Depleted Coaxium Fuel Rods appear after the durability of a Coaxium Fuel Rod in a Nuclear Reactor reaches 0.
Depleted Coaxium Fuel Rods are radioactive. A HAZMAT suit is needed to handle it, or you might die of radiation poisoning.
Run a Coaxium Fuel Rod through a Nuclear Reactor until the durability reaches 0.
Place it in a Thermal Centrifuge. You get 3/4 of the Iridium (as Iridium Dust) and Dense Diamond Plates (as Diamond Blocks) back, as well as a Semi-Processed Coaxium Fuel Rod. See it's page for more info.