Fusion transmutation.

[Guest Schwarzwald]

One side effect of fusion is that only .7% of the mass converts to energy, the rest fusions up the periodic table to a higher element.

 

So, for instance, since traveller setting fusion technology uses hydrogen, plain ordinary hydrogen, it would likely end up as helium.

 

Now, if you put some more energy into the fusion process, you can fuse helium into the next element and get energy out of the process, and so on up to iron, where the amount of energy released by the fusion woudl equal the amount put into fusing it.

 

Puting more energy into it would allow you to fuse iron to higher elements, even gold, lead, uranium, etc. But it would end up costing you energy to do that and likely take a hell of a lot more equipment that 'simply' fusing hydrogen for energy and getting helium would release.

 

Still, a lot of useful elements could likely be produced by low level fusion. A few runs thru a fusion reactor could turn hydrogen into oxygen, generally a useful thing to have on a spacecraft with a human crew, for example. Mix oxygen and hydrogen and get water, again a useful thing to have when humans are around.

 

Lithium is a useful metal that's fairly low on the periodic table, and can be used to make fusion reactor linings if I recall my science correctly.

 

So why not discuss the possibilities opened by practical fusion power in terms of building up higher elements from basic hydrogen? I believe the process is called 'nucleosysthesis" if I recall correctly.

[Nyrath]

Re: Fusion transmutation.

 

Meanwhile nuclear fission converts heavy elements into lighter ones. In this case, the process tends to stop with lead.

 

Amusingly, in his 1940's era novels, John W. Campbell wrote of a science fictional process that would burn iron or lead into pure energy. He figured if fission starts with heavy elements and fusion starts with light elements, his imaginary process could start in the middle.

[Guest Schwarzwald]

Re: Fusion transmutation.

 

Meanwhile nuclear fission converts heavy elements into lighter ones. In this case, the process tends to stop with lead.

 

Amusingly, in his 1940's era novels, John W. Campbell wrote of a science fictional process that would burn iron or lead into pure energy. He figured if fission starts with heavy elements and fusion starts with light elements, his imaginary process could start in the middle.

 

Well, as far as science knows, every element in the universe asides from hydrogen was created via fusion in stars, so a lot more transmutation via fusion occurs than transmutation via fission.

 

As far as a gamer of SF fhan goes what do you think of the idea of 'cascade fusion' creating any element needed from hydrogen? There's a hell of a good SF novel called "Bloom" by Wil McArthy that deals with it a LOT, and I highly recommedn it to you personally, Nyarth.

 

(Now I will just wait for the message telling me you've already read it...)

[theltemes]

Re: Fusion transmutation.

 

One side effect of fusion is that only .7% of the mass converts to energy, the rest fusions up the periodic table to a higher element.

 

So, for instance, since traveller setting fusion technology uses hydrogen, plain ordinary hydrogen, it would likely end up as helium.

 

Now, if you put some more energy into the fusion process, you can fuse helium into the next element and get energy out of the process, and so on up to iron, where the amount of energy released by the fusion woudl equal the amount put into fusing it.

 

Puting more energy into it would allow you to fuse iron to higher elements, even gold, lead, uranium, etc. But it would end up costing you energy to do that and likely take a hell of a lot more equipment that 'simply' fusing hydrogen for energy and getting helium would release.

 

Still, a lot of useful elements could likely be produced by low level fusion. A few runs thru a fusion reactor could turn hydrogen into oxygen, generally a useful thing to have on a spacecraft with a human crew, for example. Mix oxygen and hydrogen and get water, again a useful thing to have when humans are around.

 

Lithium is a useful metal that's fairly low on the periodic table, and can be used to make fusion reactor linings if I recall my science correctly.

 

So why not discuss the possibilities opened by practical fusion power in terms of building up higher elements from basic hydrogen? I believe the process is called 'nucleosysthesis" if I recall correctly.

 

Wouldn't be a whole lot easier to just collect the materials you wanted from the planets and asteroids in whatever star system you happen to be in?

