Uranium dissolved, distribution

Homogeneous Aqueous Reactors. As a part of the research on neutron multiphcation at Los Alamos in the 1940s, a small low power reactor was built using a solution of uranium salt. Uranyl nitrate [36478-76-9] U02(N0 2> dissolved in ordinary water, resulted in a homogeneous reactor, having uniformly distributed fuel. This water boiler reactor was spherical. The 235u... 

Prange, A. and K. Kremling. 1985. Distribution of dissolved molybdenum, uranium and vanadium in Baltic Sea waters. Mar. Chem. 16 259-274. 

Th and 231Pa are ubiquitous components of recently deposited deep-sea sediments because they are produced uniformly throughout the ocean from the decay of dissolved uranium isotopes and they are actively collected onto sinking particles. The distribution with depth of these nuclides in deep-sea sediments may be modeled to estimate rates of sedimentation extending over the past 200 to 300 thousand years. These techniques complement 14C dating methods that only extend to about 40 thousand years before the present. 

Figure 20 Distribution of dissolved uranium species in the system U-O-H2O-CO2 at 25 °C, assuming a Pco of 0.01 atm (after Drever, 1997).

Fig. 10-7. Distribution of dissolved helium in groundwaters, Edgemont uranium district, South Dakota (from Bowles ct al., 1980).

Uranium transfers at a slower rate than plutonium because uranium has a lower solubility than plutonium in the donor alloy and uranium has a lower distribution coefficient that plutonium in the donor alloy-salt system. This difference in the rate of transfer is very desirable because it provides a means for enriching the plutonium content of the product to required concentrations for recycle to the reactor core. This enrichment is obtained by terminating the circulation of the transport salt between donor and acceptor alloys before complete uranium transfer has occurred. As uranium transfers, the solid UCU5 compound dissolves into the donor alloy. After plutonium and the desired amount of uranium are separated from FP-4 elements, the remaining uranium may be separated by diverting the transport salt to a second zinc-magnesium acceptor alloy. 

Prange, A., Kremling, K., 1985. Distribution of dissolved Molybdenum, Uranium and Vanadium in Baltic Seawaters. Marine Chemistry, 16, 259-274. 

The next step in purification is separation of uranyl nitrate from the other metallic impurities in the dissolver solution by solvent extraction. Practically aU uranium refineries now use as solvent tributyl phosphate (TBP) dissolved in an inert hydrocarbon diluent. The first U.S. refinery used diethyl ether as solvent and later refineries have used methyl isobutyl ketone or organic amines, but practically all have now adopted TBP. It is nonvolatile, chemically stable, selective for uranium, and has a uranium distribution coefficient greater than unity when the aqueous phase contains nitric acid or inorganic nitrates. 

Distribution coefficients may be further modified and operating temperatures reduced by dissolving uranium fuel in a low-melting metal such as bismuth or zinc. Separation of uranium from fission products by liquid extraction between molten bismuth and fused chlorides was extensively studied at Brookhaven National Laboratory [D5] in connection with the liquid-metal fuel reactor (LMFR), which used a dilute solution of in bismuth as fuel. Extraction of fission products from molten plutonium by fused chlorides was studied at Los Alamos [L2] in connection with the LAMPRE reactor. 

As in the Purex process, the Thorex process uses a solution of TBP in hydrocarbon diluent to extract the desired elements, uranium and thorium, from an aqueous solution of nitrates. Thorium nitrate however, has a much lower distribution coefficient between an aqueous solution and TBP than uranium or plutonium. To drive thorium into the TBP, the Thorex process as first developed at the Knolls Atomic Power Laboratory [HI] and the Oak Ridge National Laboratory [G14] added aluminum nitrate to the thorium nitrate in dissolved fuel. This had the disadvantage of increasing the bulk of the high-level wastes, which then contained almost as many moles of metallic elements as the original fuel. To reduce the metal content of the waste, the Oak Ridge National Laboratory in the late 1950s [Rl, R2] developed the acid Thorex process, in which nitric acid is substituted for most of the aluminum nitrate in the first extraction section. The nitric acid is later evaporated from the wastes, as in the Purex process. 

Thorium is widely distributed in Nature and there are large deposits of the principal mineral, monazite, a complex phosphate containing uranium, cerium, and other lanthanides. The extraction of thorium from monazite is complicated, the main problems being the destruction of the resistant sand and the separation of thorium from cerium and phosphate. One method involves a digestion with sodium hydroxide the insoluble hydroxides are removed and dissolved in hydrochloric acid. When the pH of the solution is adjusted to 5.8, all the thorium and uranium, together with about 3% of the lanthanides, are precipitated as hydroxides. The thorium is recovered by tributyl phosphate extraction from >6M hydrochloric acid solution or by... 

The distribution of dissolved substances in two solvent phases is employed on a large scale in the industrial separation of mixtures of substances. Examples are the removal of unsaturated constituents from vegetable oils with furfurol or methanol, the purification of animal and vegetable oils with liquid propane, and the removal of waxes from lubricants with liquid propane or ketones. Penicillin is similarly concentrated with methyl isobutyl ketone, and aqueous glycerol is purified with xylene. Preparative and analytical separations are also performed by liquid-liquid extraction. Inorganic salts can be extracted from aqueous solutions with suitable solvents, such as ethers, ketones, and esters. This method is particularly efficient for metal halides and nitrates, e.g., the separation of uranium compounds from aqueous solutions or the fractional extraction of rare earths. 

Critically measurements and calculations on enriched (93,2 wt% ) uranium metal spheres symmetrically immersed in enriched (93.2 wt% U) uranyl nitrate solution, have been presented previously. The present Interpretation of the same data considers the criticality safety of an enriched uranium metal sphere immersed, initially, in nonflssile liquid (e.g., acid). As the metal dissolves, the fissile concentration of the solution in-creases until. the two interacting fissile regions together achieve criticality. Here, the uranium in solution is assumed homogeneously distributed. 

Another relevant general review summarizes the knowledge on the behavior of series radionuclides in soils and plants and is intended to provide a comprehensive source of information for environmental impact studies (Mitchell et al. 2013). The summary of the data on plant to soil concentration ratios that depends on the specific soil and type of plant and the distribution of uranium within the parts of the plant is especially important. The dependence of the sorption of dissolved uranium compounds on the type of soil (like the clay content) and the parameters mentioned earlier (pH, complex forming agents, anions, presence of iron, organic matter, etc.), based mainly on studies of the (distribution factor) of spiked soil samples, is discussed. It is noted that in general the uranium concentration in plants is several orders of magnitude lower than in soil, but some plants may efficiently absorb uranium and translocation within the plant is quite common (Mitchell et al. 2013). These features, and especially the soil-to-plant transfer factors, will be discussed in Section 3.4 that deals with the uranium content in plants and soil and the relation between them. 

The relative ratio between flows in processes 1 and 2 (Fig. 1) varies according to the distribution of uranium atoms in the different molecular sites in the source rock—indeed, such distribution determines the way in which uranium mobilization occurs. Uranium contained in insoluble accessories is leached to a very limited extent, so it is presumed to be transported to and concentrated in resistate sediments as clastic material. Adams and co-workers estimated that 60-85% of the uranium in igneous rocks is present in mineral phases that are resistant to chemical alteration approximately 15-40% is transported in the dissolved form by liquid water. The above fraction of uranium takes part in the processes of erosion and sedimentation (represented by the arrow in the upper part of Fig. 1 that connects the source rock box directly with the sediment box). 

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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