Helium 3 Fusion question

[Imperial528]

So I was bored today and decided to toy around with some math and calculate the energy release of helium 3 fusion. I immediately proceeded to Wikipedia and look up necessary information such as energy produced per reaction and atomic mass and such. For the energy produced by fusing two atoms of He3 I found the figure of 12.86MeV, or 2.06E-12 joules. I then scaled that up to energy produced by the fusion of a total of one mole of He3, which came out as 6.20E+11 joules, or a whopping 148 ton's worth of TNT.
Now, my issue is that frankly, that number seems very, very large. I know that fusion produces a massive amount of energy that is only exceeded by M/AM reactions and certain stellar phenomena, but the number still sets off my "is not right" alarm. This is most likely due to my lack of any formal education in nuclear fusion, and as a result I don't feel that I can confirm my results with confidence, despite having the ability to do the math.

So, to all ye nuclear nerds out there, am I right? Or did I fuck up?

If needed I can post my work, I could even upload the workbook I used to Google docs if anyone wants to see it.

[Spectre_nz]

I use very 'rough and ready' numbers when calcing fusion; 1x1015 J/kg for ideal fusion, and 1x1014 for D-T fusion. I haven't got my reference on hand to double check, but I'm pretty sure that's what I've run with as a conveniant back of the envalope figure.

So from that I get 3 Terajoules from a mol of 3g/mol ideal fusion fuel. Just shy of one kiloton. Doing the same for my rough and ready D-T aproximation gives me around 0.1 kilotons, so 100 tons of TNT equivalent. Certainly sounds like you're in the right ballpark.


Contrast that with fisson energy yeilds of 6.9x1013 J/kg or chemical energy yeilds of around about 5x107 J/kg for liquid fuels

If you trust wikipedia, it quotes 18.4 MeV and gives you the handy number of 493 MW/hrs per mol of D-He3, 1.7TJ per mol 0.4 kilotons, give or take, 400 tons TNT equivalent. You might even be a little low...

[starslayer]

Yes, that number is large, but that's what makes nuclear processes so attractive as power sources. You can also use something like this as a rule of thumb for comparing M/AM to fusion reactions:

A proton has a mass of roughly 1 GeV, and in hydrogen or helium fusion, you can expect about 10-30 MeV per reaction as a rough ballpark. This means that fusion reactions should liberate about 1-3% the energy that an M/AM reaction with the same mass of reactants would (1 GeV/10 MeV = 100) as an upper limit. Helium fusion and up is less efficient than hydrogen fusion, so that number will get smaller the higher you go. Without bothering to check your math, the energy released from a M/AM reaction of one mole of He-3 (mass ~ 3 grams) will be about 3E14 J assuming 100% efficiency. This is about 500 times your energy figure, which means your figure's in the right ballpark at the least. I'd say you did it right.

[Imperial528]

I see, and good to know that I didn't mess up. Well then, this reactor is going to be quite the beast, which is good. Now I can have a 10kt/s spinal mounted laser, mwahahaha...

Now, a question on fuels: Would it be better to switch to D+He3 fusion or stick with He3+He3 fusion? I decided to do pure He3 so it would be an (almost?) entirely aneutronic reaction, mostly for shielding and efficiency concerns. My only real concern with He3 would be the temperatures generated, but in-setting there are very good thermal conductors and radiators, so I may not have to worry about that. Although I am trying to maintain some sort of link to reality, in that what I come up with should be eventually feasible.

FYI: I'm doing this because I designed a starship and the thought occurred to me "What kind of stuff would I need to make this work?", which brought me here.

[Spectre_nz]

This really comes down to what you want to be limiting in-universe.
If you really want something anetronic there's H-11B. Quite a lot hotter however.
You could just call is 'slush fuel' and let whoever is reading imagine what isotopes its made of.

My solution was to say a wizard did it and handwave past the more detailed physics considerations that most people wouldn't be interested in, then throw in an 'efficency factor' to all the reactors.
I boiled everything down to a mass for the reactor, fuel demand per second and power output based on its efficency. If I wanted more power I just threw in an additional reactor in paralell or a larger reactor. And more fuel capacity. This may not be a hard enough answer for you however. It was workable enough for a modular starship design system for an RPG however.

[Ellindsey]

Pure 3He-3He fusion is much, much harder to achieve than D-He3 fusion. If you have the ability to sustain a pure 3He-3He reaction with net power output, you can probably do p-11B, which is also aneutronic and probably easier to find fuel for.

[dragon]

Well there's a crap load of He3 on the moon

As a result, it has been estimated that there are around 1,100,000 metric tonnes of helium-3 on the surface of the Moon down to a depth of a few metres. This helium-3 could potentially be extracted by heating the lunar dust to around 600 degrees C,

link

[Dave]

Well there's a crap load of He3 on the moon

As a result, it has been estimated that there are around 1,100,000 metric tonnes of helium-3 on the surface of the Moon down to a depth of a few metres. This helium-3 could potentially be extracted by heating the lunar dust to around 600 degrees C,

link

Yeah, in the 50 parts per billion and under range.

Because of the low concentrations of helium-3, any mining equipment would need to process extremely large amounts of regolith (over 100,000 million tonnes of regolith to obtain one ton of helium 3),[43] and some proposals have suggested that helium-3 extraction be piggybacked onto a larger mining and development operation.

I'm not sure it's worth it.

[VarrusTheEthical]

Personally, I think He3 mining makes more sense if we're talking about Uranus and Neptune rather than the moon. Granted, these planets are much further than the Moon, but you're likely to get a much better return on investment extracting He3 from Uranus' and Neptune's atmosphere than you are in digging up rocks on the moon.

[Imperial528]

I was planning to acquire it from gas giants, as while my setting is on the hard side of science fiction, they do have a form of FTL and a rather large stellar civilization so it is possible to get more than enough fuel from both the local gas giants and those in other systems.