[Dr. Anomaly]

Re: Fusion transmutation.

 

There's a hell of a good SF novel called "Bloom" by Wil McArthy that deals with it a LOT' date=' and I highly recommedn it to you personally, Nyarth.[/quote']

Hmmm. They called them "ladder" reactors, IIRC.

[Guest Schwarzwald]

Re: Fusion transmutation.

 

Hmmm. They called them "ladder" reactors' date=' IIRC.[/quote']

 

Well, they had 'ladder down' technology that allowed them to convert heavy elements into lighter elements while releasing the excess mass as energy.

 

Converting hydrogen to higher elements was, I think, called 'cascade fusion'.

 

It made heavy elements very valuable, with the GU (Gram of Uranium) as the monetary unit. tho people did NOT literally carry them around any more than we in america carry real gold in our pockets.

 

It also meant that lead was actually worth much more than gold, since it's a much heavier element.

[Guest Schwarzwald]

Re: Fusion transmutation.

 

Wouldn't be a whole lot easier to just collect the materials you wanted from the planets and asteroids in whatever star system you happen to be in?

 

Depends on 2 factors, Thelt.

 

1. How efficient, advanced and easy is your fusion tech?

2. How rare and/or hard to extract and process is the element you want?

[Curufea]

Re: Fusion transmutation.

 

With stars, I think the limit is iron (hence red stars) for stable stars, as heavier elements require more energy to form than would be gained by the process of fusion.

[Cancer]

Re: Fusion transmutation.

 

The most stable nucleus in this universe is iron-56. Nuclei lighter than that you can fuse and get energy out (as long as you don't cross that Z + N = 56 line). Nuclei heavier than that you can split and get energy out (same proviso about 56). ("Z" is the number of protons in the nucleus, equivalent to atomic number; "N" is the number of neutrons in the nucleus.)

 

Thing is, as you approach that atomic mass 56 line from either direction, the less energy you get out per unit input mass.

 

Core collapse supernovae (Type II's, and the Type Ib's and Ic's, it's now thought) are the direct result of trying to cross the mass-56 line. In a star, you start by fusing four protons to helium. This goes on at temperatures of about 20 million Kelvin. After 90% of the star's lifetime, the core (the only place it's hot & dense enough for fusion to go on ... otherwise the nuclei don't get close enough for the strong force to overcome the repulsion between the positively-charged nuclei) runs out of hydrogen.

 

To grossly oversimplify, the core contracts some, gets hotter. Ignoring "shell burning" and the effects of degeneracy for the moment, eventually conditions reach the point where helium fusion can start ... that takes about 200 million Kelvin. The hotter termperature is needed because (1) the positive charges on the helium nuclei are larger so the repulsion between them is stronger, and (2) you need to collide three heliums, not two, to make the next fusion step (to carbon-12). Once you have it started, the core fuses helium "happily".

 

Problem is that it does so at a much higher temperature and pressure, which drives the reaction rates way up, so you're expending the energy generated by fusion faster. At the same time, the total energy per gram of matter that can come out of helium -- > carbon fusion is rather lower than it is for hydrogen. Since the helium mass is only slightly less than the hyrdrogen mass consumed in the first stage, that means there's a much smaller pool of available nucelar energy. And it's being consumed faster. So this helium-fusing stage doesn't last anywhere near as long as the hydrogen-fusing stage.

 

Exhaust helium and the core contracts again. Most stars can't contract enough to reach conditions where carbon will fuse; those end up at white dwarfs.

 

Those that can get hot enough, end up going through a couple of more stages of heavy-nucleus fusion. Each is hotter and briefer than the preceding.

 

The thing to remember is that the overlying weight of the matter in the star is being held up literally by the flow of heat outward from the core towards the surface.

 

When the star arrives at a core of iron-56, there is a dead end. Nuclear reactions on that nucleus all consume rather than release energy. But, they get started anyway. That's the catstrophe; the flow of energy outward from the core reverses as the endothermic reactions go, nothing holds up the rest of the star, and it collapses, each layer of the star going approximately into free fall.

