Fluids for Milk and Magnesium have no texture in the world.

This is a config setting in NuclearCraft. You can turn the textures back on if you like - the pack is small enough that it won't cause problems. hit Mod Options, and then the Nuclearcraft mod/Config/Other Configs and find Register Fission Fluid Blocks. You may have to close out to the main menu and then load your world again.