A couple of things I might try to fault find...
After saving!
If you have a save when you had the power plant, you could use the console to get its ID and then load up a save after you got rid of it and enter 'PRID {ID number}' to see if the reference is still there
Maybe use
R2K's Gameplay Mods - 0005. Power Grid Tools 1.4
In console you can check the power grid using 'cpg' which will tell you if there are any errors, and how many power grids.
'cpg 1' will fix any errors (but tends to need you to redo settlement power lines)
you could also use 'pg 1' to get a list of all the power objects on power grid 1 to look for the reactor, (and increase the last number to look at any further power grids
If you can identify the reactors ID you can PRID its ID and try disable or markfordelete (need to travel away and reload the settlement)
A rather long method if no luck from above... (definitely not the quick and easy fix you asked for)
You could try scrapping the settlement at the workbench (under manage options)
and see if the power count is still there.
If it goes away reload a save with what you have built, go to the workbench manage options, and export a layout.
Use the log file and generate a
Layout esl
Put it in you data folder and enable the mod
In game load a save (remove stuff you have stored in any settlement container to be safe) go to the workbench and scrap the settlement, then import the layout you made.
Hopefully it will be right.
If not, you could open the layout export file in a text editor and search for fusion to see if it has an entry. Delete that entire line from the log file and then create a layout on the web page, before enabling and continuing as described above.
You can disable the layout esl once its built (you can't if it contains a SCOL but those are for things edited in the Creation Kit - like RotC city plans)