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I have been out of physics for some time now since my childhood, so please bear with me if the question below feels too novice.

I grew up with the understanding that the nuclear fusion reaction is still a dream of many people as it's a source of clean energy without the side effects of nuclear waste as we observe in nuclear fission.

Now recently I was just checking the principle on which the hydrogen bomb works, and I was shocked that it uses nuclear fusion to generate all that energy. This contradicted my understanding that nuclear fusion is not a dream but it actually is a reality.

So if we already achieved nuclear fusion why can't we create a nuclear fusion reactor out of it to generate all the power we need? Also why can't we have the small scale fusion reaction on Jupiter (as mentioned in my other question) that can help us take over the outer planets of solar system.

Also I just wanted to know if we can continue this fusion reaction to generate precious heavy metals – is it possible?

psmears
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Xinus
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    The trick is extracting the energy in a way that you can use it (and the city it is built in) more than once. – Martin Beckett May 02 '17 at 02:29
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    A bomb is an uncontrolled, short time, non-sustained reaction. A power reactor needs to be controllable, long-term, sustained reaction. Producing elements above iron would be highly endothermic and not economical. – Bill N May 02 '17 at 02:57
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    @BillN: Not necessarily sustained. I've seen a "redneck fusion reactor" idea where you'd set off hydrogen bombs underground to generate geothermal energy, which you then would use conventionally. Once the earth has cooled off, you remove the plumbing, toss in another bomb, re-drill the shaft, and put back the plumbing. – MSalters May 02 '17 at 09:33
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    People are trying it; it's the basic idea behind Inertial confinement fusion – paj28 May 02 '17 at 09:42
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    "What stops us from creating nuclear fusion reactor as we already have hydrogen bomb working on same principle of fusion?" – The "bomb" part. – Jörg W Mittag May 02 '17 at 11:18
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    Note that hydrogen bombs often do not generate most of their energy from fusion: they generate a good amount of it that way but the majority is often fission. –  May 02 '17 at 13:54
  • This is why we cannot do it: https://img.memesuper.com/e421511c1cababd252555939eedc143d_meme-nuclear-bomb-memesuper-nuclear-explosion-meme_625-468.jpeg – user64742 May 02 '17 at 22:03
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    We could capture the energy from thermonuclear explosives by digging a large underground cavern, filling it with water, periodically detonating thermonuclear explosives in the center, and piping the resulting steam through a heat exchanger to a turbine. For various good reasons this has never been tried. – Brendan Cannell May 02 '17 at 23:02
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    "and I was shocked that it uses nuclear fusion to generate all that energy" - a pedantic aside: a significant amount of the energy yield of multi-megaton class bombs can be due to the rapid fission of an ordinarily non-fissle uranium jacket induced by the large neutron flux of the fusion reaction. From the Wikipedia article "Thermonuclear weapon": "The fuel undergoing the fusion reaction emits a large flux of neutrons, which irradiates the U-238 tamper (or the U-238 bomb casing), causing it to undergo a fission reaction, providing about half of the total energy." – Alfred Centauri May 03 '17 at 01:55
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    A bomb only needs to work for a millisecond. A reactor needs to keep working for billions of milliseconds. – RBarryYoung May 03 '17 at 18:20
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    @RBarryYoung -- a billion millisecond is less than two weeks. Ideally, you'd want a reactor to last a good portion of a trillion milliseconds. – Michael Lorton May 04 '17 at 16:30
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    @Malvolio Which is why I said "billions", plural. – RBarryYoung May 04 '17 at 17:06
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    @RBarryYoung -- if you can use any plural without restriction, why not just say "milliseconds"? – Michael Lorton May 05 '17 at 06:34

3 Answers3

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The example of a Molotov bomb, a favorite of anarchists, and a car engine are a good analogy. The technology needed to contain the energies in a fusion reaction is much harder than the one needed for a car engine because of the MeV energies needed to initiate fusion. Once initiated it is explosive, so it must be engineered into small explosions from which energy can be extracted continuously.

Various ways of controlling fusion in a hot plasma of fusible materials, isotopes of hydrogen mainly, have been proposed and are being worked on. The tokamak is the basis of the international collaboration aiming to build an industrial prototype, ITER..

It is mainly an engineering problem coupled with the sociological problem of so many engineers and scientists working together in a project controlled by many research institutes. ( "too many cooks spoil the broth")

Also just wanted to know if we can continue this fusion reaction to generate precious heavy metals, is it possible?

Heavy metals are on the wrong curve for fusion, which can happen with elements up to iron or so. Each specific reaction will have to be considered, and it will be a completely different problem.

anna v
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You have several questions there, let's first focus on the main question: why is there no working fusion reactor on earth as we already have the hydrogen bomb?

This is an interesting question, as a lot of people had similar expectations when the first hydrogen bomb, Ivy Mike, was ignited in 1952. They probably had the first fission bomb, Trinity, ignited in 1945 and the first (proof-of-principle) fission reactor in their minds which went critical even a few years in advance.

Directly after World War II, fusion research was conducted in secret laboratories (UK, USA, Soviet Union). It was, however, soon realized that harnessing the energy released in a fusion reaction is a bit more complicated than initially expected and in 1955 the national laboratories involved in fusion research met for the first time at an international conference (1st UN Conference on Peaceful Use of Atomic Energy). They saw that everybody had similar problems and therefore in 1958, it was decided to declassify fusion research which was quite remarkable - keep the cold war in mind.

