# Questions on Fusion

#### Kaldanis

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!

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#### mathman

For a tokamak reactor to work, the plasma must be carefully confined. If anything goes wrong, it simply stops.

#### Pengwuino

Gold Member
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.

#### Kaldanis

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.

#### Drakkith

Staff Emeritus
2018 Award
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)

Staff Emeritus

#### Tyrannical

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.

#### phyzguy

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.

#### Tyrannical

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.

#### Kaldanis

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 realise it was the same conversion in chemical reactions, but it seems obvious now when I think about it. Thank you for pointing it out

#### Astronuc

Staff Emeritus
I actually didn't realise 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.

#### mheslep

Gold Member
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.

#### Drakkith

Staff Emeritus
2018 Award
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.

#### capanni

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.

#### Drakkith

Staff Emeritus
2018 Award
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.

#### capanni

"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.

#### Kaldanis

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!)

#### Drakkith

Staff Emeritus
2018 Award
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.

#### capanni

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.

#### Drakkith

Staff Emeritus
2018 Award
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".

#### capanni

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.

#### Drakkith

Staff Emeritus
2018 Award
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.

#### capanni

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.

#### phyzguy

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.

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