Game says Kovarex has been researched but the recipe is missing and cannot be selected.

Wonder if the nuclear fuel mod does not properly re-enable Kovarex when the mod is uninstalled?

Update: I used console commands to "un-research the technology" then to enable the technology again and now Kovarex is available.

yup. mod removes kovarex. which is bad, because mods that add kovarex enrichment as a technology prerequisite become impossible. Vanilla techs SHOULD NOT be removed or renamed, as currently, there is no method in game or by mod to ensure that a "invalid" prerequisite pointer for a tech is unresolved at runtime. I've written to the devs about this, suggesting they update logic so that, if a technology has a prerequisite for a tech that has been removed in data updates or data-final-fixes declared,that technology-prerequisite is IGNORED, but they don't seem to think that's too important. >.>

fixed by commenting out first line of technology.lua in mod's prototypes folder. >.>

This is still a problem. I have yet to find a proper solution to fix my save.

I had this problem too, then I found the solution right here on the information tab of this mod.
"Uninstallation: Re-enable Kovarex process in the mod startup options, Load and save your game, Remove this mod"
It worked for me.