Neptunium partitioning

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

Smith, K. L Blackford, M. G Lumpkin, G. R., Hart, K. P. Robinson, B. J. 1996. Neptunium-doped Synroc partitioning, leach data and secondary phase development. In Murphy, W. M. Knecht, D. A. (eds) Scientific Basis for Nuclear Waste Management XIX. Materials Research Society Symposium Proceedings, 412, 313-319. 

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. 

Krupka K. M. and Seme R. J. (2000) Understanding Variation in Partition Coefficient, Kd, Values, Volume III Review of Geochemistry and available Kd values for Americium, Arsenic, Curium, Iodine, Neptunium, Radium, and Technetium. Pacific Northwest National Laboratory, Richland, WA. 

A laboratory study was undertaken to determine the behaviour of neptunium in the WAK flowsheet, and to devise a procedure for its recovery. Based on static ( ) and counter-current experiments (J5), the conclusion was reached that about half of the Np is co-extracted with the U and Pu in the HA-HS mixer-settlers of WAK while the other half is rejected to the HAW, see Fig.1. It could also be shown that an increase of the aqueous acidity, or the addition of pentavalent vanadium as an oxidant into the lower stages of the HA mixer-settler (6), would increase the Np yield in the organic solvent. In the 1BX-1BS mixer-settlers where the partitioning of U and Pu is carried out by use of uranium (IV)nitrate - hydrazine nitrate, a splitting of the coextracted Np between the two product streams was observed the proportions of the (co-extracted) Np which ended up in the 1CU (uranium product) stream fluctuated from 30 to 93 % while the difference amount (from 7 to 70 %) ended up in the 1 BP (plutonium product) stream. 

Neptunium and plutonium are partitioned by reducing Pu(IV) or Pu(VI) to inextractable Pu(III) neptunium is simultaneously reduced to Np(IV). Neptunium is kept in the organic phase by adjusting the acid in the aqueous strip solution (1BX) and the organic-to-aqueous flow ratio to maintain the extraction factor of neptunium greater than one. 

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. 

Partitioning of plutonium. Evaporator product is made 2 M in nitric acid and extracted with four volumes of 30 v/o TBP in the plutonium partitioning unit. This leaves plutonium in the aqueous raffinate and extracts the uranium and neptunium. 

Partitioning of neptunium. Uranium and tetravalent neptunium in the extract are separated by fractional extraction with 0.5 M HNO3. The less extractable Np(IV) is returned to the aqueous phase while uranium remains in the solvent, from which it can be stripped with 0.01 AfHNOa (not shown). 

Chemical separation. Current concepts for high-efficiency separation of actinides call for improved plutonium recovery, coextraction of uranium and neptunium with subsequent partitioning by valence control, and extraction of amercium and curium from the HAW stream. There are a number of major problems to be solved before a technically feasible process will be available. 

Another benefit for the SFR fuel cycle system is the reduction of environmental burden by recycling all actinide nuclides and partitioning selected fission products (FPs). The spent fuel contains minor actinides (MAs ie, neptunium, americium, curium, etc.) as well as uranimn and plutonium. In the conventional nuclear fuel cycle, those MAs and FPs are disposed of in a deep geological repository as high-level radioactive wastes. Because of the long-lived radioactive MAs such as Am (half-life 433 years) and Np (half-life 2.1 million years), it takes several hundred thousand years to reduce the radiotoxicity of high-level radioactive waste to the level of natural uranium. 

Sometimes, the distribution ratio is referred to as the partition coefficient, which is often expressed as the logarithm. Note that a distribution ratio for uranium and neptunium between two inorganic solids (zirconolite and perovskite) has been reported. In solvent extraction, two immiscible liquids are shaken together. The more polar solutes dissolve preferentially in the more polar solvent, and the less polar solutes in the less polar solvent. In this experiment, the nonpolar halogens preferentially dissolve in the nonpolar mineral oil. 

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