suck. I'm saying it would not cause the extinction of the human race (just a massive and traumatic thinning of our numbers).

OTOH, relocating to the moon doesn't sound like such a bad idea, all things considered. By 2100, I'd be surprised if it wasn't a serious alternative.
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Old September 15 2012, 12:57 PM   #138
Mars
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Re: Envisioning the world of 2100

newtype_alpha wrote: View Post
Mars wrote: View Post
So tell me why can't you run a nuclear reactor for 5000 years?
Because running at full power, a typical fuel rod will only last for 20 (25 if you're lucky) before it decays to the point of no longer producing useable heat and subsequently becoming a serious radiation hazard.
Ah, but you forget, in space no longer usable fuel rods can be tossed overboard and never seen again, you don't need to store them anywhere or worry about their radiation.

And that's before you take into account neutron breakdown of shielding materials and the piping of the heat exchangers, which over time become brittle and have to be replaced considerably more often.
Well the nuclear power plant will only be at full power for 20 years out of the 5000 year voyage, for the rest of the voyage, it will only be used to provide light and heat for the habitat. There is no other power source available in interstellar space until the discovery of fusion.

You would essentially have to overhaul the entire reactor every ten years of continued operation, replacing the fuel rods every other overhaul. That is no easy task, even for a machine.
If humans can do it, a machine can do it, machines will probably already be doing most of this stuff for nuclear reactors anyway.

In other words, your robots will have to completely rebuild the entire reactor two hundred and fifty times before the end of the voyage. God help you if you've got multiple reactors on board.
So it seems to me to be more than equal to the taske performed by a calculator brain, you'd need an AI to oversee things, as your not going to get a human crew to do this for 5000 years, and pushing the ship to faster speeds has other problems. I'd say compared to building a starship that can reach speeds of up to 10% of the speed of light, building a slow starship like this would be easy, even when these things are taken into consideration. For the price of one speedy starship, you could probably build 100 of the slow ones.

Which goes to the overall point of this being a fundamentally impractical endeavor: that's a LOT of new capabilities being developed for a space craft that doesn't actually accomplish any concrete goal for anyone. Your stated goal is to ensure the survival of the human race 5000 years in the future, yet the sheer massive amount of resources that would be needed for a project this ambitious could be more efficiently used to eradicate world hunger, terraform Mars and tap the methane lakes of Titan to provide the world with an inexhaustible energy supply. It's a highly expensive and complicated solution to a problem that may or may not even exist.
But slow starships are relatively cheap to build, this one would be the size of an O'Neill habitat, and not much more expensive to build, it can house 10,000 people when fully occupied, and an ion drive would accelerate it up to 300 km/sec and then slow it down again at the end of the journey, this is not much in the realm of starships, certainly teraforming Mars would be more expensive than this.

As for World hunger, AIs alone could solve this problem, AI's could grow the food, AIs could do all the work, the rest is just a matter of redistribution. If we can get enough AIs to outnumber the human population, the starving millions need not even work for their food. I don't think there is any chance of the human race dying of starvation, maybe gluttony perhaps.

I do perhaps think that maybe humans need to work to stay healthy, having machines do everything for them may pose a long term risk to their survival, which is why establishing a distant colony of humans is important, and isolation from the rest of humanity is also important.

Mars wrote: View Post
The AI can slow down his consciousness so he won't get bored
And you know this how?


Neither will your generation ship if the AIs decide to chase after it. Or, for that matter, if the pilot AI gets an email from Earth containing the Cyberdyne Manifesto and decides to turn around and head back.
Machines would be in the same peril from obsolescence as humans. I think the event would be more like a forest fire, chances are it wouldn't spend much effort looking for remote spaceships in interstellar space, anyway being far away from the conflagration would be better than being right in it.

Right, because PUTTING AN AI ON THE GENERATION SHIP is isolation enough.


