BMW M3 versus Toyota Prius

[Kitdy]

I keep pretty up to date on fusion technology, or so I thought. As far as I knew, the only fusion tech we have requires more input energy than it produces.

Further, fusion requires fuel... what are you expecting to fuse, iron? Hydrogen is the only element that we can hope to efficiently use at this point, but 'free' hydrogen is hard to obtain, and limited, just like any other fuel. Cracking hydrogen from water takes as more energy than it makes, by definition.

Do you believe that with fusion we're getting something for nothing? Surely you know that fusion requires free, fusiable elements and is another fuel based energy source, much like fossil and nuclear fission power.

And you're right about space based infrastructure being expensive to create, but remember that in space a collector could be incredibly thin and light, and thus the payload for a rocket would not be that high. The hardest part of the entire process is lifting materials to orbit.



Do you have more info on the ITER project? Specifically, what are they fusing?

Again, it's likely hydrogen. If we had an unlimited source of hydrogen, we could simply burn it for energy, but that's the problem- we don't. Collecting hydrogen is very difficult because it rises to the edge of the atmosphere, and cracking it from water or other hydrogen compounds is more expensive in terms of energy than fusion can make up for.

I'm going to open up Wikipedia as I write this.

ITER will fuse deuterium and tritium, two isotopes of hydrogen with 2 and 3 neutrons respectively as this reaction yields the best energy output for the circumstances. Both deuterium and tritium are naturally occurring in hydrogen, deuterium one every 6500 or so parts, and tritium significantly less than this, yes in fact, 1 part tritium in water to 10^16 parts hydrogen. Cracking water seems to ring a bell as to gain the fuel sources but I can't remember.

Hmm. Current tritium is sourced from certain nuclear reactors on the level of a few kilograms a year. I am looking to find out how deuterium is harvested from water but I cannot find it.

[mikelzapi3]

As kitdy has said correctly, the elements that are fused are deuterium and tritium. But, they are not burnt with oxygen to obtain water, a chemical reaction that gives us energy. Instead, the two hydrogen atoms are fusioned to create a helium atom, giving much much more energy. Think that way, if you take a H bomb and burn the hydrogen, you will have a small explosion, but if you ignite the fusion of that bomb using a nuclear bomb , you have a big big boom.

I found ths from wikipedia, copy-pasted:

ITER will be designed to produce approximately 500 MW (500,000,000 watts) of fusion power sustained for up to 400 seconds[3] (compared to JET's peak of 16 MW for less than a second) by the fusion of about 0.5 g of deuterium/tritium mixture in its approximately 840 m3 reactor chamber

so they are using only 0.5g to obtain 500MW, that's 200 000 MJ of energy in all 400 seconds, from 0.5 g.

If we take 1kg of mixture, the output, would be 4E8 MJ/kg that, comparing to the energy released in the combustion of hydrogen ( 286 MJ/kg), is 1 398 601 times greater!!! so you need very few hydrogen, and lthough it is expensive to obtain deuterium and tritium, you see that it compensates.

About the process efficiency, with ITER they expect to obatain 5 or 10 times more energy that the consumed to maintain the fusion, so they are really producing energy, no wasting it.

If all goes well, the next step is DEMO, th first commercial reactor that will really produce electricity with the heat, 2000 MW continuously.

All, this is in wikipedia, if you want more info, my knowledge is limited to wikipedia

[Kitdy]

As kitdy has said correctly, the elements that are fused are deuterium and tritium. But, they are not burnt with oxygen to obtain water, a chemical reaction that gives us energy. Instead, the two hydrogen atoms are fusioned to create a helium atom, giving much much more energy. Think that way, if you take a H bomb and burn the hydrogen, you will have a small explosion, but if you ignite the fusion of that bomb using a nuclear bomb , you have a big big boom.

I found ths from wikipedia, copy-pasted:



so they are using only 0.5g to obtain 500MW, that's 200 000 MJ of energy in all 400 seconds, from 0.5 g.

If we take 1kg of mixture, the output, would be 4E8 MJ/kg that, comparing to the energy released in the combustion of hydrogen ( 286 MJ/kg), is 1 398 601 times greater!!! so you need very few hydrogen, and lthough it is expensive to obtain deuterium and tritium, you see that it compensates.

About the process efficiency, with ITER they expect to obatain 5 or 10 times more energy that the consumed to maintain the fusion, so they are really producing energy, no wasting it.

If all goes well, the next step is DEMO, th first commercial reactor that will really produce electricity with the heat, 2000 MW continuously.

All, this is in wikipedia, if you want more info, my knowledge is limited to wikipedia

My knowledge is limited to Wikipedia as well unfortunately.

I just learned that the new one would be called DEMO and that seems to be the real breakthrough in fusion technology.

It doesn't seem as though that harvesting deuterium or tritium is an issue as it hasn't been mentioned in any of the articles of future fusion use, and considering that the amount of energy fusion puts out, I am not sure this would be an issue.

