The future of helium?

[lamont]

Yes, I know it's still not there yet but they've made SOME progress right? They are about to build quite a big test reactor in France I thought. If your right about the radioactivity then I suppose that would spoil things a bit. On the other hand - how can they say that it's 'clean' and produces no waste at all if it's actually producing radioactive gas? I must admit I know very little about this - I sat up in my chair when I heard it last night though!

tritium-deuterium fusion (H2 + H3) produces helium and a neutron (He4 + n0). the neutron is a little bit nasty since it is electrically neutral and will readily combine with nuclei that it encounters. if a nucleus absorbs enough neutrons it becomes radioactive, so the walls of the containment chamber eventually become radioactive waste.

deuterium-helium3 fusion is nearly as energetic as deuterium-tritium fusion, but helium3 is extremely rare on earth because most of our helium is alpha-particles (helium4). it would be a better reaction for nuclear fusion though because it produces He4 and a proton (instead of He4 and a neutron). since the proton is electrically charged it will not as readily combine with the nuclei in the walls of the reactor.

if we start running out of helium, the best idea is probably just to mine it out of the solar wind -- either from orbit or the surface of the moon. as an added benefit, that helium will come with solar abundances of he3 which could be used for cleaner burning fusion reactions.

 

[CuriousRambler]

I guess that makes sense now that you put it that way..I still find it pretty amazing though!

What about the lifespan of the radiation? I can't remember what isotope powers nuclear plants offhand (U-235 comes to mind, but wasn't that one of the Japan bombs in WWII?) but I know it has a Looooong half life...How long does the waste from coal plants stay toxic compared to a nuclear plant?

 

[vondo]

I guess that makes sense now that you put it that way..I still find it pretty amazing though!

What about the lifespan of the radiation? I can't remember what isotope powers nuclear plants offhand (U-235 comes to mind, but wasn't that one of the Japan bombs in WWII?) but I know it has a Looooong half life...How long does the waste from coal plants stay toxic compared to a nuclear plant?

Nuclear plants are mostly U-238 which is not really the fuel and U-235 which is. U-238 has a 4 billion year half life, so it's not really radioactive. U-235 has a half life of 700 million years, so it isn't really radioactive either. The problem with fission is that it produces a fair bit of plutonium (23,000 years) and a bunch of other stuff (the fission products) with half lifes from seconds -> thousands of years.

So, long half lifes are not so bad since they give off very little radiation. Short half lifes are not so bad since you can contain it until it decays away. It's the medium lengthed stuff that is the problem (medium being 100 yrs -> 100,000, say).

The isotopes in coal were created in supernova's 5 billion+ years ago and they are still around, so the answer for them is basically "infinite." The real question is how long do they float around in the environment before ending up in a rock layer again.

 

[Charlie99]

I guess that makes sense now that you put it that way..I still find it pretty amazing though!

What about the lifespan of the radiation? I can't remember what isotope powers nuclear plants offhand (U-235 comes to mind, but wasn't that one of the Japan bombs in WWII?) but I know it has a Looooong half life...How long does the waste from coal plants stay toxic compared to a nuclear plant?

Burning the coal doesn't affect the radioactivity. It just releases it from the coal into the atmosphere. The half lifes and total radioactivity will be the same as what was in the coal before.

Long lived doesn't necessarily mean more dangerous. Very long half lifes means that the decay rates are slow, so therefore the radiation emitted is low. That's why "spent" or "depleted" uranium can be used where the extremely high density is useful --- bullets and even as counterweights in the rudder and elevator assemblies of Boeing 747 aircraft. (Depleted uranium is mostly the U-238,with the shorter halflife U-235 and 234 being mostly used up. Natural uranium is about 0.7% U-235, 99.3% U-238. That 0.7% U-235 is what emits most of the radiation)

The other important factor is what sort of emission. Some fissions, like U-238 kick out an alpha particle, which is slow and can be blocked by a couple sheets of paper, so it isn't generally a problem unless you have inhaled it or in some other way gotten it into your tissues. OTOH, things like Cobalt 60 emits relatively hard to shield gamma emissions.

 

[D1V3R]

fusion reactors are far from being a possible power source
(unless you wnat to blow **** up, then theres the good ol' h-bomb for that)
so far, the best they have done is to get a 60% return on energy
as in they used a huge amount of energy to heat tridium gas into a plasma and to control it with huge electro-magnets and they could only get 60% of that energy back from the reaction

 

[Scared Silly]

I work actively in this area as a visualization scientist with both simulation and experimental plasma physicists who are working on this problem. Currently in the US there are three experimental reactors in the US which all produce short (mirco second buring plasmas) all of which use more energy than they produce. ITER which in planned to construction is hoped to be the first experimental reactor that will produce a sustained controlled burning plasma. The key here is sustained and controlled but yet it will still be experimental. Also, ITER will not be operational for another 5 years or so as they have just barely finished the politics. Going from experimental to production is another matter. As such, I will be probably be long retired before there is production fusion reactor. But if it goes it will be a very promising future.

 

[lamont]

i get a back of the envelope estimation that a 1 GWe nuclear fusion plant would produce 66 double-130s worth of helium-4 in a year?

 

[Kim]

i get a back of the envelope estimation that a 1 GWe nuclear fusion plant would produce 66 double-130s worth of helium-4 in a year?

I'm not gonna ask...........

 

[dsteding]

i get a back of the envelope estimation that a 1 GWe nuclear fusion plant would produce 66 double-130s worth of helium-4 in a year?

I hear shops are going to start adding fusion plants to their fill stations. . .

On a side note, the reason why we only have helium-4 on earth is because helium-3 is gravitationally negative. When it outgasses from the mantle (as a non-compatible element) it escapes into space. Gravity doesn't hold it into the atmosphere. I have seen presentations by researchers who use helium-3 content of sediments in the ocean as a proxy for cosmic dust coming down from space. I'm not making this up-one guy at USC put a pool of water on top of his building and measured increases of helium-3 as more cosmic dust built up. I think that is a fairly elegant experiment.

Thread hijack over, continue on.

Yes, I am a science geek. Isotope chemistry was a former career.

 

[lamont]

"gravitationally negative" isn't quite correct. it floats to the top of the atmosphere though and then the gravitational force is insufficiently strong to keep it attatched to the planet...