But why would a fusion reactor produce 10 times more power than a fission reactor of the same cost? A fission reactor is, comparatively, very simple. Just a steel cylinder filled with fuel and control rods. No superconducting magnets, no zillion degree plasma to contain and control, low neutron field (as long as we're comparing to D-T fusion). Also heat transfer is much more efficient, enabling high power density, since you pump coolant through the entire reactor vessel instead of just the outer edges.