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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 nuc

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.

harshalaxman 6y ago
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."
- jeffreyrogers 6y ago
  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.
  - projektfu 6y ago
    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.
    - rabidrat 6y ago
      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'?
peterwoerner 6y ago
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.
- pfdietz 6y ago
  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.
- varjag 6y ago
  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.
regularfry 6y ago
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.
- jeffreyrogers 6y ago
  Thanks, that's interesting.
ksec 6y ago
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-hydrogen-boron/ https://www.intelligentliving.co/future-fusion-radiationless...
sam 6y ago
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.
- jeffreyrogers 6y ago
  Thanks, that makes a lot of sense.
  - garmaine 6y ago
    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.
    - hcknwscommenter 6y ago
      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?
      - someguyorother 6y ago
        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_capture 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.
pfdietz 6y ago
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.
- lambdatronics 6y ago
  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).
  - pfdietz 6y ago
    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.

harshalaxman 6y ago

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."

jeffreyrogers 6y ago
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.
- projektfu 6y ago
  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.
  - rabidrat 6y ago
    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'?

peterwoerner 6y ago

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.

pfdietz 6y ago
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.
varjag 6y ago
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.

regularfry 6y ago

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.

jeffreyrogers 6y ago
Thanks, that's interesting.

ksec 6y ago

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-hydrogen-boron/ https://www.intelligentliving.co/future-fusion-radiationless...

sam 6y ago

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.

jeffreyrogers 6y ago
Thanks, that makes a lot of sense.
- garmaine 6y ago
  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.
  - hcknwscommenter 6y ago
    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?
    - someguyorother 6y ago
      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_capture 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.

pfdietz 6y ago

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.

lambdatronics 6y ago
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).
- pfdietz 6y ago
  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.