PUREX

PUREX is the acronym for the Plutonium Uranium Recovery by EXtraction solvent extraction process, which is commonly used, in plutonium reprocessing, to separates uranium and plutonium from the fission by-products. This extraction is of the nitrate salts and is classed as a solvation mechanism. For example, the extraction of plutonium by an extraction agent (S) in a nitrate medium occurs by the following reaction.

Pu4+aq + 4NO3-aq + 2Sorganic --> [Pu(NO3)4S2]organic

A complex is formed between the metal cation, the nitrates and the tributyl phosphate, and a model compound of a dioxouranium(VI) complex with two nitrates and two triethyl phosphates has been characterised by X-ray crystalography.

When the nitric acid concentration is high, extraction into the organic phase is favoured, and when it is low, the extraction is reversed (the organic phase is stripped of the metal). It is normal to dissolve the used fuel in nitric acid; after removing the insoluble matter, the uranium and plutonium are extracted from the highly active liquor. It is normal to then back-extract the loaded organic phase to create a medium active liquor which contains mostly uranium and plutonium with only small traces of fission products. This mixture is then extracted again by tributyl phosphate (TBP)/hydrocarbon to form a new organic phase, the metal-bearing organic phase is then stripped of the metals to form an aqueous mixture of only uranium and plutonium.

TBP solution preferentially extracts uranium and plutonium nitrates, leaving fission products and other nitrates in the aqueous phase. Then, chemical conditions are adjusted so that the plutonium and uranium are reextracted into a fresh aqueous phase. Normally, two solvent extraction cycles are used for the separation; the first removes the fission products from the uranium and plutonium, while the second provides further decontamination. Uranium and plutonium are separated from one another in a similar second extraction operation.

Solvent extraction usually takes place in a pulse column, a several-inch diameter metal tube resistant to nitric acid and used to mix together the two immiscible phases (organic phase containing TBP and an aqueous phase containing U, Pu, and the fission products). The mixing is accomplished by forcing one of the phases through the other via a series of pulses with a repetition rate of 30 to 120 cycles/minute and amplitudes of 0.5 to 2.0 inches. The metal tube contains a series of perforated plates which disperses the two immiscible liquids.

The two stages of extraction improve the purity of the actinide product. The organic phase used for the first extraction will suffer a far greater dose of radiation. The radiation can degrade the tributyl phosphate into dibutyl hydrogen phosphate, which can act as an extraction agent for both the actinides and other metals such as ruthenium. The dibutyl hydrogen phosphate can make the system behave in a more complex manner as it tends to extract metals by an ion exchange mechanism (extraction favoured by low acid concentration), to reduce this effect, it is common for the used organic phase to be washed with sodium carbonate solution to remove the acidic degradation products of the tributyl phosphate.