 

The free fall time for the layers around the formerly iron core is very quick; in rather less than a second they crash into the center, release a stupendous amount of energy in the crash (of order 10^53 ergs, of which 99% or more escapes in neutrinos), which blasts the star apart.

 

Moral of the story: iron is not an acceptable nuclear fuel.

[Basil]

Re: Fusion transmutation.

 

The most stable nucleus in this universe is iron-56. Nuclei lighter than that you can fuse and get energy out (as long as you don't cross that Z + N = 56 line). Nuclei heavier than that you can split and get energy out (same proviso about 56). ("Z" is the number of protons in the nucleus, equivalent to atomic number; "N" is the number of neutrons in the nucleus.)

 

Thing is, as you approach that atomic mass 56 line from either direction, the less energy you get out per unit input mass.

 

Core collapse supernovae (Type II's, and the Type Ib's and Ic's, it's now thought) are the direct result of trying to cross the mass-56 line. In a star, you start by fusing four protons to helium. This goes on at temperatures of about 20 million Kelvin. After 90% of the star's lifetime, the core (the only place it's hot & dense enough for fusion to go on ... otherwise the nuclei don't get close enough for the strong force to overcome the repulsion between the positively-charged nuclei) runs out of hydrogen.

 

To grossly oversimplify, the core contracts some, gets hotter. Ignoring "shell burning" and the effects of degeneracy for the moment, eventually conditions reach the point where helium fusion can start ... that takes about 200 million Kelvin. The hotter termperature is needed because (1) the positive charges on the helium nuclei are larger so the repulsion between them is stronger, and (2) you need to collide three heliums, not two, to make the next fusion step (to carbon-12). Once you have it started, the core fuses helium "happily".

 

Problem is that it does so at a much higher temperature and pressure, which drives the reaction rates way up, so you're expending the energy generated by fusion faster. At the same time, the total energy per gram of matter that can come out of helium -- > carbon fusion is rather lower than it is for hydrogen. Since the helium mass is only slightly less than the hyrdrogen mass consumed in the first stage, that means there's a much smaller pool of available nucelar energy. And it's being consumed faster. So this helium-fusing stage doesn't last anywhere near as long as the hydrogen-fusing stage.

 

Exhaust helium and the core contracts again. Most stars can't contract enough to reach conditions where carbon will fuse; those end up at white dwarfs.

 

Those that can get hot enough, end up going through a couple of more stages of heavy-nucleus fusion. Each is hotter and briefer than the preceding.

 

The thing to remember is that the overlying weight of the matter in the star is being held up literally by the flow of heat outward from the core towards the surface.

 

When the star arrives at a core of iron-56, there is a dead end. Nuclear reactions on that nucleus all consume rather than release energy. But, they get started anyway. That's the catstrophe; the flow of energy outward from the core reverses as the endothermic reactions go, nothing holds up the rest of the star, and it collapses, each layer of the star going approximately into free fall.

 

The free fall time for the layers around the formerly iron core is very quick; in rather less than a second they crash into the center, release a stupendous amount of energy in the crash (of order 10^53 ergs, of which 99% or more escapes in neutrinos), which blasts the star apart.

 

Moral of the story: iron is not an acceptable nuclear fuel.

 

Excellent expostion!

 

Two points based on this:

1) A human-made device that can fuse helium (and beyond) is orders of magnitude more difficult than one that fuses hydrogen. Anything that can "create" elements beyond helium is going to be indescribably costly, and horrifically hard to build. I think you should drop the idea, frankly; mining and refining will be thoroughly more likely future method of acquiring materials

 

2) At the time of the explosion Cancer describes, there is so very much energy that some of it is "siphoned" off to cause endothermic fusion, thereby creating nuclei heavier than iron. Indeed, it is only in such explosions that those heavier element come to be. In short, every bit of cobalt, copper, silver, gold, iodine, lead, radium, gadolinium, bromine, uranium, etc., etc. in the universe is the ash of a supernova.

[Nyrath]

Re: Fusion transmutation.

