Plutonium processing Polymers

Experiences with aqueous chemistry and behavior of the transuranium elements obtained in nuclear fuel reprocessing and plutonium processing are only of limited relevance for PWR primary coolants with the extremely low concentrations of these elements in a boric acid—LiOH solution of varying composition. The plutonium polymers which are formed in less acid and neutral solutions and which have been reported to show the highest plate-out potential (e. g. Wilkins and Wisbey,... 

At the end of the above reaction sequence Pu(OH)4 precipitates out - Katz and Seaborg (11) have calculated the solubility product as 7 x 10-56. Polymer formation is a rapid process when the Pu4+ solution is adjusted to contain 0.4 x 10 4m to 1.2 x 10 2m in 0.1m HN03,40% of the plutonium polymerised within the first 30 minutes and within 60 minutes 55 % had polymerised. 

Uranous nitrate [U(N03)4] solution is used for the quantitative reduction of plutonium from loaded tributyl phosphate (TBP) phase [8]. Membrane cell technology was investigated for the production of 100% uranous nitrate solution [9], which is to be used in the partition cycle of the PUREX process in the fuel reprocessing plant. The membranes used hitherto have suffered from mechanical instability. A study was carried out at the BARC to obtain 100% uranous nitrate solution using a membrane-based electrolytic cell. The membrane used in this study was a thin polymer film reinforced with a Teflon fabric. The film was used as a separator between the anolyte and catholyte chambers, which are made of perfluorinated polymers, thus offering high thermal and chemical stability. 

Irradiation also affects the course of more conventional separation processes. Visible and ultraviolet light have been found to affect plutonium solvent extraction by photochemical reduction of the plutonium (12). Although the results vary somewhat with the conditions, generally plutonium(VI) can be reduced to pluto-nium(IV), and plutonium(IV) to plutonium(III). The reduction appears to take place more readily if the uranyl ion is also present, possibly as a result of photochemical reduction of the uranyl ion and subsequent reduction of plutonium by uranium(IV). Light has also been found to break up the unextractable plutonium polymer that forms in solvent extraction systems (7b,c). The effect of vibrational excitation resulting from infrared laser irradiation has been studied for a number of heterogeneous processes, including solvent extraction (13). 

The presence of DTPA should prevent the formation of plutonium polymer during the transition from pH = 9 to less than pH = 0.3. Above 1 M in hydrogen ion concentration, H5DTPA is no longer very effective as a chelating agent and thus does not affect the Dfs of actinides in the ARALEX process. In addition, DTPA in the Na2C03 scrub also prevents the formation of plutonium hydroxide when macro concentrations of plutonium are present. 

The tendency toward Pu(IV) polymerization is of considerable practical importance in process operations involving plutonium solutions. Dilution of an acidic plutonium solution with water can result in polymerization in localized regions of low acidity, so plutonium solutions should be diluted instead vdth acid solutions. Polymerization can result from leaks of steam or water into plutonium solutions or by overheating during evaporation. Polymer formation can clog transfer lines, interfere with ion-exchange separations, cause emulsification in solvent extraction and excessive foaming in evaporation, and can result in localized accumulation of plutonium that may create a criticality hazard [CS]. 

Plutonium polymer. At low acidity and high temperature, plutonium forms a polymer that deposits as an insoluble solid film on the walls of process equipment. Polymer deposition plugs lines, fouls surfaces, and may result in unanticipated accumulation of a critical mass of plutonium. Figure 10.38 summarizes [M3] the results of investigations of the combinations of low acidity and high temperature that must be avoided if plutonium polymer formation is to be prevented. 

As an additional precaution, process equipment in which plutonium polymer might form should be soaked periodically in boiling, concentrated nitric acid. If plutonium is found in solution, the presence of a polymer deposit is indicated. Complete removal may require addition of 0.01 to 0.1 MHF to the hot HNO3. 

The hydrolysis of Pu(IV) can result in the formation of polymers which are rather intractable to reversal to simpler species. This has led often to incorrect conclusions about the nature of the plutonium species present and the validity of their equilibrium constants. The kinetics of depolymerization take a different course as the polymer ages such that while freshly prepared hydroxides are easily decomposed, aged polymers require quite rigorous conditions. A reasonable model for this process involves initial formation of aggregates with OH bridging which dehydrate with aging (Choppin 1983). 

On account of its large practical importance, polymer formation in hydrolyzed plutonium(iv) solutions has attracted much interest [203]. This polymer is formed fairly rapidly [204,205]. The reaction is faster, and more extensive, the higher the temperature. As long as ionic plutonium(iv) is present in detectable amounts, the rate of polymerization is proportional to the concentration of this component, and inversely proportional to the square of the acidity. When the ionic plutonium(iv) has been consumed, the rate depends in a rather complicated manner upon the concentration of other oxidation states present [205]. If the polymer is allowed to age, depolymerization becomes very slow even if the concentration of acid is fairly high [204]. The colloid behaves very differently from ionic plutonium(iv) in the extraction and ion-exchange procedures used in the processing of plutonium, and is also apt to transform into a precipitate. The conditions should therefore be chosen so that the formation of the colloid is... 

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