Because in the collected sum of mankind's knowledge about itself, its world, the solar system that contains the world and the immediate vicinity of our stellar neighborhood, there is no reason whatsoever to believe that it WON'T.

More importantly -- and more relevantly to this thread -- I, like most human beings, don't give a damn one way or the other what might happen five thousand years from now. This is a thread about the world of 2100, less than a century into the future, at a time when my son will be watching his grandchildren go on to take meaningful careers.
Well this ship can certainly be launched by 2100, that is it can be built in that time frame with that time frame's technology, so it is an appropriate subject. Maybe you don't care if the human race perishes but I do, and I'm examining what technologies in the late 21st century might be available to save it.

So if I'm to worry about the future at all, it'll be whether or not humanity is going to survive for the next FIFTY years. Is a generation ship on a 5000 year voyage a good way to insure that? No? Then why the hell would I want to spend money building one?
Both 2100 and 5000 years is beyond my expected life span, so I worry about them equally, because I don't expect to personally live in the year 2100, so I remove myself from immediate consideration, it is the rest of humanity, those that will come after me that I am concerned about, since I both won't live in the year 2100 or 7100, I am not concerned more about one than the other, they are of equal concern to me as it is about the future of the human race.

Why does one buy life insurance, this is life insurance for the human race, I think it is worth some effort and expense.

On the other hand, it's relatively easy to control the six or seven thousand people on the entire planet who are even remotely smart enough to attempt to build an AI. We ALREADY do this with nuclear non-proliferation.
I hate to tell you this, but its not working, North Korea has acquired the nuclear bomb and Iran is acquiring it, and shunning them, or not trading with them is not going to stop them, the only way to control the entire planet is with a world government, and that's another fate which I wish to avoid, that of the human race being controlled and turned into drones, Ala "the Borg". Humans need to spread out over time and space so they don't get turned into an ant colony. A world government and trying to control us all puts all our eggs in one basket, if all our decisions get made at the top of government, then the wrong decisions could wipe us out!

On the other hand, you don't have a shred of evidence that the emergence of humanlike AI is even possible, let alone inevitable, let alone that any negative consequence would follow for humanity if it was.
Its hard to predict the unpredictable, AIs will be quite clever, they'll be agents of change, you can't predict what that change will be or what that change will do, the more change there is the greater the risk to the human race. So we have to spread the human race out over time and space to prevent us all from getting caught in one of our mistakes.

Its not going to work, because the one who violates the treaty will always be at an immediate advantage.
Until the next biggest country bombs him back into the stone age for violating the treaty.
And we're back to starting a nuclear war aren't we.
A lot more can go wrong with a community of billions of humans than with an isolated starship with frozen embryos traveling the void between the stars
Which is trivially true. The problem with this statement is that just about anything that can go seriously wrong on a space ship will usually result in the destruction of that space ship. With a population of 7 billion, a global-scale catastrophe could annihilate 99% of the human race and that would still leave more survivors than most countries have people.
Its not the numbers but how far they are spread out that matters. If you can pack one billion human beings within the radius of one nuclear bomb blast, then they all die if one goes off, It is the limited scope of humanity on one planet that is the danger. The thing about spaceships, especially this one, is if it gets destroyed, no one dies, it is simply that no one gets born afterwards. One can have multiple spaceships, and you limit your chance of having one all-consuming disaster that destroys us all.
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Old September 15 2012, 05:15 PM   #139
gturner
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Re: Envisioning the world of 2100

Mars wrote: View Post
newtype_alpha wrote: View Post
Mars wrote: View Post
So tell me why can't you run a nuclear reactor for 5000 years?
Because running at full power, a typical fuel rod will only last for 20 (25 if you're lucky) before it decays to the point of no longer producing useable heat and subsequently becoming a serious radiation hazard.
Ah, but you forget, in space no longer usable fuel rods can be tossed overboard and never seen again, you don't need to store them anywhere or worry about their radiation.
And that's why your ship will get boarded for a "health and safety inspection."

