Metadane

I don't know what I'm doing wrong but the NERVA fission engine keeps telling me I've run out of fuel (Enriched Uranium) but I definitely have it on my ship. It also shows two other fuels I can switch to Uranium-238 TWR or Plutonium TWR but I can't find anything that stores these. Please can someone offer some help. Edit: I've noticed that the EnrichedUranium slider doesn't come up on the NERVA engine like it does other nuclear engines so possibly a conflict, not sure though.