Uranium nitric acid dependency

The purification of uranium by fractional crystallization involves repeated recrystallizations of U02(N03)2 from water or nitric acid, three usually being sufficient. The composition of the crystalline product may range from U02(N03)v2H20 to U02(N03)2 6H20 depending upon the temperature, acidity and uranyl nitrate concentration of the solution.198... 

In the solvent extraction purification of the uranium ore concentrate, TBP is the extractant of choice although, in the past, Butex, diethyl ether and MIBK have also been used for this purpose. The uranium ore concentrate is first dissolved in nitric acid to produce a solution of uranyl nitrate from which insoluble impurities are separated. The consumption of nitric acid in this step will depend upon the nature of the concentrate, as shown in equations (104)-( 109). Many impurities... 

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. 

Reaction of uranium oxide with nitric acid results in the formation of nitrates U02(N03)2.x H2O (x = 2, 3, 6) the value of x depends upon the acid concentration. All contain [U02(N03)2(H20)2] molecules the nitrate groups are bidentate, so that uranium is 8 coordinate (Figure 11.5). Its most important property lies in its high solubility in a range of organic solvents in addition to water (Table 11.4), which is an important factor in the processing of nuclear waste. 

The frequent occurrence of vanadium in uranium minerals renders the separation of these two metals of importance. One method in use is based on the solubility of uranyl nitrate in ether, %vhilst vanadic and also molybdic and tungstic acids are insoluble. A solution containing these substances may therefore be evaporated to drjmess, and the uranyl salt extracted from the residue with ether. Another method depends upon the fact that uranyl nitrate is readily soluble, whilst vanadium compounds are insoluble, in acetic acid of 95 per cent, strength to which nitric acid has been added in the proportion 1 20. ... 

Commercially available Cyanex 923, or TRPO (see Table 19), has been used for the successful extraction of ions from nitric acid solutions into xylene. Extractant dependency gives a slope of two for hexavalent uranium, similar to the behavior observed for trioctylphosphine oxide... 

A study of the kinetics of the separation of the uranium isotopes and by uranium(iv)-uranium(vi) chemical exchange on cationic exchange resins in sulphuric acid has demonstrated that large-scale uranium enrichment by this method would be uneconomical. The sorption of uranium from highly concentrated nitric acid solutions on AMP resins has been shown to be variable and dependent on concentration. 

Therefore, the uranium extraction has a second-order concentration dependence on the aqueous nitrate and the organic TBP concentrations. Nitric acid extraction has a first-order dependence on the aqueous nitrate and the organic TBP concentrations. The addition of an inextractable nitrate salt (e.g., NaNOj) increases the extraction of both uranium and nitric acid. This model also predicts that uranium extraction is decreased by reducing the total nitrate concentration, which is the procedure for back-extracting uranium, by washing the extract with about 0.1 M nitric acid. Also, uranium and nitric acid compete for free TBP, as indicated in Equations (10.7) and (10.8). 

The rate of dissolution of PUO2 in nitric acid is slower than UO2 and depends on the plutonium/uranium ratio, the methods used to fabricate fuel, and the conditions of irradiation. At one extreme, plutonium produced at low concentration in UO2 by transmutation dissolves almost as rapidly as the associated UO2. At the other extreme, plutonium present as PUO2 mixed mechanically with UO2 without proper sintering dissolves much more slowly and less completely than UO2. Plutonium present as a solid solution (U,Pu)02 at the concentration of 20 to 25 percent used in breeder-reactor fuel dissolves at an intermediate rate. ... 

Figure 7 illustrates an example of dissolution curve for the UO2 powder. The rate of the dissolution of UO2 increased when the content of HNO3 in the HNO3-TBP complex was higher. A half life, t, defined as an extraction time where 50% of uranium is extracted was evaluated for each experiment, and a reciprocal number of th, which is proportional to the dissolution rate, was plotted in Figure 8. The correlated curve shows the dissolution rate is proportional to ([HN03]/[H20]). This dependency is smaller than the value (2.3) reported for UG2 dissolution in aqueous nitric acid (75), and it suggests the chemical mechanism of UO2 dissolution in TBP-HNO3-H2O-CO2 system is somewhat different from that in aqueous nitric acid.

Fig. 2.15 Dependency of the uranium distribution ratio, Ku, on TBP concentration in [Ciomim] [NTf2] at constant (3 M) nitric acid concentration (Dietz and Stepinski 2(K)8)...

Urine QCs (see below), standards, samples, and ultrapure water blanks are acidified with ultrapure nitric add. The volume of urine depends on the concentration range and instrument setup. For a magnetic sector instrument with an Aridus desolvation system and nitrogen addition, 2-4 mL of urine is the typical volume required for samples with uranium concentrations as low as 0.003 pg/L. For samples with uranium concentrations <10 ng/L, 4mL of urine is preferred. Four milliliters of addified urine is the maximum volume that will also fit in the Eichrom 0.21 g TRU resin SPE columns without the additional funnel. Three hundred seventy-five microliters of concentrated nitric acid per milliliter of urine is added to each unknown urine, QC sample, or water blank before introduction onto the column. 

The recovery of uranium from ores uses SX to reject impurities and concentrate the uranium in solution so that it can be economically recovered (Gupta and Singh 2003 Lloyd 1983). The choice of extractant depends on the lixiviant used in the upstream leaching operation, which, in turn, depends on the type of ore in which the uranium is found. Most nranium-bearing ores are readily leached in sulfuric acid and the uraninm is recovered by SX using amines or dialkylorganophosphorus acids. Phosphate ores (snch as those in Florida) are leached in a mixture of sulfuric and phosphoric acids or in phosphoric acid alone. Hot nitric acid has also been used as a lixiviant for nraninm ores (as at Phalaborwa, South Africa). The two common extraction systems for the recovery of uranium(VI) from sulfate leach liquors are compared in Table 5.6. 

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