Questions on Fusion

  • Thread starter Kaldanis
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  • #1
Kaldanis
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I was hoping someone could help clear up a few things about fusion for me.

I've read that fission produces around 1,000,000 times more energy than any chemical recation and that fusion produces 3 to 4 times more energy than fission. I can only find 'estimated' numbers like these, but is there an actual comparison anywhere that says "A certain amount of oil gives X energy, the same mass used for fission gives Y and for fusion gives Z"?

Also, I read that when using a tokamak reactor it isn't possible to start a sustained fusion chain reaction, making it much safer than fission. Is this true?

I thought these questions would be simple enough to find the answer for but I can't find anything!
 

Answers and Replies

  • #2
mathman
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For a tokamak reactor to work, the plasma must be carefully confined. If anything goes wrong, it simply stops.
 
  • #3
Pengwuino
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No but surely you can do some googling and piece together the information on your own. I doubt many people would go beyond "gasoline vs. uranium" because it is quite staggering to think about how much mass is saved using uranium as power source instead of gasoline or coal or something of the sorts. However, I don't think you'll find too many people making such calculations because they're somewhat meaningless. Any form of nuclear power is orders of magnitude higher energy/mass than traditional methods and that's basically all there is to say.
 
  • #4
Kaldanis
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Yeah, I was beginning to think maybe people just didn't compare the data because it was pointless. Oh well! Thanks for both of the answers.
 
  • #5
Drakkith
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If you compare an individual fission reaction in Uranium to an individual Fusion reaction in, say tritium-dueterium, the fission reaction produces more energy. HOWEVER, uranium is hundreds of times heavier than hydrogen, so the energy density is much greater using Fusion. (More bang for the amount of fuel you use)
 
  • #8
Tyrannical
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There are many different types of fission / fusion reactions, and the different reactions can provide different amounts of energy output. But what they all share in common is that a portion of matter is transformed into energy. Given the famous E=MC^2, a tiny amount of matter contains a LOT of energy.
 
  • #9
phyzguy
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There are many different types of fission / fusion reactions, and the different reactions can provide different amounts of energy output. But what they all share in common is that a portion of matter is transformed into energy. Given the famous E=MC^2, a tiny amount of matter contains a LOT of energy.

One thing many people misunderstand (I'm not saying that you misunderstand, but your post implies it) is that they think nuclear reactions convert mass into energy via E=mc^2, but that chemical reactions don't. E=mc^2 is always satisfied, so that any reaction that generates an energy E will result in products that weight less than the initial reactants by the amount E/c^2. It is just that this mass deficit is much smaller for chemical reactions than nuclear reactions, and hence usually unmeasurable.
 
  • #10
Tyrannical
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Yeah, it is often over looked that there is a matter to energy conversion in chemical reactions too because the mass difference is so small.
 
  • #11
Kaldanis
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Thanks for all the help. Especially the wikipedia links, they seem to be exactly what I was looking for! The hyperphysics site is great too, I can't believe I forgot to check there for information on it.

Tyrannical said:
There are many different types of fission / fusion reactions, and the different reactions can provide different amounts of energy output. But what they all share in common is that a portion of matter is transformed into energy. Given the famous E=MC^2, a tiny amount of matter contains a LOT of energy.
One thing many people misunderstand (I'm not saying that you misunderstand, but your post implies it) is that they think nuclear reactions convert mass into energy via E=mc^2, but that chemical reactions don't. E=mc^2 is always satisfied, so that any reaction that generates an energy E will result in products that weight less than the initial reactants by the amount E/c^2. It is just that this mass deficit is much smaller for chemical reactions than nuclear reactions, and hence usually unmeasurable.

I actually didn't realize it was the same conversion in chemical reactions, but it seems obvious now when I think about it. Thank you for pointing it out
 
  • #12
Astronuc
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I actually didn't realize it was the same conversion in chemical reactions, but it seems obvious now when I think about it. Thank you for pointing it out
Keep in mind that nuclear reaction energies are on the order of MeV as opposed to energies of eVs for chemical reactions. The mass defect in chemical reactions is negligible.
 
