9 ms·

Except the article is wrong, because it makes fundamental assumptions about the vacuum wall and lithium blanket, and even the superconductors. The ARC reactor,

Except the article is wrong, because it makes fundamental assumptions about the vacuum wall and lithium blanket, and even the superconductors. The ARC reactor, for example, doesn't use a solid sandwich style blanket, it uses a volumetric blanket of FLiBe, a liquid that acts as the moderator that absorbs neutrons, the breeder, the magnetic shield, and the heat exchanger -- and has proven to work in molten salt reactor prototypes. And unlike the designs he claims that are "essentially unchanged", this significantly reduces the cost and mass of the reactor, reduces the so-called "danger" of lithium, and leverages the fact that high energy fusion neutrons travel further, as it increases the efficacy of the liquid blanket. The claim that neutrons will damage the metal structure is also handled by many aspects of the design: easy reparability thanks to modular, segmented magnets, being completely submerged in the liquid blanket, low activation steel for structure, and the pulse design which runs the reactor in a way that reduces overall temperature of the walls. Plus, he brings up the ole aneutronic fusion dream, except that creating a confined Boron-11 plasma is much more difficult, outside the realm of tokamaks, and typically proposes using laser internal confinement fusion at the Petawatt scale. More than 20 papers were published on ARC, heavily vetted in journals like Nature, and pretty much all of the peer review concluded the design will work, and if it does work, then it will be much cheaper than ITER, simpler than ITER, and thus refute your paper. We'll know in about 4 years.

pfdietz 6y ago
No. The article makes no assumptions about superconductors. The argument makes no assumption even about plasma confinement. It's all about the limits on areal power density at the reactor walls. The conclusion he reaches, that fusion reactors will have a volumetric power density at least an order of magnitude lower than a fission reactor, is not contradicted by ARC. The volumetric power density of ARC (from the ARC paper I linked to elsewhere in these comments) is 0.5 MW(th)/m^3 (counting the volume of the entire reactor, not just the plasma) vs. 20 MW(th)/m^3 for the primary reactor vessel of a PWR -- a factor of 40 worse. His suggestion to look at aneutronic plasmas didn't pan out, as you note, but that doesn't save DT fusion. I'll note that it's his PhD student Todd Rider who shot down the non-Maxwellian schemes that aneutronic fuels would have needed. After that, Lidsky switched to working on fission for the rest of career and life.
- cromwellian 6y ago
  The latest ARC claims from Whyte's last presentation was 3MW/m^3 density, and that's with currently achievable magnets, not those people believe are ultimately achievable. Counting just the PWR reactor vessel, and not the entire containment facility, seems a huge cheat. We're talking economics of energy. PWR Fission reactors have enormous footprints, averaging 1 square mile. An ARC reactor wouldn't need anywhere need that size. The very nature of using pressurized water means your containment facility has to be a factor of 1000 larger than the reactor vessel to handle a flash boil over. Current costs to build a 1GW fission plant are $6-9 billion. Predicted costs to build a 200MW ARC reactor are $4-5 billion, so the ARC reactor costs ~5x more at the high end, and 77% more at the low end, for the same output (e.g. build 5 ARC reactors to equal one fission plant). The economics are still viable, and your costs for storage, disposal, and de-comissioning are far lower, as is your land footprint. Note that these ARC costs are for the first reactor, include the sunk development costs, and not the economies of scale that would occur from dropping HTS costs. > It's all about the limits on areal power density at the reactor walls. No, he makes lots of assumptions about how the magnets are shielded, how breeding works, and how heat transfer works. All of these are addressed by FLiBe, there's no "structure" to the heat blanket to damage, it acts simultaneously as radioactive shielding for the magnets, heat transfer, and a breeder. The "blanket" in ARC isn't a surface sandwiched with a super structure, it's a volume that's continuously pumped and recirculated. He also makes the implicit assumption that reactor wall damage is a huge problem. Instead of seeing it as a problem, you should see it as a solution, like an ablative heat shield on a rocket or capsule. As long as the cost of maintenance or replacement is low, and reactor downtime is limited, it is not a problem. Current estimates for ARC, which is designed to be modular, so it can be easily maintained, is that the reactor walls will last about 1 year. He doesn't even mention the biggest unsolved engineering problem, removing the "ash" (Helium) that accumulates in the plasma. His article has all the trappings of reusable rocket naysayers in the 80s and 90s, who claimed reusables could never work, because carrying wings, landing legs, heat shielding, and structural reinforcement to make rapid reusability would cut down on your usable payload, and thus doom reusable rockets to always be substantially inferior. And this is because of an obsession with a single variable, like power density, or payload fraction, and ignoring the end to end economics of the entire system, including safety margins, maintenance, footprint, etc. And that's why I believe ARC will prove Lidsky wrong.