Well the nuclear power plant will only be at full power for 20 years out of the 5000 year voyage, for the rest of the voyage, it will only be used to provide light and heat for the habitat. There is no other power source available in interstellar space until the discovery of fusion.
A molten salt reactor is probably going to be better than a conventional nuclear reactor, for a host of reasons.

1) A conventional reactor runs at very high operating pressures, requiring a heavy containment vessel, yet the containment vessel has to open to allow replacement of the fuel rods. That's very heavy and complicated.

A molten salt reactor can operate at atmospheric pressure, so the reactor vessel itself doesn't need to contain high pressures.

2) A conventional reactor has all the short half-life highly radioactive breakdown products trapped in the fuel rods, which not only causes problems with fuel poisoning, but the issue of a massive release of radiation during a melt-down.

Molten salt reactors allow the seperation of breakdown products from the fuel during operation because they boil out of the liquid fuel, making it easy to vent them somewhere else for storage, if even venting them overboard as a gas.

3) Having the reaction products boil out as a gas also eliminates most of the problems with reactor poisoning, an issue primarily with xenon-135, which has a 9 hour half-life and a huge neutron capture cross section, potentially sucking up so many neutrons that the reactor won't restart.

Reactor poisoning isn't much of an issue unless you have to rapidly cycle the ship's power levels for maneuvering to avoid space objects, but if you might have to do that, then designing to avoid poisoning is critical.

Naval powerplants use highly enriched uranium (instead of very low enriched uranium like commercial reactors) because they have to guarantee full power operation for combat maneuvers regardless of the output power levels over the previous few hours.

But going that route means you've massively increased the cost of the fuel, and created further handling problems because it's much, much more fissionable than commercial fuel.

4) Conventional solid fuel rods will need to be reprocessed, which is technically difficult.

If you toss them overboard, you're throwing away 97% of your potential fuel, which means you have to store over thirty times as much initial fuel on board.

The fuel rods, obviously, can't be stored too close together or you could risk a chain reaction, especially if a moderator is introduced (such as from a broken water pipe). So each future fuel rod has a shielding or storage space overhead.

Unnecessarily upping the mass and space requirements for your fuel storage by perhaps several hundred times isn't a good design, and the ship will have to be able to reprocess fuel anyway, because if it can't, it will one day run out of fuel without the ability to manufacture any more no matter how much uranium the crew can dig up.

5) If the molten salt reactor uses thorium, which is extremely likely, then you not only can reprocess the fuel on the fly, as part of the normal reaction cycle, but you can store tons of thorium as a giant lump, because it won't support a nuclear chain reaction no matter how pure it is. Thorium is also 100% fertile, so it can all be burned up. With the uranium cycle, you've generally got a lot of excess U-238 to deal with.

6) Molten salt reactors can run at much higher temperatures, making them more thermodynamically efficient. That means more available power to the drive system for a given thermal output.

On Earth, where the heat rejection is limited by outside ambient temperature, they could hit roughly 50% efficiency in a single fluid design. Water cooled uranium reactors run at about 35% efficiency.

In deep space, both reactors could have their efficiency impoved by further stages using lighter gases with lower boiling points, but the molten salt reactor would retain the advantage.

7) Molten salt reactors have a much higher specific power density, which is the energy output per mass. Conventional nuclear reactors work well in ships, which ran pretty well with coal-fired steam piston engines, but molten salt reactors were first designed to power an Air Force bomber. In aerospace applications, that's an advantage that's hard to ignore.

8) Conventional reactors have to be shut down for long periods to replace the fuel rods. That means you have to have multiple reactors to guarantee that power will always be available to keep the ship from freezing.

Molten salt reactors can circulate the fuel in and out as part of normal operations, so they never actually need to shut down. They can also be shut down, the fuel drained, and then refilled and restarted up to full power in just a few hours, as opposed to weeks or months with a conventional reactor.