  • #13
mheslep
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For a tokamak reactor to work, the plasma must be carefully confined. If anything goes wrong, it simply stops.
With the plasma maybe so, with a 10 Tesla superconducting magnet going suddenly normal maybe not.
 
  • #14
Drakkith
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With the plasma maybe so, with a 10 Tesla superconducting magnet going suddenly normal maybe not.

The quenching of a magnet, while not a good thing, isn't that dangerous overall. At worst you would have to replace the magnet and possibly some surrounding components. There won't be a huge explosion or anything.
 
  • #15
capanni
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Also, I read that when using a tokamak reactor it isn't possible to start a sustained fusion chain reaction, making it much safer than fission. Is this true?

The energy in the super conductive magnets in a tokamak (one which is big enough to potentially break even) is approximately 1/40 that of the first atomic bomb dropped in anger.

If that energy is releases it goes somewhere. Possibly helping to spread the lithium blanket which surrounds the plasma chamber, around the neighbourhood.

That is before you even consider the energy inside the reaction chamber.

These are not nice materials to have raining down, and as a bonus the materials that have become radioactive due to the fusion by products will be coming down too.
 
  • #16
Drakkith
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The energy in the super conductive magnets in a tokamak (one which is big enough to potentially break even) is approximately 1/40 that of the first atomic bomb dropped in anger.

If that energy is releases it goes somewhere. Possibly helping to spread the lithium blanket which surrounds the plasma chamber, around the neighbourhood.

That is before you even consider the energy inside the reaction chamber.

These are not nice materials to have raining down, and as a bonus the materials that have become radioactive due to the fusion by products will be coming down too.

The energy is divided between the magnets. Even if all of them suddenly quenched at the same time, I don't think you would get a massive explosion. I'm guessing most of the energy would be released as Heat into the surrounding equipment. Alot of equipment damage maybe, but I don't think its nearly as destructive as you imagine it.
 
  • #17
capanni
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"energy would be released as heat into the surrounding equipment"
Yes and we are talking about a massive amount of heat, I certainly would not want to be anywhere near it.
 
  • #18
Kaldanis
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Thanks again to everyone who posted in this thread but I have another question.

For a section in my report I'm trying to describe the potential dangers of fusion. From research and posts here it seems that there are very little to none? If conditions aren't exact or something goes wrong, the reaction just stops. The elements used and produced aren't very radioactive or dangerous (If lithium some how escaped it doesn't stay in the body long, has a short halflife and doesn't release strong radiation). The only 'bad' things are the neutrons that get through the lithium blanket and could cause the reactor walls to become radioactive over time. These can be safely disposed of and don't stay radioactive for very long, so it's not really a huge problem.

Is this correct or am I missing any dangers to human life or the environment? Is it possible that more fusion reactors around the world could make it easier for people to create hydrogen bombs, or is that just ridiculous? (i'm trying desperately to find negatives here!)
 
  • #19
Drakkith
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With tritium-deuterium fuel, or deuterium-deuterium fuel, you would have significant neutrons produced, just like you said. Thats about the worst negative I can think of, and even that is much safer that current fission power plants as the waste, while initially more radioactive, only decays for about 50 years or so and isn't in the form of iodine and such. COULD it result in more people making thermonuclear bombs? I can't say no for sure, but I find it hard to see how you can go from a controlled reaction in a power plant to a hugely uncontrolled explosion without using something like a fission bomb for the initial fuel. It MIGHT be possible, but I don't think we will know until we get fusion power.
 
  • #20
capanni
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Assuming you are talking about a plasma magnetic confinement system.

If you are comparing the reactive elements used to a fission reaction then the fusion elements are safer. This does not mean they are safe.

The surrounding materials will become radioactive over time as you say and there is a lot of material. It is not enough to just say "These can be safely disposed of". You are still looking at long term storage of tones of material and/or extracting radioactive material to reduce the amount stored.

