3 ms·

> Scale is everything for fusion (plasma). No. Q is everything in fusion. Scale is one way to improve Q in a Tokamak. The problem with improving Q by increasin

> Scale is everything for fusion (plasma). No. Q is everything in fusion. Scale is one way to improve Q in a Tokamak. The problem with improving Q by increasing scale is that it makes the reactors uneconomical. A reactor the size of ITER cannot ever be economically viable, even was capable of producing massive amounts of electricity. Luckily, scaling up is not the only way to increase Q. The better way is to use more powerful magnetic fields. Put more eloquently than I can: https://www.youtube.com/watch?v=L0KuAx1COEk&t=43m47s https://www.youtube.com/watch?v=L0KuAx1COEk&t=43m47s

infogulch 8y ago
Wow that's a really great talk, thanks for sharing!
pfdietz 8y ago
Unfortunately, even with higher magnetic fields, DT fusion is unlikely to ever be practical. That's because the power density of the reactors is inherently limited by wall loading and minimum size considerations from neutron cross sections. Look at the numbers. The power density of a PWR fission reactor core is 100 MW/m^3. If you consider the volume of the primary reactor vessel instead, it's 20 MW/m^3. The power density of ITER is 0.05 MW/m^3 (counting gross fusion power). Even if you just include the volume of the plasma, it's 0.6 MW/m^3. What about higher field concepts like MIT's ARC reactor? If you look at the paper on arxiv where details are given, the power density is 0.5 MW/m^3 -- 40 times worse than a PWR. The low power density is devastating to the economic case for fusion. Magnetic fusion reactors will be complex, expensive things, with superconducting magnets, complex cooling systems, breeding blankets, heating and control systems. They will be much more expensive per unit mass or volume than a simple PWR reactor vessel. And much of that complex system will need to be periodically replaced due to neutron damage. If the power density is 40x worse they cannot possibly compete. Yes, (SP)ARC is better than ITER. But ITER is so horribly bad that a reactor can be an order of magnitude better and still not have any real chance of competing.
- tim333 8y ago
  Well, from a 2016 article on ARC they say cost 4 to 5 billion USD, output to the grid 200MW, which is not competitive with existing reactors but that would be the first one producing power built, the designs may improve. (https://www.computerworld.com/article/3028113/sustainable-it/mit-takes-a-page-from-tony-stark-edges-closer-to-an-arc-fusion-reactor.html https://www.computerworld.com/article/3028113/sustainable-it...)
- londons_explore 8y ago
  I don't see why the number of cubic meters matters? If you give me a machine that outputs 50kilowatts, I'll happily give up a cubic meter of my house for it...
  - pfdietz 8y ago
    Because size (and complexity) directly correlate with cost. So unless fusion can enable cost reduction elsewhere, if the nuclear island is inherently larger, more complex, and hence more expensive than in a fission plant, power from it will be more expensive than from a fission plant. And in that case, why would any utility want one? New and risky (in the sense of having a significant chance of not working as well as hoped) technologies like fusion will be adopted only if they are significantly less expensive than more proven alternatives. The size and complexity also directly affect reliability. There is more to go wrong in a fusion reactor than in a fission reactor, and repairing anything there will be difficult because hands-on work will be impossible.
- pas 8y ago
  Agreed. Aneutronic fusion is the end goal and the real prize in the fusion game, and we're very very far from that. Yet with just waiting for it, we'll never get closer. And of course eventually clean (as in proper reprocessing/recycling) fission energy would be great too, but that again is stalled, largely due to nuclear armament proliferation concerns.

infogulch 8y ago

Wow that's a really great talk, thanks for sharing!

pfdietz 8y ago

Unfortunately, even with higher magnetic fields, DT fusion is unlikely to ever be practical. That's because the power density of the reactors is inherently limited by wall loading and minimum size considerations from neutron cross sections. Look at the numbers. The power density of a PWR fission reactor core is 100 MW/m^3. If you consider the volume of the primary reactor vessel instead, it's 20 MW/m^3. The power density of ITER is 0.05 MW/m^3 (counting gross fusion power). Even if you just include the volume of the plasma, it's 0.6 MW/m^3. What about higher field concepts like MIT's ARC reactor? If you look at the paper on arxiv where details are given, the power density is 0.5 MW/m^3 -- 40 times worse than a PWR. The low power density is devastating to the economic case for fusion. Magnetic fusion reactors will be complex, expensive things, with superconducting magnets, complex cooling systems, breeding blankets, heating and control systems. They will be much more expensive per unit mass or volume than a simple PWR reactor vessel. And much of that complex system will need to be periodically replaced due to neutron damage. If the power density is 40x worse they cannot possibly compete. Yes, (SP)ARC is better than ITER. But ITER is so horribly bad that a reactor can be an order of magnitude better and still not have any real chance of competing.

tim333 8y ago
Well, from a 2016 article on ARC they say cost 4 to 5 billion USD, output to the grid 200MW, which is not competitive with existing reactors but that would be the first one producing power built, the designs may improve. (https://www.computerworld.com/article/3028113/sustainable-it/mit-takes-a-page-from-tony-stark-edges-closer-to-an-arc-fusion-reactor.html https://www.computerworld.com/article/3028113/sustainable-it...)
londons_explore 8y ago
I don't see why the number of cubic meters matters? If you give me a machine that outputs 50kilowatts, I'll happily give up a cubic meter of my house for it...
- pfdietz 8y ago
  Because size (and complexity) directly correlate with cost. So unless fusion can enable cost reduction elsewhere, if the nuclear island is inherently larger, more complex, and hence more expensive than in a fission plant, power from it will be more expensive than from a fission plant. And in that case, why would any utility want one? New and risky (in the sense of having a significant chance of not working as well as hoped) technologies like fusion will be adopted only if they are significantly less expensive than more proven alternatives. The size and complexity also directly affect reliability. There is more to go wrong in a fusion reactor than in a fission reactor, and repairing anything there will be difficult because hands-on work will be impossible.
pas 8y ago
Agreed. Aneutronic fusion is the end goal and the real prize in the fusion game, and we're very very far from that. Yet with just waiting for it, we'll never get closer. And of course eventually clean (as in proper reprocessing/recycling) fission energy would be great too, but that again is stalled, largely due to nuclear armament proliferation concerns.