9) Since their reactor vessels doesn't have to hold high pressures, they are thin and lightweight, which means they are vastly easier to store, move, or fabricate, and multiple vessels could be carried for inflight swapping if neutron embrittlement becomes an issue.

The reactor shielding (the room's walls) doesn't have to be structural, so it never has to be swapped.

So neutron embrittlement is at least easier to cope with in a molten salt reactor that has to run for centuries, because fabricating a very thick, high pressure vessel is always difficult, or requires carrying a whole lot of extra steel.

10) Thorium is more abundant than uranium, and doesn't require enrichment, so any human colony using thorium just needs to mine it, instead of trying to build an isotope seperation facility.

Canadian CANDU reactors don't require fuel enrichment, but they do require a source of deuterium, which they extract from seawater. Though not technically difficult, it does require processing about 6,000 times as much water as would otherwise be required.

And if the destination solar system is much older (or derived from older source materials) the U-235/U-238 ratio will be lower, possibly preventing even a CANDU from running without some level of fuel enrichment.

****

A final note is that from an engine standpoint, you've got the initial and final acceleration phases (which burn fuel), and your coast phase where you keep the ship from freezing (which burns fuel). Once you've got hard numbers for the power requirements of those phases, you'd optimize for the minimal total energy consumed during the flight. If the deep-space heating and lighting is a huge demand and delta V isn't that expensive, you'd accelerate more to shorten the trip (100 years of lighting takes 50 times less fuel than 5000 years of lighting).
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Old September 15 2012, 07:12 PM   #140
publiusr
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Re: Envisioning the world of 2100

Because, last time I checked, the STS is no longer operational and its replacement won't be ready before the end of the decade.

And its enemies would love to kill it thereby wasting money when they should be supporting it.

It was THINKING BIG that gave us the shuttle program...effectively cost us our first and orbiting launch platform when the shuttle mission failed to replace Apollo.

Apollo was thinking big. STS was thinking reusable.

Any prudent business manager knows that "Thinking Big" is not something you do when you're just starting out

We are not just starting out--and how is that starting out when folks want to kill it and not give it a chance

It would be a pretty sweet payoff if the SLS actually flies on time and on schedule... but what exactly does NASA plan to do if it blows up on the launch pad?

What does NASA doe when any rocket blows up? Good grief--you're just being difficult.

If you were in any way concerned about the opinions of real experts you wouldn't be spamming unsourced essays from the nasaspaceflight.com forum.

The piece was from Covault, BTW. Of Av Week and space, and the folks he had on there are hardly unknowns

So Griffin--who wrote AIAA texts is unsourced. A man on Augustine who trashed HLVs, gave us the Roton.

Pot, Kettle

And for those of you who think this isn't relavent--what do you think launches heavy reactors--Delta IIs? Get the ride first--the reactor later.
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Old September 15 2012, 08:01 PM   #141
gturner
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Re: Envisioning the world of 2100

publiusr wrote: View Post
Because, last time I checked, the STS is no longer operational and its replacement won't be ready before the end of the decade.

And its enemies would love to kill it thereby wasting money when they should be supporting it.
Its cost is going to kill it, not its enemies. The budget axes are going to start falling all over government, and NASA is going to have trouble maintaining a drawn-out development program for something that doesn't even have a payload when other branches are cutting essential services. It's going to face the same gauntlet as Ares and Constellation.

The Block 0 SLS can lift 70 tonnes and throws away both the solids and 3 RS-25D engines. The estimated cost of the Block 0 is $1.4 billion, which is $44,000 a pound to LEO, and its first launch is in 2018. Under the plan with the highest flight rate, it will then fly once a year till the launch of SLS #5 (which is upgraded to 100 or 130 tons), in 2021, and under the curent Senate authorization will fly once a year till 2026. That's nine flights for the next 14 years. Under the President's current budget we get 4 flights over the next 14 years. That's 930 metric tons delivered to LEO by 2026.

http://www.spacepolicyonline.com/pag...on_2011-08.pdf

In the 1990's the Shuttle was making about 7 flights a year, and just counting the roughly 55,000 lbs in the cargo bay, that comes to 175 metric tons delivered to orbit, per year, which would've been 2450 metric tons delivered between now and 2026, instead of the SLS's 930 tons under the Senate plan, or the 280 tons under the President's budget.

