Plutonium(IV) precipitates formed in alkaline media in the presence of various anions

Abstract

SummaryThis investigation was undertaken to obtain new data on the composition and properties of . plutonium(IV) compounds formed under diverse conditions in strongly alkaline media in the presence of various anions. Such information is important to understand P u o behavior and the forms of its existence in the alkaline sludges of Hanford Site radioactive waste tanks. The knowledge then may be applied to assess plutonium disposition in the 'storage, retrieval, and treatment of Hanford Site tank wastes with respect to its criticality hazards and contribution to the transuranic waste inventory.In all studied cases, P u o precipitates from 0.2 to 10 M NaOH solutions at 10 to 200°C to form nearly amorphous compounds of general composition Pu02'xH20. Extended aging of the compounds does not produce well-crystallized phases. Aging was accelerated by coagulation at elevated temperatures, including hydrothermal conditions. The number of water molecules, x, associated with the Pu02 solid depends strongly on the PuO~.xH20 drying conditions but is not sensitive to the precipitation method (direct or reverse) or the coagulation time. For PuO~.xH20 solids prepared at 10 to 60°C and dried at 20°C in desiccators over KOH pellets (1.7 torr H20 vapor pressure) or over 25% H2SO4 (14.6 torr H20 water vapor pressure), mean values of x are 1 . 6 and 2.8, respectively. The large difference in these values confims the strongly hygroscopic properties of h02.xH20. The x values decrease with increasing temperature. ' The composition of PuO~xH20 obtained at 180 to 200°C and dried over KOH has an average x of 0.60. Thermogravimetric scans of various PuO~.xH20 samples were similar regardless of compound preparation conditions. In all cases, the mass loss occurred monotonously, in one step, in the range 50 to 250°C. All differential thermal analyses showed mildly endothermic peaks at about 110°C. This confirmed that the water in PuOrxH20 does not have discrete states but has a continuous range of bonding energy.Anhydrous P u 0 2 , produced by heating Pu02'xH20, is strongly hygroscopic. The hygroscopicity remained even after prolonged heating at 500"C, disappearing only at 800°C. The minor influence of temperature on Pu02'xH20 hygroscopicity can be explained by the rather high thermal stability of the primary crystallites. This supposition was confirmed by direct estimates of PuOrxH2O crystallite size by an X-ray powder diffraction method. It was found that PuOn.xH20 crystallite size increases only from about 2.5 to 7 nm in the range 20 to 800°C. * Infrared (IR) spectra of PuO~.xH20 solids prepared under different conditions closely resemble each other and show an intense band with maximum at 3400 cm-', three weak bands in the range 1700 to 1250 cm-', and an additional strong and generally split band between 600 and 350 cm-'. The band at 3400 cm-', arising from valent water vibrations in the PuO~.xH20 structure, is relatively wide and smooth and without shoulder. This observation confirms the assumption that water molecules have no discrete...

Effects of aging on PuO2·xH2O particle size in alkaline solution

“…Overall, the XRD measurements in the present study, the published laboratory findings [12][13][14][15][16][17], and theoretical predictions based on the solubility products of the nascent PuO 2 ·xH 2 O and fully crystalline PuO 2 as interpreted by work of Schindler [18] concur that the PuO 2 ·xH 2 O birth particles produced by precipitation from solution are about 2-5 nm in diameter.…”

“…The PuO 2 ·xH 2 O particle size formed by plutonium nitrate salt precipitation by alkaline and neutral solution has been measured in several previous studies [12][13][14][15][16][17]. Work done in the Manhattan Project showed that PuO 2 ·xH 2 O solids precipitated from acidic Pu(IV) nitrate solution with excess ammonium hydroxide solution and then air-dried at 70 • C had 3-nm particle size as measured by XRD reflection line broadening [12].…”

Effects Influencing Plutonium-Absorber Interactions and Distributions in Routine and Upset Waste Treatment Plant Operations

“…However, precipitated plutonium silicate deposits might occur (Section 5.3.1 and 6.2 of Delegard and Jones (2015)). Increased dehydration also occurs for PuO 2 •xH 2 O as hydrothermal aging temperature increases at least over the range of 10 to 200° C (Krot et al 1998c;Yusov et al 2000a). The differential dissolution of plutonium and solids of the absorber elements from such thermally aged materials is not known, however, except to the extent that tank wastes also have been thermally aged by decades-long storage at temperatures that sometimes have exceeded boiling.…”

“…The separation of plutonium from iron-bearing solids (amorphous ferric hydroxide and goethite, FeOOH) by leaching with alkaline EDTA solutions was studied by Krot et al (1998c). They examined the physical characteristics of the bulk precipitates and the susceptibilities of the precipitated products to leaching by use of 0.1 M EDTA in 1 M NaOH containing 5 M NaNO 3 .…”

Criticality Safety Evaluation of Hanford Tank Farms Facility

“…Anions (Krot et al 1998); (Fedoseev et al 1998); (Peretrukin et al 1998). (Vail 1997b), which documents the evaluation of transferring waste from DST C-106 to DST AY-102, provides a good review of the chemical processes that ensure criticality safety in waste disturbing operations.…”