A fusion system is much more than just the reactive elements and there are plenty of environmental & biological dangerous materials involved that could be widely disperse by an accident.

The energy levels are huge a sudden failure parts of this could be devastating.

However it is unlikely that it will help anyone create a hydrogen bomb. An H bomb uses a deuterium/tritium mixture as the fusion component but is triggered, and derives a lot of its energy from a fission trigger. The trigger is of A bomb materials, uranium and plutonium. As such anyone with H bomb capability must already have A bomb materials.

Yes it will make a bigger explosion but if this is a DIY project, it is adding a higher likelihood of failure. Someone sufficiently technically skilled to make it work would probably be able to get tritium without stealing it from a power station, which would add to the risk of them getting caught.

Additionally very little tritium is needed for fusion reactions and due to its shortish half life it is better to generate it as you need it, in small quantities. Rather than generate it and store it.
 
  • #21
Drakkith
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The energy levels are huge a sudden failure parts of this could be devastating.

Devastating to what exactly? As far as I understood it, if the containment completely failed then the plasma would simply scorch the inside of the reactor at worst. If the superconductors get quenched, you have a large amount of power coming back into them and burning them out and such, but still nothing that I would say is "Devastating".
 
  • #22
capanni
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I am referring to the energy levels used for confinement. The plasma is in very small quantities. The energy in the magnets is huge, and this is a complicated systems with lots of lovely stuff in it, including cryogenics. The material is not just made radioactive and can be disposed of. The structures are weakened by its own operation and this adds to the chance of sudden failure.

Any effect is likely to be far more localised than an accident at a fission facility. But energy has to go somewhere and this will be into anything and anyone around it.
 
  • #23
Drakkith
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Any effect is likely to be far more localised than an accident at a fission facility. But energy has to go somewhere and this will be into anything and anyone around it.

Of course it will. IF there was a sudden failure of something the energy in the magnets would either: A. Be bled off like it normally is when the magnets get turned off. B. IF they magnets quenched the result would be destruction of the magnets and possibly the surrounding structure/components. (The electrical and mechanical components, not the actual structure of the reactor itself)

The LHC had a quench in one of their magnets which resulted in having to replace it and the surrounding components. But it was far from devastating. Would such an accident injure people at the plant? Possibly, but only if they were in the vicinity of the incident.
 
  • #24
capanni
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Well, the LHC is not a fusion plant, its operation and the equation of its superconductors to a fusion plant's ones is not direct. Fortunately no one was injured/killed, because no one was in the vicinity. The devastating part from the LHC point of view would be the large amount of liquid helium that rendered the location inaccessible and weeks of in-operation.
 
  • #25
phyzguy
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Additionally very little tritium is needed for fusion reactions and due to its shortish half life it is better to generate it as you need it, in small quantities. Rather than generate it and store it.

I'm not sure where you are getting this information. Studies that I have seen show that a typical fusion reactor would need to inventory about 1 kg of Tritium, which is about 50 million curies - not an insignificant amount of radioactive material. In addition, tritium has a large biological impact since hydrogen is a significant component of living cells. There is also a large amount of structural material that is made radioactive by the high flux of 14 MeV neutrons, and this needs to be disposed of every couple of decades because the neutrons cause it to lose structural strength. All in all I'm not sure that a fusion reactor will generate that much less radioactive material than a fission reactor.
 
  • #26
Drakkith
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There is also a large amount of structural material that is made radioactive by the high flux of 14 MeV neutrons, and this needs to be disposed of every couple of decades because the neutrons cause it to lose structural strength. All in all I'm not sure that a fusion reactor will generate that much less radioactive material than a fission reactor.

Remember the Japan incident. Fission reactors have to be kept cool and controlled, and can relatively easily go out of control without the proper safeguards. While you may need to replace things in a fusion reactor every few decades, the security you'd have knowing that the plant will not suddenly explode is very nice. You can prepare and plan for replacing things at intervals, you cannot always plan for a disaster.

