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If we are able to create a fusion reactor so there is a net gain in energy how can we turn the heat into electricity.

I know how it is done normally for fission, steam, but how could this be adapted for a fusion reactor where all of the heat is confined?

Are there any concepts for the stage after fusion becomes net positive?

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  • $\begingroup$ Check out the French - they are building one, but it has gone through many revisions and still not complete. Can't think of the name at the moment. As for the heat then steam or any fluid that can transport the energy at the required temperature. $\endgroup$ – Solar Mike May 14 '17 at 21:07
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    $\begingroup$ Cadarache, a Tokamak design and an ITER project. $\endgroup$ – Solar Mike May 14 '17 at 21:12
  • $\begingroup$ it depends how the heat is available, and the complexity of the container and generator. $\endgroup$ – DeltaEnfieldWaid May 14 '17 at 21:31
  • $\begingroup$ I'm not sure I understand why you say the heat is confined. The plasma that is undergoing fusion is confined by magnetic fields, but the heat of the fusion reaction can escape the area the plasma is confined to. It is likely that we would continue using the heat to generate superheated steam and drive turbines. $\endgroup$ – J. Ari May 15 '17 at 13:37
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    $\begingroup$ You can see this thread (quora.com/…) and the sources section of the Wikiepedia page for details on energy capture. The heat from the reaction would raise the temperature of the plasma if left alone, but if you had a water jacket (drastic simplification), then radiative heat transfer would take place and you would use the jacket to maintain a certain temperature and that would fix the amount of power you could draw from the reactor. $\endgroup$ – J. Ari May 15 '17 at 14:28
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This is an excellent question, and one that has not yet been fully answered by people working in this area of research. Let's look at some of the bigger projects and see what their proposals are:

ITER, the tokamak (giant doughnut) being built in France is supposed to be the first reactor to ever beat the break even point, unless someone either beats them to it beforehand or it doesn't work like we saw with the hotly anticipated NIF (National Ignition Facility) fusion experiment.

Their goal is to simply beat the break even energy barrier, there are no good plans, that I have seen at this time, on how to easily extract energy for power production, but it will likely use a steam process. Their current best shot is to use the high energy neutrons resulting from the fusion reactions to lose energy in the walls, thereby heating it and having that heat removed by a high pressure water blanket on the outside. This will be extraordinarily challenging, but not as difficult an engineering problem as solving fusion in the first place.

The NIF (National Ignition Facility) uses small spherical capsules of super cooled deuterium/tritium in a hohlraum (precision made tube) that converts the energy of the worlds largest laser into x-rays in order to compress the fusion fuel to a density of around 1000g/cm^3 causing fusion to occur.

Their plan was to make a system where these capsules would be loaded into the chamber in quick succession to maintain a "constant" energy output. Problems with this are cost, complexity, and inability (so far) to be able to get past the break even threshold. They likely would implement a steam cycle for power generation if ever successful.

Then there are some private sector ventures. Most are not doing much in my opinion (they can't even manage basic fusion reactions, let alone anything near the break even barrier), although there are a couple exceptions.

General Fusion has probably the second best idea in fusion for electrical production. They are using a massive sphere full of molten lead that swirls around to create a vortex that they shoot a sub fusion temperature plasma into. Once the plasma is in the center of the swirling lead, a series of pistons SMACK hammers that in turn produce a massive shock wave that then collapses the plasma resulting in fusion reactions. This in turn would heat the lead which has heat exchangers built into the reactor which in turn boils water for a steam cycle.

There are several advantages to their design. First, they don't have to worry about melting down the containment walls (like with a tokamak) as they are already molten. Second, the high energy neutrons from the deuterium/tritium reactions can be moderated by the large volume of lead (it's a poor moderator compared to other elements, but having such a large physical cross sectional area it works). Third, the lead can be made into an amalgam with Li6 to absorb the now thermal neutrons to produce more tritium (most expensive component of their fuel at ~$30,000.00/g) while also providing a nice 'boost' in power. Fourth, the pistons could be directly driven by the steam created from the reactor for greater efficiency.

Another company, Tri Alpha Energy (although they are pushing for being called TAE now) is trying proton/boron 11 fusion to create an unstable C12 that blows itself apart into 3 helium 4 nuclei that are referred to as alpha particles, hence the name 'Tri Alpha' Energy.

Being in the industry, I have had ample opportunity to speak with some of their people. It seems their big focus (no pun intended), as I understand it, is on trying a new technique for compressing and maintaining a plasma through self generated magnetic fields (as opposed to shoving it together externally). The issue from what they have told me is their equipment is off by a minimum of an order of magnitude in order to hit fusion temperatures, even after 20 years of development (fusion is hard after all).

I mention them because they did have a unique idea on electrical production which ties directly to your question. They claimed to be working on a 'reverse cyclotron' where they would input the alpha particles 'backwards' into a cyclotron to generate electrical power. There are about a million challenges to this problem, e.g. how do you get the alpha particles to enter the device precisely enough to generate a magnetic field to produce electrical current and do it without cooling the plasma to such a degree that the plasma can no longer maintain fusion reactions.

Put simply, the big goal right now is just try to figure out how to create excess energy, by any means necessary, and work out power production after.

Hope this answers your question!

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  • $\begingroup$ So, just about every bit you mention has the words somewhere in it : "just try to figure out".... $\endgroup$ – Solar Mike Jul 12 '18 at 18:04
  • $\begingroup$ Yup, be nice when the day comes when that turns into : "now that we figured it out...". $\endgroup$ – eatscrayons Jul 14 '18 at 4:24
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Thermal energy is, in part, obtained or collected in the breeder blanket concept. Here neutrons with high energy are used to produce heat and breed valuable tritium by interaction with lithium ceramic breeder pebbles. From interaction with a high energy neutron, Lithium fissions into helium and tritium. The lithium ceramics (usually Li4SiO4 or Li2TiO3 or mixtures thereof) are present in the form of a pebble bed, cooled by helium. Heat, tritium and helium are produced within the bed, thus providing further fuel and useful heat for the generation of electricity.

Basically to sum it up, the neutrons emitted in the fusion reaction serve to deliver energy to the external system that is isolated from the plasma (in which the fusion takes place).

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