Current materials limit reactors to around 300ºC. At that temperature steam turbines are ~30% effiency.
Supercritical coal plants can run at 550-600ºC and ~45% efficiency.

Why not "after heat" the steam of nuclear reactors to 600ºC using fossil fuel? overall eff would increase to 45%.
That would add ~450MWe for a 900MWhe reactor without new materials or fuel consumption.
The problem would be a "peg" in nuclear:fossil (natural gas/coal) consumption.

>3000 MWt (Nuclear, ~300ºC) -> 900MWe (~30%)
>+ ~3000 MWt (fossil, 300 -> 500-600ºC) eq. ~ 1350 MWe (non nuclear plant)
> total 6000MWt
> 2700 MWe combined (~45%) vs. 2250 MWe (not combined fossil 45% + nuclear 30%)