ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets

> Their problem is all the rare earths The fraction of mass in a HTS magnet that is rare earth is surprisingly small. Most of the mass is mechanical support, or substrate on which the thin HTS film is deposited. The entire ARC reactor might contain 10 kilograms of REBCO (2015 paper), out of a reactor mass of over 7 gigagrams. https://arxiv.org/pdf/1409.3540

You can construct any narrative you want if you have selective perception and decide to not actually research or learn anything. https://arxiv.org/pdf/1409.3540v1.pdf

SPARC will not generate electricity[0]. It's intended to show that the ARC concept, which should be able to generate net electricity[1], is viable. If SPARC works, ARC might be built as early as 2030. 'As early as' being the keywords here. Showing that the difficult physics work is important to do before moving on to the difficult engineering. [0]https://www.psfc.mit.edu/sparc/faq [1]https://arxiv.org/pdf/1409.3540.pdf

> That weight figure (7190 tonnes, not 4000) for ARC is just for the reactor. https://arxiv.org/pdf/1409.3540.pdf (page 30) Thanks. As I hadn't seen that paper, I used the Fletcher class destroyer, which was the most numerous class the US built, and it was slightly on the larger side at about 2000 tons; common destroyers were around 1500 tons IIRC. I also assumed, wrongly it seems, that 500 MW was the electrical power. Anyways, if we're really interested in material requirements for various power plant types, one can check e.g. the DOE quadrennial energy review 2015, Table 10.4 on page 390. One can see that fission does pretty well, about an order of magnitude better than wind (and solar being somewhat worse than wind). I would guess fusion wouldn't do as well as fission, but it shouldn't be a huge difference either considering that except the reactor itself it should be pretty similar.

The arxiv paper describing the design concept is here: https://arxiv.org/abs/1409.3540 Some additional features (besides the magnets) that jump out at me: The reactor vessel has joints to allow it to open up for maintenance access (previous tokamak designs have a reactor vessel that cannot be opened up once constructed, so maintenance inside has to be done through small access ports using remote manipulators). The external current drive for heating the plasma to ignition looks like it is more efficient than previous designs. The blanket around the reactor vessel is liquid instead of solid, using a fluorine-lithium-beryllium compound.

The ARC paper proposes that the lithium in the salt should be enriched to 90% lithium 6 to reach a tritium breeding ratio of 1.1. See section 5.3, "Tritium breeding." https://arxiv.org/pdf/1409.3540.pdf I presume that the breeding ratio would fall below 1.0 with natural lithium.

I'm finding it difficult to Google how efficiency scales with magnetic flux. I've ended up at a thirty five page pdf https://arxiv.org/pdf/1409.3540. Equation 1 on page 5 states the fourth power dependency. So using REBCO magnets, instead of Niobium-Tin, to get twice the magnetic field looks like a really big deal. Page 6 adds a twist to the story. "For REBCO superconductors operating far below their critical temperatures, the toroidal field is generally limited by mechancical stress rather than the critical current density, which typically limits standard Nb3Sn." So further increases in magnetic field might be waiting on advances in mechanical engineering design, not on better superconductors.

ARC has a mass of 7,190 tonnes (not including generators or radiators). This is about the mass of three US WW2 destroyers. It's not room sized; the thing is something like 20m tall (once you include the support structure). https://arxiv.org/pdf/1409.3540.pdf A 190MW(e) fission reactor would be considerably smaller. There is really no use case for DT fusion in space, especially if they have to make their own tritium.

The ARC design (see https://arxiv.org/abs/1409.3540 for details) uses surprisingly little yttrium. The superconductor is only about 1% of the tapes, and the tapes are only a faction of the coils, and the coils mass is just a fraction of the mass of the coil support structure. I think the total amount of yttrium will be in the tens of kilograms, if that. I have a suspicion that this is being funded at all because the magnet technology would be useful in non-fusion contexts (hybrid electric aircraft, superconducting generators in wind turbines.) In contrast, IIRC a single ARC reactor of that design would have 90 tonnes of beryllium (although that could be reduced by half if the secondary loop used a different molten salt.)

That weight figure (7190 tonnes, not 4000) for ARC is just for the reactor. https://arxiv.org/pdf/1409.3540.pdf (page 30) The net output of ARC in that design is 190 MW(e), which comes to 26 kW(e)/tonne. It doesn't include anything outside the reactor, which would likely increase the weight by at least an order of magnitude. It doesn't include (for example) the cryo plant, tritium processing, heat exchangers, turbines, cooling system, the BUILDING, and facilities for installation and disposal of vacuum vessels.

The volume is the volume of the reactor, including blanket, magnets, and the structure needed to support the JxB forces on the magnets, not the volume of the plasma. See table 11, page 30, in the ARC paper: https://arxiv.org/pdf/1409.3540.pdf For PWRs: https://ocw.mit.edu/courses/nuclear-engineering/22-06-engine... (take the dimensions given for the primary reactor vessel, compute the volume as a cylinder with spherical end caps, and divide that into 3400 MW(th). The result is slightly below 20MW/m^3. Note also the power density of the core itself is given as greater than 100 MW/m^3.)

According to the 2014 arxiv.org paper on the ARC reactor https://arxiv.org/pdf/1409.3540.pdf the concept (which would produce 190MW(e)) uses 90 tonnes of beryllium. According to the USGS, the current total world annual production of Be is 220 tonnes, and total global resource is estimated at 100,000 tonnes. The world uses ~20 TW of primary energy, so roughly 100,000 ARC reactors would be needed to supply the world, using about 100x as much Be as the estimated amount available for mining. Unless Be supplies can be drastically increased, this concept is at best a niche player.

According to the 2014 arxiv.org paper on the ARC reactor https://arxiv.org/pdf/1409.3540.pdf the concept (which would produce 190MW(e)) uses 90 tonnes of beryllium. According to the USGS, the current total world annual production of Be is 220 tonnes, and total global resource is estimated at 100,000 tonnes. The world uses ~20 TW of primary energy, so roughly 100,000 ARC reactors would be needed to supply the world, using about 100x as much Be as the estimated amount available for mining. Unless Be supplies can be drastically increased, this concept is at best a niche player.

Here's the paper (no paywall): http://arxiv.org/abs/1409.3540 Interesting that the liquid blanket is the same as in the molten-salt thorium reactors (lithium beryllium fluoride (FLiBe)).