What stops us from creating a nuclear fusion reactor as we already have the hydrogen bomb working on the same principle of fusion?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

The physics or engineering is not stopping us from building such a reactor, but rather international treaties.

There is the design of a plant that works on the principle of using a hydrogen bomb to create energy, this is called a PACER power plant. The electric power output is about 1600 MW (5000 MW thermal) and it is designed for about 5 explosions of Deuterium-Deuterium nuclear explosives per day inside of a water-steam environment. Because of the steam environment the heated steam neutron and gamma radiation dont travel that far, the cavern for the explosion would have a diameter of about 200 m at a depth of about 1.5 km.

Notably most other nuclear fusion designs use deuterium-tritium because of the much larger cross section, however tritium is quite rare but could be created by surrounding a fusion reactor by a breeding blanket. If a fusion reactor can breed enough tritium to sustain itself is still an active area of research.

Below you can see a picture of this kind of design outlined in the book "Nukleare Sprengkörper Bedrohung oder Energieversorgung für die Menschheit?" ("Nuclear explosives - threat of powersupply for humanity?") by Walter Seifritz.

However I think that building such a plant would be considered a political challenge as using nuclear explosives is banned even for peaceful purposes. Also I would expect quite some prejudice from the general population and good communication would be required, a significant challenge for the nuclear community.