4 ms·

What are some of the others ? I’ve heard even if everything worked the generated neutrons would eventually destroy the reactor.

pfdietz 6y ago
That's one. Another showstopper is that all the energy has to be radiated through the wall of the reactor. By limits on this areal power density and the square cube law, the volumetric power density of fusion reactors will suck. Compare ITER (0.05 MW/m^3) or ARC (0.5 MW/m^3) vs. a commercial PWR fission reactor primary reactor vessel (20 MW/m^3). Stronger magnets don't save a reactor from this. The large size and complexity of a fusion reactor also means their reliability is a huge problem. There are many parts and joins there, and the machine will be so radioactive hands on access will be impossible. A single leak of coolant into the vacuum chamber renders a fusion reactor inoperable (while a fission reactor can keep operating even with multiple fuel rod leaks.)
- thewarrior 6y ago
  I’ve heard the sun has a very low power density it’s just that it’s massive. Maybe it’s just easier to create a star and live in orbit around it
  - pfdietz 6y ago
    The key point is the Sun is already paid for. :)
    - elcritch 6y ago
      Are you certain? ;) Sounds like the plot of a sci-fi in the vein of Hitchhiker's guide where galactic aliens show up with an overdue bill for the sun.
  - NortySpock 6y ago
    Or find an existing unsettled one and move in! Tricky bit will be getting to that nearby star...
    - nine_k 6y ago
      If only we had an engine with high enough Isp to accelerate to a decent velocity! Maybe something based on nuclear fusion...
      - adrianN 6y ago
        That's not that hard actually. We already know how to do fusion, we just have a hard time confining it into a reactor. In space you don't have that problem, bombs work fine.
  - sdflhasjd 6y ago
    It's true, but at least partially deceptive because the fusion only happens in the core, which is <2% by volume
    - nine_k 6y ago
      The rest is needed to keep the pressure and the magnetic field.
  - hutzlibu 6y ago
    Well .. we do already live in the orbit of a star. (intended joke?) And we have lots of desert land, that could be covered with solarthermal plants. https://en.m.wikipedia.org/wiki/Desertec https://en.m.wikipedia.org/wiki/Desertec Also not easy, but sounds more predictable than a possible fusion future.
- HPsquared 6y ago
  A good comparison would be, what's the capital cost of a hypothetical fusion plant, compared to equivalent renewables (e.g. battery-backed solar/wind)
  - Veedrac 6y ago
    https://arxiv.org/abs/1409.3540 https://arxiv.org/abs/1409.3540, Table 11 and Section 6.6.
    - mikeyouse 6y ago
      Yikes. For a 250MWe reactor: > *Assuming the higher cost estimate for the REBCO tape, the materials costs for ARC total $428M and the total fabricated component cost estimates total $5.56B. Given it's a novel nuclear project - it's probably safe to double/triple that cost estimate. Still wildly uncompetitive. That's actually really disappointing.
      - Veedrac 6y ago
        The fabricated cost estimates were extrapolated from other novel nuclear fusion projects, so doubling it would be double-counting.
      - pfdietz 6y ago
        Their cost estimation methodology in that paper seems absurdly simplistic, though.
- Tuna-Fish 6y ago
  > Compare ITER (0.05 MW/m^3) or ARC (0.5 MW/m^3) vs. a commercial PWR fission reactor primary reactor vessel (20 MW/m^3). Stronger magnets don't save a reactor from this. Stronger magnets absolutely would save the reactor from this, because for a fusion reactor of any given size the power density goes to at least the fourth power of the magnetic field. If we could have ITER, but with 40T in the on-axis toroidal field, it would have a power density of ~150 MW/m^3. This is of course ludicrous because the thermal output of 1.5TW would not be possible to contain, not even considering how the coils are supposed to be kept together. But this clearly show the path that also allows beating the square-cube scaling: You scale magnetic field up, and reactor size down, until you have reasonable power outputs from reasonably priced reactors. There is potential future in fusion, but it specifically requires better superconducting magnets. ARC at least is research in the right direction of high-field superconductors, even if the REBCO magnets they are using are not quite up to the task of making a reactor that would be economically viable.
  - pfdietz 6y ago
    > Stronger magnets absolutely would save the reactor from this, because for a fusion reactor of any given size the power density goes to at least the fourth power of the magnetic field. ... right up until you reach the limit on what your first wall can handle. At that point, making the magnetic field stronger gains you little or nothing, because your reactor does not survive operating at the fusion power density it would enable. The claim I am making is that this power/area limit causes DT fusion to be inferior in volumetric power density to fission. The factor of inferiority is roughly (minor radius of fusion reactor)/(radius of fission reactor fuel rod), and the numerator there at least about a meter due to the need to stop neutrons. The factor is independent of magnetic field. What strong magnetic fields might do is allow you to go to advanced fuels. The hope there is avoiding a thermal power cycle entirely, saving on the non-nuclear side of the power plant. Tokamaks would be hopeless for this, though; their beta is too low.
- deleted 6y ago
  [deleted]
kaybe 6y ago
I'm also thinking of the highly diffusive radioactive tritium gas which is a PITA to contain and work with. https://en.wikipedia.org/wiki/Tritium https://en.wikipedia.org/wiki/Tritium
moogly 6y ago
They're hoping to use a lithium "blanket" that would absorb and use the emitted neutrons to breed tritium (that's needed for D+T fusion anyway).
- tsimionescu 6y ago
  That doesn't save the confinement walls from becoming more and more brittle, it just limits the damage to those areas, and it helps recover some of the tritium it leaks.
- pfdietz 6y ago
  It's not that they hope to breed tritium; tritium breeding is absolutely required for DT fusion to make any sense at all. Making the tritium in fission reactors for a 1 GW(e) DT fusion reactor would cost $20B/year.