pfdietz 6y ago

No. The article makes no assumptions about superconductors. The argument makes no assumption even about plasma confinement. It's all about the limits on areal power density at the reactor walls. The conclusion he reaches, that fusion reactors will have a volumetric power density at least an order of magnitude lower than a fission reactor, is not contradicted by ARC. The volumetric power density of ARC (from the ARC paper I linked to elsewhere in these comments) is 0.5 MW(th)/m^3 (counting the volume of the entire reactor, not just the plasma) vs. 20 MW(th)/m^3 for the primary reactor vessel of a PWR -- a factor of 40 worse. His suggestion to look at aneutronic plasmas didn't pan out, as you note, but that doesn't save DT fusion. I'll note that it's his PhD student Todd Rider who shot down the non-Maxwellian schemes that aneutronic fuels would have needed. After that, Lidsky switched to working on fission for the rest of career and life.

cromwellian 6y ago
The latest ARC claims from Whyte's last presentation was 3MW/m^3 density, and that's with currently achievable magnets, not those people believe are ultimately achievable. Counting just the PWR reactor vessel, and not the entire containment facility, seems a huge cheat. We're talking economics of energy. PWR Fission reactors have enormous footprints, averaging 1 square mile. An ARC reactor wouldn't need anywhere need that size. The very nature of using pressurized water means your containment facility has to be a factor of 1000 larger than the reactor vessel to handle a flash boil over. Current costs to build a 1GW fission plant are $6-9 billion. Predicted costs to build a 200MW ARC reactor are $4-5 billion, so the ARC reactor costs ~5x more at the high end, and 77% more at the low end, for the same output (e.g. build 5 ARC reactors to equal one fission plant). The economics are still viable, and your costs for storage, disposal, and de-comissioning are far lower, as is your land footprint. Note that these ARC costs are for the first reactor, include the sunk development costs, and not the economies of scale that would occur from dropping HTS costs. > It's all about the limits on areal power density at the reactor walls. No, he makes lots of assumptions about how the magnets are shielded, how breeding works, and how heat transfer works. All of these are addressed by FLiBe, there's no "structure" to the heat blanket to damage, it acts simultaneously as radioactive shielding for the magnets, heat transfer, and a breeder. The "blanket" in ARC isn't a surface sandwiched with a super structure, it's a volume that's continuously pumped and recirculated. He also makes the implicit assumption that reactor wall damage is a huge problem. Instead of seeing it as a problem, you should see it as a solution, like an ablative heat shield on a rocket or capsule. As long as the cost of maintenance or replacement is low, and reactor downtime is limited, it is not a problem. Current estimates for ARC, which is designed to be modular, so it can be easily maintained, is that the reactor walls will last about 1 year. He doesn't even mention the biggest unsolved engineering problem, removing the "ash" (Helium) that accumulates in the plasma. His article has all the trappings of reusable rocket naysayers in the 80s and 90s, who claimed reusables could never work, because carrying wings, landing legs, heat shielding, and structural reinforcement to make rapid reusability would cut down on your usable payload, and thus doom reusable rockets to always be substantially inferior. And this is because of an obsession with a single variable, like power density, or payload fraction, and ignoring the end to end economics of the entire system, including safety margins, maintenance, footprint, etc. And that's why I believe ARC will prove Lidsky wrong.