Uranium dioxide dissolution

Both humic acids and fulvic acids have a strong affinity for particulate and crystalline substances possessing oxygen atoms at their surfaces and they have been reported to bring about the dissolution of iron phosphate, calcium phosphate (61), uranium dioxide (65), hydrated magnesium alumino-silicates (66) and limonite, a complex mixture of hydrated ferric oxides (67). 

Since the water movement will be very slow compared with the rate at which the wastes dissolve, we are concerned first and foremost with equilibrium solubility. Also, if only to relate behaviour on the geological time scale to that on the laboratory time scale, we will need to know about the mechanisms and kinetics of dissolution and leaching. The waste forms envisaged at present are glass blocks containing separated fission products and residual actinides fused into the glass and, alternatively, the uranium dioxide matrix of the used fuel containing unseparated fission products and plutonium. In the... 

Oversby, V. M. 1999. Uranium Dioxide, SIMFUEL, and Spent Fuel Dissolution Rates - A Review of Published Data. Swedish Nuclear Fuel and Waste Management Co., Stockholm, Sweden, TR-99-22. 

De Pablo J, Casas I, Clarens F, El Aamrani F, Rovira M. (2001) The effect of hydrogen peroxide concenttation on the oxidative dissolution of unirradiated uranium dioxide. Mater Res Soc Symp Proc 663 409-416. 

The product of this reaction appeared to have smaller particles than the original uranium dioxide, which settled rapidly to the bottom of the reaction tube. These particles were recovered by aqueous dissolution of the solidified eutectic and filtration of the insoluble uranium-containing species. No hydrolysis of the insoluble uranium-containing species is believed to occur with aqueous dissolution of the salt matrix. To preclude hydrolysis in experiments where it was more likely, the nitrate salts were dissolved in 0.5 M HNO3. 

A second test was conducted under the above conditions. Reaction of the uranium dioxide was assumed to be complete when gas evolution ceased. At that point, the temperature of the melt was reduced to 200°C, and nitric acid vapor was added to the melt. The nitric acid vapor was carried from a heated vessel containing 100% nitric acid with the inert gas sparge. Transfer lines were heated to minimize condensation. The quantity and transfer rate of the nitric acid were not determined. Addition of the nitric acid produced a reaction with the solids present, shown by gas evolution from the solid s surface, which yielded a soluble uranium species in the nitrate melt. The total solids were dissolved, which produced a characteristic uranyl color in the melt. After complete dissolution of the uranium species, the nitric acid sparge was removed, and the melt was open to the atmosphere. 

We have verified that a soluble uranium species is produced by the addition of 100% nitric acid vapor to a nitrate melt containing uranates formed by reaction of the melt with uranium dioxide. The temperature range of dissolution and the thermal stability of the soluble species have been approximately defined. Neither the identity nor the solubility limit of the uranium species has been determined. 

C1334. (2010). Standard specification for uranium oxides with content of less than 5% for dissolution prior to conversion to nuclear-grade uranium dioxide. West Conshohocken, PA ASTM. 

Oxidation of UO2 by CI2 in molten salts leads to the formation of uranyl(VI) ions [16], but at low uranium concentration, when the melt has equilibrated with uranium dioxide, a significant amount of U(V) ions can be formed [17] due to the reduction of uranyl(Vl) ions by uranium dioxide. Uranyl(V)-containing melts can also be obtained by anodic dissolution of UO2 [18]. 

Komarov, V.E. and Nekrasova, N.P. (1981) Equilibrium electrode potentials and anodic dissolution of uranium dioxide in cesium chloride (in Russian). Elektrokhimiya, 17(4), 488-493. 

TI Trofimov, MD Samsonov, SC Lee, NG Smart, CM Wai. Ultrasound enhancement of dissolution kinetics of uranium dioxides in supercritical carbon dioxide. J Chem Technol Biotechnol 76 1223-1226, 2001. 

In order to make use of thorium as a nuclear resource for power generation, development of efficient separation processes are necessary to recover 233U from irradiated thorium and fission products. The THORium uranium Extraction (THOREX) process has not been commercially used as much as the PUREX process due to lack of exploitation of thorium as an energy resource (157,180). Extensive work carried out at ORNL during the fifties and sixties led to the development of various versions of the THOREX process given in Table 2.6. The stable nature of thorium dioxide poses difficulties in its dissolution in nitric acid. A small amount of fluoride addition to nitric acid is required for the dissolution of more inert thorium (181). 

There are also some non-analytical uses of USASTD. One is in metallurgy, where minerals are sub]eot to exhaustive dissolution treatments that oan be used for analytical purposes. Sueh is the case with the digestion of phosphate rook in HCI [56] or HNO3 [57], pyrite ores in sulphuric acid [58] or uranium oxides in supereritieal earbon dioxide [59]. 

Metal-complexation/SFE using carbon dioxide has been successfully demonstrated for removal of lanthanides, actinides and various other fission products from solids and liquids (8-18), Direct dissolution of recalcitrant uranium oxides using nitric acid and metal-complexing agents in supercritical fluid carbon dioxide has also been reported (79-25). In this paper we explored supercritical fluid extraction of sorbed plutonium and americium from soil using common organophosphorus and beta-diketone complexants. We also qualitatively characterize actinide sorption to various soil fractions via use of sequential chemical extraction techniques. 

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