According to the plans the SLS isn't even as good at putting mass into orbit as the Shuttle was over the course of the next decade and a half, by a factor of about 3 to 10.

If that's the plan the execution certainly won't exceed it.

That's just one reason why the SLS program gives me cause for concern.
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Old September 15 2012, 08:27 PM   #142
publiusr
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Re: Envisioning the world of 2100

It's what we have. My concern is this habit of starting something--then someone killing it--then starting something else, and its enemies killing that. SLS won't have that pesky orbiter to contend with. The SLS up mass will be all payload. This means that ISS would actually have been done more quickly with fewer large modules than with a lot of smaller 20 ton modules. That puts it ahead of an STS that wasted a lot of power on placing an orbiter in LEO.

I just don't like the idea of a lot of liquid sloshing around up there in a depot with boil-off problems. Now newtype wants a return to hypergolics--and a hypergolic depot would be fine--but we aren't going down that path. That decision has been made. The plus that comes from larger LVs for exploration is that you do all your fueling on ground level--put a heck of an upper stage up there with a big rocket--and get rid of those liquids as fast as you can. Things do wear in space--we saw that with LDEF and ISS solar panels, micrometeoroids, etc.

The plan is political, but there is nothing to say that if STS can fly 7 times of year with an orbiter--that SLS couldn't match that without one. That's a budgetary question--but that affects everything. We might get a president that destoys NASA--and then we get only what Musk can fund privately--and his LV was 80-90% gov't, so is that really private spaceflight if he is just another contactor?

Having 70 tons in space (orbiter free) allows for simpler missions, no Rube Goldberg assembly --at least for awhile--until we can afford a Mars ship that is more responsible than Musk's laughable one-way missions. I've been a big fan of Cassini's Ms. Porco, and she and others see easier sample return missions, icy moon landers and the like coming from SLS. It is capability--not frequency of launch--that is most important.

STS was an HLLV with only a Titan IV payload--and thus redundant. Free it of the orbiter, and you open up BEO in a responsible and engineering friendly fashion without depots that could easily make space debris worse by being a prolonged target to expansion, debris hits, etc. I don't want one of those things blowing up and making LEO a minefield.

Let's put that off--or at least have the depot a more sturdy SLS launched design with some meteor bumpers, and not an EELV launched balloon tank eggshell of a Centaur. That spooks the crap out of me. We had a small Briz failure a few years ago and it put more debris up there than the China ASAT test and the US sat-shootdown combined.

We have a space race exactly because R-7 was more than an ICBM needed to be. Where we shrank the payload, the Soviet response was--make the rocket bigger.

And you know something? It worked.

Last edited by publiusr; September 15 2012 at 08:38 PM.
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Old September 15 2012, 09:11 PM   #143
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Re: Envisioning the world of 2100

SLS costs more per kg of payload than a smaller launcher. End of argument.
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Old September 15 2012, 10:24 PM   #144
gturner
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Re: Envisioning the world of 2100

sojourner wrote: View Post
SLS costs more per kg of payload than a smaller launcher. End of argument.
I'm not sure SLS ever gets below $20,000 a pound to LEO. SpaceX is already selling Falcon 9 launches at $2,400 per pound to LEO. And that's their expensive, early system. He thinks $500 a pound is doable with expendables, and hopefully get down to $100 a pound if his grasshopper goes well.

