A layman's question about nuclear fusion

[justawriter]

Hi,

I’m a hobbyist writer with a slight science fiction slant to my stories. Currently my work involves some reference to the much expected nuclear fusion power. I have been doing some research online, but since I’m not a physicist this naturally resulted in a number of questions.

I’m not trying to recreate a fully realistic scenario in my story (that would make me some denomination of scientist), but I would like to have at least some level of credibility.

So here are my fusion questions (hope someone can shed some light on them):

1: Could Deuterium be extracted from seawater at an industrial level?
2: If a Deuterium-Deuterium fuel is used in a fusion reactor, would the by-product be Helium-3?
3: is it imaginable (could there be an economic reason) that a power plant combined a secondary (low power) reactor fuelled by Deuterium-Deuterium and then a primary (high power) reactor fuelled by the resulting Helium-3?

I hope the forumites will indulge a layman and let me hear your thoughts about the above.

 

[Mattenerinfo]

1) Yes, the water is called 'heavy water' and can be easily extracted from seawater
2) About half of the product would be Helium 3 and a neutron, the other half would be tritium and a proton
3) Helium 3 is not an efficient reactant, however tritium-deuterium is much more efficient generating nearly five times more energy then deuterium-deuterium, so a secondary reactor utilizing the tritium generated by the D-D reaction would be extremely efficient.

 

[justawriter]

Hi Mettenerinfo,

Thanks for your reply, which cleared up a my questions well.

I was under the impression that Helium-3 was the holy grail of nuclear fusion power, since I read about possible lunar extraction sometime in the far future – although this seems to eliminate one of the advantages of fusion power.

 

[GeorgeRaetz]

...3) Helium 3 is not an efficient reactant, however tritium-deuterium is much more efficient generating nearly five times more energy then deuterium-deuterium, so a secondary reactor utilizing the tritium generated by the D-D reaction would be extremely efficient.

Just one subtle point: Deuterium-Tritium fusion is about 100 times "easier" than pure Deuterium fusion. So the most of the Tritium product of a pure Deuterium reactor would react before it escapes. That renders a secondary reactor unneccessary.

 

[GeorgeRaetz]

...
I was under the impression that Helium-3 was the holy grail of nuclear fusion power...

The He3-Deuterium reaction produces no neutrons.(The reaction is "aneutronic".) The neutrons produced in the easier DD and DT reactions are a nuisance beacause:
1) they are a very deadly radiation.
2) they travel long distances through most materials generating low level radioactive waste as they go.
3) Because of (2) it's hard to extract useful heat energy from them.
The absence of neutrons is what makes the reaction attractive as a(speculative) energy source.
The big disadvantage of the Helium3-Deuterium reaction is that it's about five times "harder" then pure Deuterium, and no one has even come close to extracting net energy from pure Deuterium (Except for the "Ivy Mike" hydrogen bomb back in 1952).

 

[justawriter]

The He3-Deuterium reaction produces no neutrons.(The reaction is "aneutronic".) The neutrons produced in the easier DD and DT reactions are a nuisance beacause:
1) they are a very deadly radiation.
2) they travel long distances through most materials generating low level radioactive waste as they go.
3) Because of (2) it's hard to extract useful heat energy from them.
The absence of neutrons is what makes the reaction attractive as a(speculative) energy source.
The big disadvantage of the Helium3-Deuterium reaction is that it's about five times "harder" then pure Deuterium, and no one has even come close to extracting net energy from pure Deuterium (Except for the "Ivy Mike" hydrogen bomb back in 1952).

Yes, and we all saw how that went

Thanks GeorgeRaetz,

Another few points to absorb, thank you.

I understand Helium-3 is produced by the decay of tritium, surely that would be impossible/uneconomical to first produce tritium, and then let it decay to Helium-3 on an industrial scale.

Getting it from the moon, where it has yet to be confirmed in significant quantities (as far as I understand), seem to go against the idea of “readily available and safe” power production that’s attached to nuclear fusion. Is there another way to produce Helium-3, or might it be a fuel we dismiss?

When you say that the secondary reactor is unneccessary, does that mean, that a D-D reactor would continue to fuse the resulting tritium and thus generate power from both primary and secondary (by-product) fuel?

If I have done my armchair-calculations right, this would rule out a deuterium-helium3 fusion, as the deuterium would fuse and create tritium well before the deuterium-helium3 fusion takes place. The resulting tritium would also fuse with the deuterium and create the deuterium-tritium fusion, rather than a true aneutronic fusion as was expected by a deuterium-helium3 fuel.

edit for second question and adding final thought.

 

[GeorgeRaetz]

I understand Helium-3 is produced by the decay of tritium,...

That seems correct. To obtain tritium you need neutrons, and the usual source of neutrons is a fission reactor.

...Is there another way to produce Helium-3, or might it be a fuel we dismiss?

There appears to be no adequate natural source of He3 source on Earth. For more info see the Wikipedia article on He3 (http://en.wikipedia.org/wiki/Helium_3).

...does ... a D-D reactor continue to fuse the resulting tritium and thus generate power from both primary and secondary (by-product) fuel?

Yes, most proposed fusion reactors are essentially a tank or pellet of deuterium heated to very high temperatures(~100E6K or more) so the deuterium nuclei acquire enough kinetic energy to slam into each other and "fuze". Now the DD fusion reaction produces a tritium nucleus that flies out at fairly high kinetic energy. This high energy tritium nucleus now finds itself in a sea of Deuterium and readily fuzes.(Actually, the tritium initially has too much kinetic energy to efficently "fuze", but it quickly loses energy through collisions and acquires an optimum speed).

...The resulting tritium would also fuse with the deuterium and create the deuterium-tritium fusion, rather than a true aneutronic fusion as was expected by a deuterium-helium3 fuel.

Very true, nuclear fusion in a tank of Deuterium-He3 wouldn't be aneutronic at all. The (highly speculative) proposals for He3-D fusion involve nuclear beams so as to minimize the neutronic side reactions.

 

[justawriter]

Thanks to both of you for the clarifications.