Also, it's not about the amount of material that is radioactive, it is more about the type of material, how it's stored, how easy to contain it is, and how dangerous it is biologically. Something that is more radioactive isn't necessarily worse than something else if it is much easier to contain and doesn't get absorbed into the body easily.

In addition, tritium has a large biological impact since hydrogen is a significant component of living cells.

Read this on wikipedia: HTO has a short biological half life in the human body of seven to 14 days, which both reduces the total effects of single-incident ingestion and precludes long-term bioaccumulation of HTO from the environment.

Looks like Tritium is dangerous, but probably much less dangerous than something like Iodine is. Wouldn't want to get too much of either though.
 
  • #27
capanni
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I'm not sure where you are getting this information. Studies that I have seen show that a typical fusion reactor would need to inventory about 1 kg of Tritium, which is about 50 million curies - not an insignificant amount of radioactive material.

I have seen studies that quote 1 and 2 kg as the Tritium requirements of reactors also. Now, I did not say this was an insignificant amount of radioactive material, I said very little tritium is needed for fusion reactions. I was considering the discussion of fission and fusion and thinking of comparable amounts for each.

If you look at other studies such as the ITER, they claim "A future fusion plant producing large amounts of power will be required to breed all of its own Tritium." Hence my remark about generating it as needed.

In addition, tritium has a large biological impact since hydrogen is a significant component of living cells. There is also a large amount of structural material that is made radioactive by the high flux of 14 MeV neutrons, and this needs to be disposed of every couple of decades because the neutrons cause it to lose structural strength. All in all I'm not sure that a fusion reactor will generate that much less radioactive material than a fission reactor.

I concur with Drakkith comments on the biological impact. In that Tritium is far less dangerous and long lasting than many fission products. I do acknowledge that it is still not a good thing and a Tritium leak should be avoided. Note that tritium is the fuel in use here and it does not produce radioactive waste, not the case in fusion.

However as you say it does result in radioactive structural materials which are problematic.
 
  • #28
Drakkith
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My opinion on this. No gurantee about the accuracy, just accumulated knowledge that I think is correct.

Just looking at the possible catastrophic disasters that could happen to a Fusion reactor compared to a Fission one immediately brings into view the difference in the two.

Fission: Incident at an active reactor or fuel storage area can result in large amounts (kilograms or more) of VERY biologically dangerous isotopes to be released, such as Iodine-131, in addition to a multitude of other radioactive gases and particles. Just look at Chernobyl and the Japan incidents.

Fusion: Breach in the reactor vessel itself only results in a very small amount (measurable in grams) of materiel to be released, the only dangerous ones are Tritium. A leakage in a tritium storage tank could release large amounts of tritium, however, tritium is FAR less dangerous than something like Iodine-121 is. (And that's assuming that it is actually stored in a tank somewhere and not bred in small amounts and fed right back into the reactor) It has a much greater half life and doesn't accumulate in any specific tissue, unlike Iodine. The irradiated structure of the reactor don't form extremely hazardous isotopes (As far as i know.) and can be stored relatively easily for a shorter amount of time than fission waste products.
 
  • #29
capanni
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That is fine if you are only focusing on tritium. A leak of which is still dangerous but lesser than fission materials. However if it is produced on site, there are different ways of doing this, some more dangerous than others.

Also it depends on how you quantify a material as "dangerous". Some Iodine isotopes have very short half-life, others have very long ones. Iodine is dangerous if ingested because of how it affect the body, and is especial dangerous for the very young. Other materials involved in fission have no radioactive dangers but are toxic.

The irradiated structure of the reactor results in a huge amount of material to deal with. Again, not as dangerous as fission but is is still something to be managed.

Given the history of large scale engineering management of dangerous materials, especially when economics is a factor. This cannot be taken lightly.

But on the big scale of things I think fusion is much safer in operational and accidental terms than fission. Of course so far, not in financial terms and it does not actually work on a net-energy production basis.
 

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