If the government can launch 200 times the payload for the same dollar, they would be fiscally insane to retain the SLS.

publiusr wrote:
--and then we get only what Musk can fund privately--and his LV was 80-90% gov't, so is that really private spaceflight if he is just another contactor?
Of the $1.2 billion in SpaceX (now valued at about $2.4 billion), NASA has contributed about $500 million, which is 20 to 40 percent, not 80 or 90%, and their private Iridium launch contract is $492 million.
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Old September 16 2012, 06:47 AM   #145
Mars
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Re: Envisioning the world of 2100

gturner wrote: View Post
Mars wrote: View Post
newtype_alpha wrote: View Post
Because running at full power, a typical fuel rod will only last for 20 (25 if you're lucky) before it decays to the point of no longer producing useable heat and subsequently becoming a serious radiation hazard.
Ah, but you forget, in space no longer usable fuel rods can be tossed overboard and never seen again, you don't need to store them anywhere or worry about their radiation.
And that's why your ship will get boarded for a "health and safety inspection."

Well the nuclear power plant will only be at full power for 20 years out of the 5000 year voyage, for the rest of the voyage, it will only be used to provide light and heat for the habitat. There is no other power source available in interstellar space until the discovery of fusion.
A molten salt reactor is probably going to be better than a conventional nuclear reactor, for a host of reasons.

1) A conventional reactor runs at very high operating pressures, requiring a heavy containment vessel, yet the containment vessel has to open to allow replacement of the fuel rods. That's very heavy and complicated.

A molten salt reactor can operate at atmospheric pressure, so the reactor vessel itself doesn't need to contain high pressures.

2) A conventional reactor has all the short half-life highly radioactive breakdown products trapped in the fuel rods, which not only causes problems with fuel poisoning, but the issue of a massive release of radiation during a melt-down.

Molten salt reactors allow the seperation of breakdown products from the fuel during operation because they boil out of the liquid fuel, making it easy to vent them somewhere else for storage, if even venting them overboard as a gas.

3) Having the reaction products boil out as a gas also eliminates most of the problems with reactor poisoning, an issue primarily with xenon-135, which has a 9 hour half-life and a huge neutron capture cross section, potentially sucking up so many neutrons that the reactor won't restart.

Reactor poisoning isn't much of an issue unless you have to rapidly cycle the ship's power levels for maneuvering to avoid space objects, but if you might have to do that, then designing to avoid poisoning is critical.

Naval powerplants use highly enriched uranium (instead of very low enriched uranium like commercial reactors) because they have to guarantee full power operation for combat maneuvers regardless of the output power levels over the previous few hours.

But going that route means you've massively increased the cost of the fuel, and created further handling problems because it's much, much more fissionable than commercial fuel.

4) Conventional solid fuel rods will need to be reprocessed, which is technically difficult.

If you toss them overboard, you're throwing away 97% of your potential fuel, which means you have to store over thirty times as much initial fuel on board.

The fuel rods, obviously, can't be stored too close together or you could risk a chain reaction, especially if a moderator is introduced (such as from a broken water pipe). So each future fuel rod has a shielding or storage space overhead.

Unnecessarily upping the mass and space requirements for your fuel storage by perhaps several hundred times isn't a good design, and the ship will have to be able to reprocess fuel anyway, because if it can't, it will one day run out of fuel without the ability to manufacture any more no matter how much uranium the crew can dig up.

5) If the molten salt reactor uses thorium, which is extremely likely, then you not only can reprocess the fuel on the fly, as part of the normal reaction cycle, but you can store tons of thorium as a giant lump, because it won't support a nuclear chain reaction no matter how pure it is. Thorium is also 100% fertile, so it can all be burned up. With the uranium cycle, you've generally got a lot of excess U-238 to deal with.

6) Molten salt reactors can run at much higher temperatures, making them more thermodynamically efficient. That means more available power to the drive system for a given thermal output.

On Earth, where the heat rejection is limited by outside ambient temperature, they could hit roughly 50% efficiency in a single fluid design. Water cooled uranium reactors run at about 35% efficiency.

