3 ms·

> 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

> 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.

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.