[Bob]

ITER will fuse deuterium and tritium, two isotopes of hydrogen with 2 and 3 neutrons respectively as this reaction yields the best energy output for the circumstances. Both deuterium and tritium are naturally occurring in hydrogen, deuterium one every 6500 or so parts, and tritium significantly less than this, yes in fact, 1 part tritium in water to 10^16 parts hydrogen. Cracking water seems to ring a bell as to gain the fuel sources but I can't remember.

Hmm. Current tritium is sourced from certain nuclear reactors on the level of a few kilograms a year. I am looking to find out how deuterium is harvested from water but I cannot find it.

so they are using only 0.5g to obtain 500MW, that's 200 000 MJ of energy in all 400 seconds, from 0.5 g.

It doesn't seem as though that harvesting deuterium or tritium is an issue as it hasn't been mentioned in any of the articles of future fusion use, and considering that the amount of energy fusion puts out, I am not sure this would be an issue.

My knowledge is based on a whole lot of reading in SciAM, and on the web. I got credit for a couple of college physics classes in 9th grade (and graduated this year) so it's taking a while to remember.

So I'm going to do the math.
.5 g 3H + 2H = 200 000 MJ - 10 to 20 % = 1.6*10^11 J to 1.8*10^11 J
that means .5g deuterium and tritium yields between 1.6E11 J and 1.8E11 J

world consumption is ~ 5*10^20 J
this means we would need (5*10^20 J) * (.5 kg deuterium/tritium) / (1.8 *10^11 J) or 1.38*10^9 kg of deuterium and tritium per year, to meet current demand at this effieciency.
or 1 388 888 metric tons of deuterium tritium per year.

tritium is the rarer of the two, so lets look at that- the mix is 60 % tritium, meaning
1.38*10^9 * 60% = 8.33 * 10^8 kg of tritium are needed per year.

That's a lot- and cracking it from water DESTROYS the efficiency because it is an energy intensive process.

So with deut - tritium fusion, your talking about again using a fuel, albiet a different one, that is rare in any form and EXTREMELY rare in useable form.

Curiously enough, there is an extremely efficient way to collect large amounts of hydrogen - gas giants. And at some point we will likely use a hydrogen powered fusion drive with fuel collected from Jupiter/Saturn. But until we have massive quantities of free hydrogen, or lesser quantities of isotopic H, fusion is not viable.

I'm far from proving my point and, indeed, there are sources of deut and tritium available- but they're much much less accesible than some people would have you believe.

[Kitdy]

I suppose that the experts have thought this out before however, and considered sources for deuterium and tritium.

I am interested in seeing just exactly how energy intensive cracking water is to get deuterium in comparison with say mining for uranium, getting coal etc.

[mikelzapi3]

So I'm going to do the math.
.5 g 3H + 2H = 200 000 MJ - 10 to 20 % = 1.6*10^11 J to 1.8*10^11 J
that means .5g deuterium and tritium yields between 1.6E11 J and 1.8E11 J

world consumption is ~ 5*10^20 J
this means we would need (5*10^20 J) * (.5 kg deuterium/tritium) / (1.8 *10^11 J) or 1.38*10^9 kg of deuterium and tritium per year, to meet current demand at this effieciency.
or 1 388 888 metric tons of deuterium tritium per year.

If i'm correct, you have done the math using grams, and then give the results in Kg. So the whole result is divided by 1000, and the amount of mixture needed per year would be 1388 tons.

Is not that much, comparing to all the petroleum and coal that we burn, but getting the tritium seems very difficult. In the case of deuterium, it must be much more easy, as they use deuterium to make heavy water, so I supose they use some kind of filter or centrifugation to separate deuterium from "normal" hydrogen, like they do with uranium.

[Bob]

I suppose that the experts have thought this out before however, and considered sources for deuterium and tritium.

I am interested in seeing just exactly how energy intensive cracking water is to get deuterium in comparison with say mining for uranium, getting coal etc.

It's possible - fusion as a power source could end up being completely workable. However, at this point the experts are still focusing on creating the technology to fuse elements reliably and efficiently. Remember that these are all proposed numbers - so far the effieciency of fusion is much lower, because the input energy to heat something to a few million degrees is pretty high

And because the experts are so interested in the technology, they're somewhat ignoring the issue of practicallity and fuel. It's like saying that we can continue on hydrocarbon deposits for centuries because we've only used a small percentage of the world's deposits - which is true, but ignores the fact that much of the world's remaining oil is highly inaccesible.

If i'm correct, you have done the math using grams, and then give the results in Kg. So the whole result is divided by 1000, and the amount of mixture needed per year would be 1388 tons.

Is not that much, comparing to all the petroleum and coal that we burn, but getting the tritium seems very difficult. In the case of deuterium, it must be much more easy, as they use deuterium to make heavy water, so I supose they use some kind of filter or centrifugation to separate deuterium from "normal" hydrogen, like they do with uranium.

You are correct... oops didn't read the units carefully enough. And you're right that compared to today's fuels that amount is tiny. Unlike coal or oil, though, tritium and deuterium can't just be dug up from the ground in anything close to pure form. At tritium's natural occurence of 1 part in 10 thousand million million (10^16), and considering tritium is only 3/18 of the mass of a water molecule, that means there is one kg of tritium in 3 * 10^16 kg of pure water.