Can you expand on the issue you see with throttling nuclear units?
As far as steam plants go, they are (?) designed for maximum efficiency at full power, so running throttled would seem to be less efficient (in other words, more Btu/kw-hr, or wasted fuel).
For nuclear units, the fuel cost is almost insignificant, so a loss in steam cycle efficiency doesn't translate to higher cost for the power. At least not as much as it does for a coal or nat gas fired plant where the operating cost is almost entirely due to the fuel cost.