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Comment by jeffreyrogers | Hacker News Reader

The thing I never understand about these fusion reactors is how they plan on generating energy from them, assuming they exceed break-even. In a conventional nuclear reactor the coolant is piped to a steam generator which then drives steam turbines to generate power. This is a relatively obvious design and it was pretty well understood from the start of development how the power generation would work. Of course, there were a lot of engineering challenges to overcome to make it work in practice. While fusion itself is a harder problem (as the lack of success for decades has demonstrated), it seems like the power generation aspect has many new challenges as well.

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The typical design for a fusion power plant that runs on deuterium tritium fuel is to place a lithium "blanket" around the plasma. 80% of the energy released in the deutrium - tritium fusion reaction comes out in the energy of a neutron which would be absorbed in the blanket, heating it up and also generating tritium fuel which could then be fed back in as half of the fuel (the other half being deuterium which is abundant in seawater).

You would then run a heat exchanger from the hot lithium to create steam to then turn a turbine and make electricity.

Thanks, that makes a lot of sense.

There is also direct-capture in some fusion designs where the charged by-products are electrically decelerated, producing current. I don't believe this is possible in the standard Tokamak design, but the efficiency gain is a selling point for those designs which do allow it.

Direct capture cannot capture a neutron, as it is not a charged particle. Charged particles are relatively trivial to block with the lithium blanket. Not sure how direct capture could possible be more efficient. Do you have a reference that explains what you are talking about?

You run a fusion reaction that mainly produces charged particles, and then convert the KE of the charged particles directly into electricity. See https://en.wikipedia.org/wiki/Aneutronic_fusion#Energy_captu... .

There's just the matter of getting the temperature up to 10x of an ordinary D-T fusion plasma. Non-equilibrium reactors like the polywell try to bypass the problem entirely, but (to my knowledge) it's very hard to maintain a non-thermal state.

That’s right, there are other fusion fuels that have only charged fusion products which could be directly converted to electricity. This is in contrast to deuterium - tritium which releases 80% of its energy in a neutron which has zero charge.

Examples of fusion fuels whose main reaction produces only charged products are deuterium helium-3 and proton boron-11. These reactions however require higher temperatures and better confinement characteristics.

The reason why deuterium tritium is the major focus of most (though certainly not all) research is that it has the highest reactivity at the lowest temperature compared to other fuels. Unfortunately it produces a high energy neutron which makes the conversion to electricity more complex.

The people working on fusion generally think it is a solved problem. Namely, fusion is a heat source and we have numerous technologies which can convert a heat source to generate electricity a number of which can be connected to the fusion generator with "some engineering". That's why they talk about the parts which aren't yet solved.

I used to work with under a guy who worked on ITER, they were all aware of that part of the problem. It just wasn't part of the research they were engaged in.

The power density of ITER will be a horribly low 0.05 MW/m^3. Of course getting heat out is easier if there's so little heat being produced. But even then, the divertor at ITER will be challenging, as will dealing with heating from disruptions.

Generally, a layperson can safely assume that whatever problem they come up with on the spot, it has already been thought of by specialists as well.

True for any field you are not an expert in.

As mentioned elsewhere in these comments, the usual plan is to convert the fusion energy to heat in a blanket, through which coolant flows.

This sounds simple, but actually shows one of the biggest problems with fusion. The wall can handle only a certain power/area before the engineering becomes too difficult. And because the wall has to be outside the plasma, the ratio of the wall area to plasma volume will be quite low. In contrast, in a fission reactor, the "wall" is the walls of the fuel rods, which are only about 1 cm in diameter. There is lots of surface area through which to convey the heat.

The square/cube law comes into play here. Fusion reactors will have terrible volumetric power density, compared to fission reactors (or, even worse, internal combustion engines like gas turbines.) As a result, fusion reactors will be massive, expensive things, compared to the competing source of heat.

The figures for existing reactor proposals illustrate this. The volumetric power density of a PWR primary reactor vessel is ~20 MW/m^3. In constrast, the power density of the ARC proposal from MIT (counting the volume of the reactor + blanket + magnet shield + magnets + support structure) is around 0.5 MW/m^3.

If someone breathlessly hypes some fusion reactor design, ask them what its volumetric power density is.