pfdietz 6y ago

That's one. Another showstopper is that all the energy has to be radiated through the wall of the reactor. By limits on this areal power density and the square cube law, the volumetric power density of fusion reactors will suck. Compare ITER (0.05 MW/m^3) or ARC (0.5 MW/m^3) vs. a commercial PWR fission reactor primary reactor vessel (20 MW/m^3). Stronger magnets don't save a reactor from this. The large size and complexity of a fusion reactor also means their reliability is a huge problem. There are many parts and joins there, and the machine will be so radioactive hands on access will be impossible. A single leak of coolant into the vacuum chamber renders a fusion reactor inoperable (while a fission reactor can keep operating even with multiple fuel rod leaks.)

thewarrior 6y ago
I’ve heard the sun has a very low power density it’s just that it’s massive. Maybe it’s just easier to create a star and live in orbit around it
- pfdietz 6y ago
  The key point is the Sun is already paid for. :)
  - elcritch 6y ago
    Are you certain? ;) Sounds like the plot of a sci-fi in the vein of Hitchhiker's guide where galactic aliens show up with an overdue bill for the sun.
- NortySpock 6y ago
  Or find an existing unsettled one and move in! Tricky bit will be getting to that nearby star...
  - nine_k 6y ago
    If only we had an engine with high enough Isp to accelerate to a decent velocity! Maybe something based on nuclear fusion...
    - adrianN 6y ago
      That's not that hard actually. We already know how to do fusion, we just have a hard time confining it into a reactor. In space you don't have that problem, bombs work fine.
- sdflhasjd 6y ago
  It's true, but at least partially deceptive because the fusion only happens in the core, which is <2% by volume
  - nine_k 6y ago
    The rest is needed to keep the pressure and the magnetic field.
- hutzlibu 6y ago
  Well .. we do already live in the orbit of a star. (intended joke?) And we have lots of desert land, that could be covered with solarthermal plants. https://en.m.wikipedia.org/wiki/Desertec https://en.m.wikipedia.org/wiki/Desertec Also not easy, but sounds more predictable than a possible fusion future.
HPsquared 6y ago
A good comparison would be, what's the capital cost of a hypothetical fusion plant, compared to equivalent renewables (e.g. battery-backed solar/wind)
- Veedrac 6y ago
  https://arxiv.org/abs/1409.3540 https://arxiv.org/abs/1409.3540, Table 11 and Section 6.6.
  - mikeyouse 6y ago
    Yikes. For a 250MWe reactor: > *Assuming the higher cost estimate for the REBCO tape, the materials costs for ARC total $428M and the total fabricated component cost estimates total $5.56B. Given it's a novel nuclear project - it's probably safe to double/triple that cost estimate. Still wildly uncompetitive. That's actually really disappointing.
    - Veedrac 6y ago
      The fabricated cost estimates were extrapolated from other novel nuclear fusion projects, so doubling it would be double-counting.
    - pfdietz 6y ago
      Their cost estimation methodology in that paper seems absurdly simplistic, though.
Tuna-Fish 6y ago
> Compare ITER (0.05 MW/m^3) or ARC (0.5 MW/m^3) vs. a commercial PWR fission reactor primary reactor vessel (20 MW/m^3). Stronger magnets don't save a reactor from this. Stronger magnets absolutely would save the reactor from this, because for a fusion reactor of any given size the power density goes to at least the fourth power of the magnetic field. If we could have ITER, but with 40T in the on-axis toroidal field, it would have a power density of ~150 MW/m^3. This is of course ludicrous because the thermal output of 1.5TW would not be possible to contain, not even considering how the coils are supposed to be kept together. But this clearly show the path that also allows beating the square-cube scaling: You scale magnetic field up, and reactor size down, until you have reasonable power outputs from reasonably priced reactors. There is potential future in fusion, but it specifically requires better superconducting magnets. ARC at least is research in the right direction of high-field superconductors, even if the REBCO magnets they are using are not quite up to the task of making a reactor that would be economically viable.
- pfdietz 6y ago
  > Stronger magnets absolutely would save the reactor from this, because for a fusion reactor of any given size the power density goes to at least the fourth power of the magnetic field. ... right up until you reach the limit on what your first wall can handle. At that point, making the magnetic field stronger gains you little or nothing, because your reactor does not survive operating at the fusion power density it would enable. The claim I am making is that this power/area limit causes DT fusion to be inferior in volumetric power density to fission. The factor of inferiority is roughly (minor radius of fusion reactor)/(radius of fission reactor fuel rod), and the numerator there at least about a meter due to the need to stop neutrons. The factor is independent of magnetic field. What strong magnetic fields might do is allow you to go to advanced fuels. The hope there is avoiding a thermal power cycle entirely, saving on the non-nuclear side of the power plant. Tokamaks would be hopeless for this, though; their beta is too low.
deleted 6y ago
[deleted]

kaybe 6y ago

I'm also thinking of the highly diffusive radioactive tritium gas which is a PITA to contain and work with. https://en.wikipedia.org/wiki/Tritium https://en.wikipedia.org/wiki/Tritium

moogly 6y ago

They're hoping to use a lithium "blanket" that would absorb and use the emitted neutrons to breed tritium (that's needed for D+T fusion anyway).

tsimionescu 6y ago
That doesn't save the confinement walls from becoming more and more brittle, it just limits the damage to those areas, and it helps recover some of the tritium it leaks.
pfdietz 6y ago
It's not that they hope to breed tritium; tritium breeding is absolutely required for DT fusion to make any sense at all. Making the tritium in fission reactors for a 1 GW(e) DT fusion reactor would cost $20B/year.