In deep space, both reactors could have their efficiency impoved by further stages using lighter gases with lower boiling points, but the molten salt reactor would retain the advantage.

7) Molten salt reactors have a much higher specific power density, which is the energy output per mass. Conventional nuclear reactors work well in ships, which ran pretty well with coal-fired steam piston engines, but molten salt reactors were first designed to power an Air Force bomber. In aerospace applications, that's an advantage that's hard to ignore.

8) Conventional reactors have to be shut down for long periods to replace the fuel rods. That means you have to have multiple reactors to guarantee that power will always be available to keep the ship from freezing.

Molten salt reactors can circulate the fuel in and out as part of normal operations, so they never actually need to shut down. They can also be shut down, the fuel drained, and then refilled and restarted up to full power in just a few hours, as opposed to weeks or months with a conventional reactor.

9) Since their reactor vessels doesn't have to hold high pressures, they are thin and lightweight, which means they are vastly easier to store, move, or fabricate, and multiple vessels could be carried for inflight swapping if neutron embrittlement becomes an issue.

The reactor shielding (the room's walls) doesn't have to be structural, so it never has to be swapped.

So neutron embrittlement is at least easier to cope with in a molten salt reactor that has to run for centuries, because fabricating a very thick, high pressure vessel is always difficult, or requires carrying a whole lot of extra steel.

10) Thorium is more abundant than uranium, and doesn't require enrichment, so any human colony using thorium just needs to mine it, instead of trying to build an isotope seperation facility.

Canadian CANDU reactors don't require fuel enrichment, but they do require a source of deuterium, which they extract from seawater. Though not technically difficult, it does require processing about 6,000 times as much water as would otherwise be required.

And if the destination solar system is much older (or derived from older source materials) the U-235/U-238 ratio will be lower, possibly preventing even a CANDU from running without some level of fuel enrichment.

****

A final note is that from an engine standpoint, you've got the initial and final acceleration phases (which burn fuel), and your coast phase where you keep the ship from freezing (which burns fuel). Once you've got hard numbers for the power requirements of those phases, you'd optimize for the minimal total energy consumed during the flight. If the deep-space heating and lighting is a huge demand and delta V isn't that expensive, you'd accelerate more to shorten the trip (100 years of lighting takes 50 times less fuel than 5000 years of lighting).
Thank you for your expertise in this matter. I am not wedded to any particular reactor design, I just assumed the fuel would be uranium because that is what powers commercial reactors.
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Old September 16 2012, 05:09 PM   #146
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Re: Envisioning the world of 2100

^ Ironically, LFTR (Liquid Fluoride Thorium Reactors) were a dead relic of the 1950's and 60's until Kirk Sorensen of NASA was looking into reactor designs more suited to space travel. He started http://energyfromthorium.com/ and then founded Flibe energy to build reactors, inititally focusing on the military market because they have their own seperate certifications (If military reactors had all the red-tape and roadblocks as commercial nuclear powerplants, it would take decades to launch a new submarine).

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Old September 17 2012, 01:48 PM   #147
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Re: Envisioning the world of 2100

How do you propose handling the corrosion problems of thorium-cycle molten salt reactors? Isn't that the main thing that makes them impractical for long-term use?
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Old September 17 2012, 03:21 PM   #148
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Like I said, whatever works, whatever off the shelf commercially available nuclear reactor at the end of the 21st century, I admit I am not a nuclear engineer, but I know about nuclear power plants, and the Fusion program just seems to be soaking up money and their progress is so slow with them talking multiple decades just to build an experimental reactor the ITER, and multiple decades more to build a commercial power plant with what was learned. I think if we are to do a starship by the end of this century, it could only reliably count on atomic fission as its main power source, I've calculated by the way it would take 1000 metric tons of fissionables to light and heat a 500 meter diameter Island One Bernal Sphere habitat for every 1000 years of the journey, and at 300 km/sec, that journey to Alpha Centauri would take 4100 years, because Alpha Centauri is approaching us, so figure 4100 metric tons of fusion fuel for lighting and heating the habitat.
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Old September 17 2012, 05:43 PM   #149
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Re: Envisioning the world of 2100

Robert Maxwell wrote: View Post
How do you propose handling the corrosion problems of thorium-cycle molten salt reactors? Isn't that the main thing that makes them impractical for long-term use?
It's not a horribly difficult problem, and one that has some pretty good solutions.