 

As far as a gamer of SF fhan goes what do you think of the idea of 'cascade fusion' creating any element needed from hydrogen? There's a hell of a good SF novel called "Bloom" by Wil McArthy that deals with it a LOT, and I highly recommedn it to you personally, Nyarth.

 

(Now I will just wait for the message telling me you've already read it...)

Yes, I've already read it. I liked it enough to read-read it periodically.

 

(BTW: it is "Nyrath", not "Nyarth", nor "Nyarlathotep")

[Nyrath]

Re: Fusion transmutation.

 

2) At the time of the explosion Cancer describes' date=' there is so very much energy that some of it is "siphoned" off to cause endothermic fusion, thereby creating nuclei heavier than iron. Indeed, it is only in such explosions that those heavier element come to be. In short, every bit of cobalt, copper, silver, gold, iodine, lead, radium, gadolinium, bromine, uranium, etc., etc. in the universe is the ash of a supernova.[/quote']

 

Yes, this was part of the MacGuffin in Poul Anderson's novel Mirkheim . Anderson reasoned that if all the known elements were created in supernovae, perhaps some unknown elements were as well -- specifically superheavy elements from the legendary "island of stability". These elements would no doubt have valuable properties.

 

Of course, the fact that such elements would be produced in small quantities compared to conventional elements, and the fact that all of these would be scattered to the four winds by the nova blast made this academic.

 

Until somebody found a supernova remanent with the frizzled remains of gas giant still in orbit. The little cinder of a planet had all sorts of elements plated over the surface, including ones from the island of stability.

[theltemes]

Re: Fusion transmutation.

 

Now I'm just a fusion power reactor engineer, so take what I say with a grain of salt

 

Once you get out of the realm of these reactions:

 

• D-T
  
• D-D
  
• D-He3
  
• He-3He3
  
• p-B11
  

 

You are definitely in the realm of "magic tech" if you are thinking about generating elements using fusion that we can sustain with any conceivable technology based on real-world physics.

 

Also, you would only generate very small amounts of matter on a scale of thermal waste that we know how to deal with. For example, a 3000MWth DT inertial confinement fusion reactor (the same output as a modern fission reactor) will use up 30mg of DT per second producing a bit less than 10mg per second He4. I mention inertial confinement here because a tokomak really won't work for nucleosynthesis - a major problem that we have now is how to get the fusion products out of the reactor while keeping the fuel inside (the products tend to poison the reaction in a tokomak plasma).

[Nyrath]

Re: Fusion transmutation.

 

Agreed, and there are those who have their doubts about p-B11.

 

The catch is, you have to arrange for the protons to impact with 300 keV of energy, and even then the reaction cross section is fairly small. Shoot a 300 keV proton beam through a cloud of boron plasma, and most of the protons will just shoot right through. 300 keV proton beam against solid boron, and most will be stopped by successive collisions without reacting. Either way, you won't likely get enough energy from the few which fuse to pay for accelerating all the ones which didn't.

 

Now, a dense p-B plasma at a temperature of 300 keV is another matter. With everything bouncing around at about the right energy, sooner or later everything will fuse. But containing such a dense, hot plasma for any reasonable length of time, is well beyond the current state of the art. We're still working on 25 keV plasmas for D-T fusion.

[theltemes]

Re: Fusion transmutation.

 

Agreed' date=' and there are those who have their doubts about p-B11.[/quote']

 

p-B11 and 3He-3He would be under the "rubber tech" classification.

[Zeropoint]

Re: Fusion transmutation.

 

It also meant that lead was actually worth much more than gold, since it's a much heavier element.

 

Lead:

Atomic Number: 82

Atomic Weight: 207.2

Density: 11.34 g/cc

 

Gold:

Atomic Number: 79

Atomic Weight: 196.9665

Density: 19.32 g/cc

 

Depending on whether you're talking per atom or per cubic centimeter, claims that lead is heavier than gold may be flat wrong.

 

In any case, calling it "much" heavier seems inaccurate.