I know that, because we already got ITER, which is widely expected to be energy positive. Moreover, the ITER approach is known to scale upwards: in fact, the ITER is so large, because it's the smallest size that is expected to work.
ITER is expected to be energy positive, but it's a kind of pyrrhic breakeven. The gross power density of ITER will be 1/400th power density of a PWR reactor vessel.