Here's a 2010 report from Idaho National Labs on the issue, which is covered on pages 5 to 10. (Highly recommended, easy to read with lots of graphs)

http://www.inl.gov/technicalpublicat...ts/4502649.pdf

It looks like chromium content in a molten salt with graphite causes most problem with favored alloys, with a chromium carbide intermediary and chromium plating out on the graphite. Nickel is largely unaffected, and cobalt and molybdenum should likewise vastly improve things.

Some of the higher chromium alloys had a corrosion rate rangine up to 1 mm/year with graphite, but Hastelloy N was 0.045 mm/year. Incoloy 800H without graphite only had a corrosion rate of 0.0033 mm/year (300 years/mm).

The report also says at a nickel coating stops the corrosion. They tried spray on moly and diamond coatings but they spalled, which should be a simple surface bonding/structural issue that could be fixed (getting Teflon to stick to a frying pan wasn't easy, either). Silicon carbide coatings also seem to eliminate the issue.

On approach I'd at least take a look at is using sacrificial anodes, perhaps aluminum or even zinc, depending on which element offers protection and is easily removed and reprocessed from the fluid. And of course since molten salts are so conductive, they could try putting an electric charge between the vessel walls and the graphite.

I've also read that some of the reaction products (like gold), plate out, making recovery for reprocessing difficult. But that could also be an advantage, if the reactor is coating its plumbing instead of corroding it (like hard water protecting copper pipes with a thin layer of scale).

So, as is true in most applications, you can pick the wrong alloy and run into corrosion trouble, or switch to an alloy that largely avoids the problem, while developing better alloys and coatings based on further experience and experiments.

If we just gave up on simple metallurgy and coatings problems, jet engine turbine temperatures would still be stuck at early 1940's levels.
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Old September 17 2012, 05:59 PM   #150
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Re: Envisioning the world of 2100

Mars wrote: View Post
Like I said, whatever works, whatever off the shelf commercially available nuclear reactor at the end of the 21st century, I admit I am not a nuclear engineer, but I know about nuclear power plants, and the Fusion program just seems to be soaking up money and their progress is so slow with them talking multiple decades just to build an experimental reactor the ITER, and multiple decades more to build a commercial power plant with what was learned. I think if we are to do a starship by the end of this century, it could only reliably count on atomic fission as its main power source, I've calculated by the way it would take 1000 metric tons of fissionables to light and heat a 500 meter diameter Island One Bernal Sphere habitat for every 1000 years of the journey, and at 300 km/sec, that journey to Alpha Centauri would take 4100 years, because Alpha Centauri is approaching us, so figure 4100 metric tons of fusion fuel for lighting and heating the habitat.
I think most people take a wait-and-see approach to fusion. If they get a reactor working for a year or two we'll have an idea what it would take, power-to-weight ratios, thermal efficiencies, neutron damage problems, and maintenance issues. Till then, it might as well be a warp drive.

But I also wouldn't design for a 4,500 year journey to the nearest star, for the simple reason that someone else could just devote a little more fuel to acceleration and get there in 500 years. That means that when your slow ship arrives, people will have already been living there for 4,000 years.

Even with a population increase of just 0.5% per year, it means that your ship's passengers will be greeted by 400 million descendants for every person who departed on the 500 year ship. But of course 50 years after the launch of the 500 years ship, somebody will launch a 100 year ship, and 50 years after it launches, someone will launch a 20 year ship.

Ironically, for a while ships from Earth will probably arrive at the nearest star in the reverse order that they launched.
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