[pfdietz]

Or volumetric power density, or simply the cost of the non-nuclear part of the power plant.

But of course no one has solved the neutron damage issue. How could they? To develop a material, if that's even possible, that could withstand 14 MeV neutrons well enough, one would need a fusion reactor to make those neutrons to test the materials (and tritium breeding to keep it running). This circular dependency would be very difficult to traverse.