What is the name of the standard chemical process used at a recycling plant to separate useful plutonium and uranium from the rest of the waste in used nuclear fuel?

The standard chemical process used at a recycling plant to separate useful plutonium and uranium from the rest of the waste in used nuclear fuel is called PUREX. PUREX stands for Plutonium Uranium Redox Extraction. This process is the most widely adopted method for nuclear fuel reprocessing, which is the chemical separation of spent nuclear fuel into its constituent elements. Used nuclear fuel refers to the depleted fuel rods removed from a nuclear reactor after energy production, containing residual uranium, newly formed plutonium, and highly radioactive fission products. The PUREX process begins by mechanically shearing the used fuel rods into small pieces and then dissolving them in hot nitric acid. This creates an aqueous solution, meaning a water-based solution, containing various dissolved metal nitrates, including uranium nitrate, plutonium nitrate, and the nitrates of radioactive fission products. The core of PUREX is a technique known as solvent extraction, which is a chemical method for separating compounds based on their differing solubilities between two immiscible liquids—liquids that do not mix, such as oil and water. In this specific application, the aqueous nitric acid solution containing the dissolved fuel is intimately mixed with an organic solvent. This organic solvent typically consists of tributyl phosphate (TBP) diluted in an inert hydrocarbon, such as kerosene. During the mixing, the uranium and plutonium selectively transfer from the aqueous phase into the organic solvent phase. This preferential transfer occurs because TBP forms stable chemical complexes with uranium and plutonium nitrates, making them significantly more soluble in the organic solvent than in the aqueous solution. Most of the highly radioactive fission products and other unwanted elements, however, remain in the aqueous phase, thereby achieving a separation from the valuable uranium and plutonium. Following this initial separation, the uranium and plutonium are then selectively stripped, or extracted back, from the organic solvent into different aqueous solutions. This is accomplished by carefully adjusting the chemical conditions, such as the acidity or by introducing specific reducing agents for plutonium. For instance, plutonium can be reduced to a lower oxidation state, which makes it less soluble in the organic solvent and causes it to transfer back into a new aqueous phase, separating it from the uranium. This multi-stage process of dissolution, solvent extraction, and selective stripping allows for the recovery of high-purity uranium and plutonium. The recovered uranium and plutonium are valuable fissile materials, meaning they are capable of sustaining a nuclear chain reaction, and can be reused in new nuclear fuel, such as mixed-oxide (MOX) fuel. Reprocessing through PUREX reduces the volume and long-term radiotoxicity of high-level nuclear waste.