The Nuclear Boiler is used to process Uranium Ore or Yellowcake to produce Uranium Hexafluoride, requiring both power and Hydrogen Fluoride. The input for Hydrogen Fluoride (or water for other recipes) is on the side which has a blue port. You can also pump buckets into it.
If placed next to a powered Centrifuge, Uranium Hexafluoride will be automatically loaded into it and processed into Uranium-235 and Uranium-238. For this to work however you need to connect the two green inputs/outputs. These are found on the sides of the machines.