You have "Uranium Hexafluoride" in Reactor Control Window as fuel mode.
However, reactor itself has Uranium Nitride storage and still works. That seem quite confusing.

Tested in 1.12.19 on KSP 1.2.2 with KER and VesselMover

yes, it one of those lose end I never fully implemented. Thanks for reminding me.

it can now use any nuclear fuel but required UF at startup