Purex uranium-plutonium partitioning

Anyun, Z., Jingxin, H., Xianye, Z., Fangding, W. 2001. Hydroxylamine derivatives in PUREX process, VI. Study on the partitioning of uranium/neptunium and uranium/ plutonium with N,N-diethylhydroxylamine in the purification cycle of uranium contactor. Solvent Extr. IonExch. 19 (6) 965-979. 

In the Purex process, plutonium and uranium are coextracted into an organic phase and partitioned by reducing plutonium(IV) to the aqueous-favoring plutonium(III). This has been achieved chemically by use of a suitable reductant such as ferrous sulfamate ( 1) or uranium(IV). (2, 3, 4, 5) The use of ferrous sulfamate results in accelerated corrosion of the stainless steel, due to the presence of ferric ions and sulfuric acid, and in an increase in the volume of wastes. The use of natural uranium(IV) can cause dilution of the 235U in slightly enriched uranium, thus lowering the value of the recovered uranium. 

Figure 10.29 shows the principal steps in applying the Purex process to irradiated LMFBR fuel, step 7 of Fig. 10.28. The flow scheme and the compositions and locations of solvent, scrubbing, and stripping streams have been taken from the process flow sheet of a 1978 Oak Ridge report [Oil] describing a planned experimental reprocessing facility designed for 0.5 MT of uranium-plutonium fuel or 0.2 MT of uranium-plutonium-thoiium fuel per day. As that report gave process flow rates only for the uranium-plutonium-thorium fuel. Fig. 10.29 does not give flow rates for the uranium-plutonium fuel of present interest. This flow sheet shows the codecontamination step, in which flssion products are separated from uranium and plutonium the partitioning step, which produces an aqueous stream of partially decontaminated...

Pandey, N.K. Koganti, S.B. Simulation of electro-mixer-settler for the partitioning of uranium and plutonium in PUREX process, Ind. J. Chem. Technol. 11 (2004) 535-547. 

The control of the actinide metal ion valence state plays a pivotal role in the separation and purification of uranium and plutonium during the processing of spent nuclear fuel. Most commercial plants use the plutonium-uranium reduction extraction process (PUREX) [58], wherein spent fuel rods are initially dissolved in nitric acid. The dissolved U and Pu are subsequently extracted from the nitric solution into a non-aqueous phase of tributyl phosphate (TBP) dissolved in an inert hydrocarbon diluent such as dodecane or odourless kerosene (OK). The organic phase is then subjected to solvent extraction techniques to partition the U from the Pu, the extractability of the ions into the TBP/OK phase being strongly dependent upon the valence state of the actinide in question. 

The Purex Process has been used either with "early" or with "late" Pu-U partitioning (Fig. 1). The early split in the first cycle avoids the co-stripping of uranium and plutonium which may result in higher plutonium losses. In other flowsheets, the partitioning is delayed until the second cycle. While no particular trend has been obvious, some of the recent flowsheets in which the plutonium is reduced prior to re-extraction (10,11) will necessarily require a second cycle partitioning. 

Ferrous sulfamate has been the reductant for plutonium during partitioning of uranium and plutonium in the Purex process at SRP since startup. In recent years, a desire to reduce waste volumes has led to studies of alternative reductants or combinations of FeSA with other reductants. The FeSA in the Pu strip solution produces Fe(OH) 3 and Na2S0i in neutralized waste these compounds account for a large percentage of the solid material in Purex low activity waste. In an effort to reduce these wastes, we investigated HAN as a substitute for part or all of the FeSA in the Purex first cycle. 

Hanfoid [D3]. Nitrite concentration in feed to the HA column of a standard Purex plant was adjusted to route most of the neptunium in inadiated natural uranium into the extract from the HS scrubbing column. Sufficient ferrous sulfamate was used in the partitioning column to reduce neptunium to Np(IV), which followed uranium. This neptunium was separated from uranium by fractional extraction with TBP in the second uranium cycle. The dilute neptunium product was recycled to HA column feed, to build up its concentration. Periodically, irradiated uranium feed was replaced by unirradiated uranium, which flushed plutonium and fission products from the system. The impure neptunium remaining was concentrated and purified by solvent extraction and ion exchange. 

Process selection. The processes just described recovered neptunium only partially and in variable yield because of the difficulty in controlling the distribution of neptunium valence between 5 and 6 in the primary extraction step with nitrite-catalyzed HNO3 and the incomplete reduction of neptunium from valence 5 to 4 in the partitioning step with feirous ion. This section describes a modified Purex process that could be used if more complete recovery of neptunium were required. It is based on process design studies by Tajik [Tl]. The principal process steps are shown in the material flow sheet Fig. 10.32. In the primary decontamination step, pentavalent vanadium oxidizes neptunium to the extractable hexavalent state. In the partitioning step, tetravalent uranium reduces plutonium to the inextractable trivalent state while converting neptunium to the still-extractable tetravalent state. 

A variation of the PUREX process is being proposed by the US Department of Energy as a possible alternative partitioning scheme for the transmutation of wastes. This aqueous process called UREX, only removes uranium from spent commercial LWR fuel and leaves plutonium in the HLW stream with the other minor actinides and fission products. Nonaqueous pyroprocessing, a variation of the ANL electrorefining process, is then proposed to be used to separate both the plutonium and minor actinides so that they can be transmuted in an ADS. For further information related to potential modifications of this process for other accelerator transmutation of waste applications, see ANL-99/15 (1999). 

Once plutonium and uranium are coextracted and codecontaminated, plutonium is separated from uranium in the partitioning contactor by reduction to Pu(III) with a reduc-tant. Over the years, a number of plutonium reductants have been proposed. The most widely used reductant to partition plutonium from uranium in the PUREX process was (Fe(S03NH2)2) other alternates were proposed such as hydrazine-stabilized ferrous nitrate or uranous nitrate, and hydroxylamine salts. 

Homer, D.E. 1969. The use of ferrous nitrate as a plutonium reductant for partitioning plutonium and uranium in PUREX processes. Report ORNL-4383. Oak Ridge, TN Oak Ridge National Laboratory. 

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