> The latest ARC claims from Whyte's last presentation was 3MW/m^3 density,

That's per volume of PLASMA, not volume of REACTOR. Since we're talking about the size of the things we have to pay for (the reactor) the latter is appropriate.

> Counting just the PWR reactor vessel, and not the entire containment facility, seems a huge cheat.

Fusion reactors will also require containment vessels, to prevent escape of tritium in accidents (unless you're ok with an accident rendering ground water in a large area around the reactor undrinkable for decades.) The containment vessel will have to be large enough to contain all the cryogens heated to accident temperature. Unlike in a PWR like the AP1000, these volatiles cannot be condensed by water cooling. In ITER, for example, complete volatilization of the cryogens would requires several times the volume of the AP1000 containment building.

The ARC design adds to accident fun by having a large volume of titanium hydride shielding near the molten salt. TiH2 decomposes to Ti and hydrogen gas at 450 C, well below the temperature of the molten salt blanket. Fully decomposed, this TiH2 would release 15 tonnes of hydrogen, occupying 180,000 cubic meters at STP.

Beyond that, the containment building around a fusion reactor will have to mitigate tritium leakage by many orders of magnitude. This will be expensive (in particular, polymer seals cannot be used on penetrations as they permeate tritium.)

ARC will also require a large volume for disassembly of the reactor for vacuum vessel replacement, and for remote control demolition of activated vacuum vessels for disposal. All that volume will become contaminated and off limits to human access.

> PWR Fission reactors have enormous footprints, averaging 1 square mile. An ARC reactor wouldn't need anywhere need that size.

Really good farmland in the US is less than $10K/acre, or about $6M/square mile. Cheap range land is an order of magnitude less; really useless land another order of magnitude cheaper. The cost of land there is a very minor part (0.1%) of the cost of the power plant.

https://www.howmuchisit.org/acre-of-land-cost/

> No, he makes lots of assumptions about how the magnets are shielded, how breeding works, and how heat transfer works.

Again, you have not understood his argument. His argument would apply even if you had 100T magnets made of unobtainium and a plasma configuration with beta = 1. All that he requires is an upper limit on how much energy per area can be transmitted through the surface of the reactor. This is a function of the materials of the first wall, and has absolutely nothing to do with magnets or breeding.

> Predicted costs to build a 200MW ARC reactor are $4-5 billion

If you are referring to the cost estimate from the arxiv paper, that is just the estimated cost of the reactor itself, not the power plant containing the reactor. I will admit I found the estimation methodology in that paper very cursory, so it's possible they were overestimating the cost (80% of the cost was the stainless steel magnet support structure.)

A fission reactor core is much cheaper than that estimate, per MW.

> He doesn't even mention the biggest unsolved engineering problem, removing the "ash" (Helium) that accumulates in the plasma.

And why should he? I mean, why does he have to kill the corpse a third time (after the power density and reliability arguments)?

> And that's why I believe ARC will prove Lidsky wrong.

As I said earlier, the power density figures from ARC show he was right.