This is underappreciated. Achieving sufficient confinement pushes toward larger volume/surface area, but getting high power density pushes the other way. In a way, the fact that 80% of the DT power output is in fast neutrons becomes beneficial -- that fraction is absorbed volumetrically, so only 20% must be conducted through the surface (or convected, if it's a liquid).

The neutrons cause their own problems, though, and these problems interact with the purely thermal ones.

For example: the plasma facing surfaces on ITER are backed by a CuCrZr alloy with high thermal conductivity. The amount of direct thermal power this innermost wall can withstand, from the plasma and photons, is proportional to this thermal conductivity, since stress there is proportional to the thermal gradient.

But in an actual reactor, designed to operate for long periods of time, this alloy cannot be used, as it becomes too activated. Instead, DEMO is going to use a kind of reduced activation steel (Eurofer 97) in which the thermal conductivity is an order of magnitude lower. This is one of the things pushing DEMO far outside the range of potentially competitive cost.

Yup. There's also the need to maintain transparency of the first wall to neutrons so the tritium breeding ratio stays above 1. However, a very thin wall is easy to breach in the event of plasma eruptions, or even long-term erosion due to ion bombardment.

It seems to me that a thick (~ cm at least) flowing liquid first wall is probably the way to go for achieving high power density. The liquid has its own problems though, such as corrosion compatibility with the underlying structural material, the need to have low vapor pressure at high temperature, the possibility of splashing, and MHD effects if the liquid is conductive.

I agree. Also, liquid surfaced walls are probably required for any pulsed fusion approach, which at a given average power/area will have much higher instantaneous power/area. This drives one toward LINUS-like approaches.

Compact Fusion Systems recently got funding to push this approach. On LinkedIn they announced purchase of surplus high-energy capacitors (from the old Shiva Star at AFRL) to build a prototype.

They seem to have taken to hear some lessons learned from previous experiments (cannot assume lossless compression, must pay attention to RT instabilities, must think about vaporization of first-wall).

One downside to a fully liquid blanket: the peak temperature is limited by the vapor pressure of the first wall, which in turn is limited by the impurity tolerance of the plasma. In a non-imploding device, it would be possible to run the majority of the blanket at a higher temperature & separate that from the cooler first-wall liquid. Of course, this puts some solid material closer to the neutrons, so YMMV.

https://arpa-e.energy.gov/sites/default/files/ALPHA_TURCHI.p... https://arpa-e.energy.gov/sites/default/files/06_TURCHI.pdf

I've seen their slides before, and I was impressed by their engineering-oriented approach. The engineering obstacles to fusion are so great that (IMO) one should use them to drive the design, rather than taking a physics-oriented approach and worry about the engineering later.

From the article "The plants would convert the raw fusion energy into heat, then use the heat to boil water, spin a turbine, and ultimately to generate electricity that can light your home and charge your phone."

Yeah, but raw fusion energy to heat is not nearly as trivial as it sounds. And given how tricky it is to sustain a fusion reaction it seems like it will be even harder to do it when you're dealing with moving heat from the source of the fusion reaction to wherever your steam generator is.

From my guess, the plasma is already quite hot, so you could do whatever you do to cool it normally, just more so because it’s self-sustaining.

I believe there are two main ways of cooling: convection and radiation. Convection requires a gas or liquid carrier to be in contact with the hot substance; it seems like it would be very difficult to get water (steam) into direct contact with the plasma to transfer the heat, and then to carry the heat away without the radioactive plasma along with it.

Radiation is a function of temperature (to the 4th power) and surface area. It seems even harder to channel the heat from the plasma to the engine this way.

So how would you do either of these 'more so'?

I don't know how this one in particular is planning to do it, but the Polywell wiffle-ball design uses magnet coolant as a heat transfer medium. I'm sure I've heard of a design that uses charged particles spat out of the fusion reaction directly as charge carriers, too.

Thanks, that's interesting.

Hydrogen-Boron Fusion [1], allows for using much smaller and simpler generators because it converts it directly into electricity.

Note: I am not a physicist and has no idea if it really works. But I guess when it comes to Nuclear Fusion no one really does.

[1] https://www.intelligentliving.co/future-fusion-radiationless...

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