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The majority of energy produced by nuclear fusion is harnessed by neutrons or protons that split out from the product.

Given the dominant fusion method today is Deuterium + Tritium which produces He and a neutron (a neutron that has most of the energy from the fusion), what do current experimental fusion reactors do to harness the energy from said neutrons?

This question is asked using the context that neutrons cannot be controlled using electromagnetic forces. Hence, the energy contained in neutrons would (in my mind, at least) be difficult to capture without resorting to some sort of fission technique.

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    $\begingroup$ Actually, neutrons can be controlled by magnetic forces, my mistake. However, it is very weakly controllable, so I was wondering what methods current experimental reactors can potentially use to control neutrons other than magnetic forces, which are tenuous upon neutrons. $\endgroup$
    – Young Jun Lee
    Oct 27, 2022 at 20:30
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    $\begingroup$ Well, current experimental fusion reactors don't really try to harness the energy. They are experiments on the fusion bits, not the energy production bits. Ultimately one needs to transfer the energy from fusion into a working fluid to drive (likely) a more conventional steam cycle. Various proposals have been put forward. $\endgroup$
    – Jon Custer
    Oct 27, 2022 at 20:36
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    $\begingroup$ @jon-custer So essentially, neutrons have very little role in helping nuclear fusion achieve its breakeven point? Or maybe, do the neutrons still create heat somehow, maybe contributing to the reheating of the plasma? $\endgroup$
    – Young Jun Lee
    Oct 27, 2022 at 20:40
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    $\begingroup$ Yes, we need to extract the KE from the neutrons, but we also want to use them to breed fuel, and to minimise the damage they cause to the reactor housing. $\endgroup$
    – PM 2Ring
    Oct 27, 2022 at 22:40

2 Answers 2

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This is known as the "first wall" problem of fusion: what do you wrap a fusion reactor with (the first wall), so as to capture the neutron energy without being destroyed by the intense neutron flux?

More specifically, the objective of the first wall is to rattle the neutron flux around so as to "thermalize" the neutrons (transfer their kinetic energy into lattice vibrations which show up as heat, which then can be carried off by some heat transfer medium to boil water into steam, etc.) without being ruined (from a materials science standpoint) by damage from the neutrons. Doing so is essential from an energy balance standpoint to make the fusion reaction products all "pay their way" towards breakeven by harvesting their kinetic energy before they zoom right out of the reactor volume and escape.

This remains as an unsolved problem in fusion technology. For example, superalloy metals get their constituent atoms knocked out of their lattice positions from neutron impacts, which interferes with ductility mechanisms (rendering the metal incapable of exhibiting resistance to thermal and mechanical shock). In addition, neutron capture leads to transmutation of the alloy constituents into new elements which lack high-temperature corrosion resistance while also generating hydrogen atoms within the lattice which lead to swelling and embrittlement.

This is an extremely difficult business!

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    $\begingroup$ Thank you, I was searching for this problem but almost nothing regarding the neutrons came up (on Google and Youtube, at least). Regarding the neutrons; as they have a magnetic moment, do you think it could be possible to get them to stop in front of a thick sheet of copper, much like how magnets abruptly stop in front of copper sheets? Example seen here: youtube.com/… $\endgroup$
    – Young Jun Lee
    Oct 28, 2022 at 4:23
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    $\begingroup$ @YoungJunLee This is (part of) why ITER is being built: To create a source of such intense neutron flux, so we can experiment on it on a macroscopic scale and find out what all we do not yet know about materials under the neutron flux created by fusion. $\endgroup$
    – DevSolar
    Oct 28, 2022 at 10:55
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    $\begingroup$ why not shroud the reactor in falling water that is heated? I mean you can't break water. $\endgroup$
    – JEB
    Oct 28, 2022 at 13:39
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    $\begingroup$ @JEB: Water behind the first wall would be useless as the first wall already got pummeled by neutrons. Water in front of the first wall would be inside the vacuum of the reaction chamber... which would then no longer be a vacuum and unable to create or sustain a fusion reaction. $\endgroup$
    – DevSolar
    Oct 28, 2022 at 16:19
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    $\begingroup$ @DevSolar I think the idea was to make the first wall something that's terrible at absorbing neutrons $\endgroup$
    – user253751
    Oct 28, 2022 at 20:55
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In fusion power plants the energy from neutrons is captured in the blanket, which is immediately behind the first wall that surrounds the plasma. One candidate for the blanket is FLiBe molten salt, which is also being studied in the context of fission and concentrated solar power, and another candidate is dual-coolant Lead-Lithium/Helium. An important aspect of the blanket is also to ensure very little neutron energy is absorbed by the superconducting magnets and support structures. The neutron energy is carried away as thermal energy in the liquid blanket and then used to power a steam turbine.

The neutron energy is also used to breed Tritium in the liquid blanket with the reaction:

$^6\text{Li} + \text{n} \rightarrow \alpha + \text{T} + 4.78 \text{ MeV}$

This Tritium can then be used as fusion fuel, while the Tritium decay product Helium-3 is rare on earth and important for neutron radiation detectors and magnetic resonance imaging of the lung. While Tritium is currently bred inside fission reactors (e.g. by replacing some control rods with rods containing Lithium), a major research goal is for fusion power plants to generate their own Tritium.

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    $\begingroup$ Thank you, if we are to breed Tritium, I guess we are using some sort of fission technique to harness the neutron's power. Is there any effort to harness neutrons' energy by magnetic or other means? I did state in my question that I am curious about non-magnetic methods; however, since you are a researcher in the field, I thought that it would be a good opportunity to ask you about the general direction of fusion research. Similarly, do you project a fundamental breakthrough eventually (much like how the discovery of neutrons allowed nuclear fission)? $\endgroup$
    – Young Jun Lee
    Oct 28, 2022 at 5:15
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    $\begingroup$ In the D _ T reaction, the 14 MeV neutrons are the main vectors (80%) for transporting the plasma energy to the outside of the enclosure. They are therefore necessary, useful and there are no simple or necessary means of acting on these neutrons: they must lose their energy in the external environments before ending up captured in these same environments. Fission is not used, it is not necessary, there is no fissile material in fusion machines. Fusion reactions are well known, they are difficult to initiate for machines built on Earth, there is nothing revolutionary to expect in theory. $\endgroup$
    – Jean Jacques
    Oct 28, 2022 at 5:53
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    $\begingroup$ There are some interesting research areas that may lead to fundamental breakthroughs like predicting the heating power required to transition from low to high confinement mode in plasmas, or spin-polarized fusion, or increasing magnet performance with breakthroughs in high-temperature superconductors $\endgroup$
    – Tom Neiser
    Oct 31, 2022 at 18:46
  • $\begingroup$ @JeanJacques - except, of course, in fission-fusion hybrids, where there's all sorts of fissionable materials. $\endgroup$
    – Maury Markowitz
    Nov 17, 2022 at 0:56

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