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What defines the volume to get to that number? From what I can tell from the ARC specs the plasma volume is 141 m3, and the expected power output is 200-300 MWe, which makes it 1.5-2 MWe/m3.

Do you have some numbers on a PWR reactor vessel? I was trying to look up some details on the APR-1400, but could not find any.

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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.)

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