Now, what are the main differences of a reactor to a fusion bomb? In principle, anna v has answered everything. In a bomb, you do not really care about efficiency, you just want a huge amount of energy released instantaneously. In a reactor, however, efficiency is quite important. Let's have a brief look at the fusion process.

In order to fuse two light nuclei, they need to come very close together to overcome the electrostatic repulsive force. Only if their distance is on the order of their radius, the strong force starts to act and a new nucleus is formed. If you successfully fuse two light nuclei, the resulting nucleus has a smaller mass than the sum of the two original nuclei - the difference is released as energy according to $E=mc^2$.

To bring them close together, the particles need a very high speed. Higher particle speed means higher temperature and to give some numbers, for the currently envisaged fusion reaction, deutrium + tritium, temperatures on the order of 150 Mio °C are required. At such high temperatures, matter is ionized and consist mostly of charged particles and is referred to as a plasma. Achieving these temperature is not a problem. To understand what are the problems, let's look at the fusion reaction a bit more in detail.

Deutrium + Tritium $\rightarrow$ Helium + neutron + energy

In a reactor, the released energy serves two purposes: produce electricity and keep the fusion reaction running. In the fusion reactor concept which seems to be the most promising at the moment, magnetic confinement fusion, we use a magnetic field to confine the plasma in a toroidal shape. Since the neutron is not affected by the magnetic cage, it just leaves the plasma and hits the wall (thus heating the wall and the heat can then be used to produce electricity). The Helium-nuclei, however, is influenced in its motion by the magnetic field and we need it to heat more Deuterium and Tritium to temperatures high enough to perform more fusion reactions. This requires a good confinement and as it turns out it is not so easy to keep particles at such high temperatures long enough in the magnetic cage. The key parameter here is the confinement time which has been constantly increase since the 1950 but is still slightly too small to achieve break even.

Break even is defined here as the point where the released power (in the fusion reactions) is larger than the external heating power. The record was achieved at JET, currently the largest tokamak in the world, the achieved value was $0.6$. The goal of ITER is to release for the first time more power than the initial heating power.

So the main difference in the reactor is that we need a sustained reaction process where the energy of the reaction products need to be transferred to the plasma and this can only be achieved if we have a large enough confinement time.

As for the other questions, I would suggest to ask them in separate questions.

Update 1: The Q value, which is ratio of the released power to the externally applied heating power, is planned to reach 10 in ITER in the last stage of operation. A running power plant will probably have a slightly higher value, something around 30. I will dig through my references-folder when I'm back home and see if I find something more precise there.

David Richerby
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Alf
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    The achieved value was $0.6$... what is the minimum desired value? $1.0$? – Ruslan May 02 '17 at 10:57
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    1.0 means you need to spend as much energy to keep the reactor running as it produces, before containment breaks down. The minimum desired value is far larger than 1.0; after all, you need to not only recoup the cost of energy to run the reactor, but also the cost to design and build it, and all the research that came before – and let's not forget the cost of destroying it after it has ended its useful life. – Jörg W Mittag May 02 '17 at 11:17
  • Yes please, I want to look through the ITER $Q$ value projections. – bright-star May 04 '17 at 04:59
  • @JörgWMittag Never mind maybe making a small profit. Economics is funny like that. – user May 04 '17 at 08:08
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Bad luck. In the case of fission, there is a chain reaction, or at least easily initiable process what we can control. To initiate a nuclear reaction, an activation energy in the order of MeVs should be somehow produced on an easy way. In the case of the fission, it is solved by that the neutrons, being neutral, can enter the nuclei on a "backdoor".

With fusion, there isn't such a trick. There could be, but this time the laws of the nature simply aren't configured for our luck.

For example, if the half life of the muon would be in the order of $10^{-5} s$ instead of $10^{-6} s$, muon-catalyzed cold fusion power plants could already exist. A muon decaying 10 times slower would have practically no effect to the Universe (on the best current knowledge).

Fusion bombs are working by using a fission bomb as initiator. The analogue solution in the peaceful nuclear technology would be to use ordinary fission chain reaction to somehow catalyze deuterium fusion. It doesn't work, neutrons can't make anything to fuse.

peterh
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    No effect on the Universe, really? And what about hydrogen clouds, which would be able to begin fusion without extreme pressure and temperature? – Ruslan May 02 '17 at 07:01
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    @Ruslan If the muon half life would be in the order of $10^{-5} s$ instead of $10^{-6} s$, it wouldn't change anything. I inserted in the post. It wouldn't make any hidrogen cloud fusable, because nothing would insert the muons into them. – peterh May 02 '17 at 07:51
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    @Ruslan Furthermore, the muon-catalyzed fusion works for deuterons, which is a very rare thing in the interstellar material (compared to the ordinary hydrogen). – peterh May 03 '17 at 09:47
  • @Ruslan Furthermore, as far I know, the models of the very early Universe show that our would be very similar to our current one, if the 2nd and 3rd lepton/quark generations hadn't ever existed. This is a big open problem, nobody really knows, what are these particles. They seem perfectly useless. – peterh May 09 '17 at 21:17