Uranium determination dioxide

Jhe nature of the defects causing nonstoichiometry in uranium dioxide has not been unequivocally established, although much previous work is consistent with an oxygen interstitial model. Possibly the most straightforward approach to the problem is the study of density variations as a function of composition. Such a study requires a determination of accurate and precise lattice constants and densities, and a knowledge of the phase relationships. 

Electron microscopy and X-ray microanalytical methods showed that uranium as uranyl nitrate hexahydrate penetrated the stratum corneum within 15 minutes and accumulated in the intracellular space between the viable epidermis and the stratum corneum (De Rey et al. 1983). As is the case with inhalation and oral absorption, water solubility is an important determinant of absorption, and no penetration was observed with the insoluble compounds uranium dioxide, uranyl acetate, or ammonium diuranate. After 48 hours, uranium applied as uranyl nitrate was no longer found in the skin and toxicity developed, indicating that the uranium had been absorbed into the blood. 

The biological half-time of uranium dioxide in human lungs (occupational exposure) at German fuel fabrication facilities was estimated to be 109 days. Body burden measurements of uranium taken from 12 people who handled uranium oxides for 5-15 years were used for this determination. Twice a year for 6 years, a urinalysis was conducted on workers exposed to uranium. In vivo lung counting was performed on the last day before and the first day after a holiday period. Levels of uranium in feces were measured during the first 3 days and the last 3 days of a holiday period and the first 3 days after the restart of work. For some employees, the levels of uranium in feces was measured during 3 days one-half year after the holiday period (Schieferdecker et al. 1985). 

In addition, the sequestration patterns of the different uranium compounds are important determinants for the target organ chemical and radiological toxicities of these compounds. The site of deposition for the soluble uranium compounds (uranyl nitrate, uranium tetrachloride, uranium hexafluoride) is the bone, while the insoluble compounds (uranium hexafluoride, uranium dioxide) accumulate in the lungs and lymph nodes (Stokinger 1953). 

The effects of uranium in animal experiments were also compound-dependent, the more water-soluble compounds (e.g., uranyl nitrate) causing much greater renal toxicity than insoluble compounds (e.g., uranium dioxide) when the dose contained equivalent amounts of uranium. ATSDR has determined that the toxicity database for uranium justifies the derivation of separate MRLs for soluble and insoluble forms of uranium for certain durations and routes of exposure. This is based on toxicokinetic evidence that absorption of uranium (and concentration in target tissue) is significantly greater during exposure to the more water-soluble compounds. Soluble forms include uranyl fluoride, uranium tetrachloride and uranyl nitrate hexahydrate insoluble forms include uranium tetrafluoride, uranium dioxide, uranium trioxide, and triuranium octaoxide. Where the database is not extensive enough to allow separate MRLs, the MRL for the soluble form should be protective for health effects due to all forms of uranium. 

Uranium Dioxide in Molten Lithium Nitrate/Potassium Nitrate Eutectic. The behavior of uranium dioxide in a molten lithium nitrate/potassium nitrate eutectic was investigated. Our goal was to determine whether a soluble uranium species could be produced using a nitric-acid vapor sparge. The materials used were a lithium nitrate/potassium nitrate eutectic mixture, pure nitric acid (100% HNO3), and powdered uranium dioxide. The mass ratio of uranium dioxide to the nitrate melt was 1 100. 

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. 

Alkali Metal Uranate. Preliminary studies to determine the composition of the alkali metal uranates formed in the various alkali metal nitrate melts have also been conducted at PNL. Uranium dioxide was reacted with the following melts sodium... 

Empirical formulas for the products from each melt system were obtained we assumed that oxygen was the other constituent of the compounds. The elemental ratios are subject to some variation because of uncertainties in the analytical data. The uranium analyses are estimated to be valid within 2%. Independent analytical determinations have shown that the original uranium dioxide contained approximately 0.5% iron, plus a trace of silica. Adjustment of the analytical data for these minor impurities was not done. 

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. 

We have determined that plutonium dioxide does not react with molten nitrates under the same conditions that uranium dioxide does. We have also determined that plutonium dioxide is not soluble in molten nitrates with the addition of 100% nitric acid vapor under conditions which did produce soluble uranium. This observation must be further verified under the various conditions which can produce the soluble uranium species. 

The method based on the use of PAR was used for determining vanadium in biological materials [27,100], sewage [24], natural waters [26,101,102], silicate rocks [103], petroleum [104], titanium tetrachloride [25], cerium dioxide [105], steel [29], uranium alloys [31], and titanium alloys [29,31]. 

C H. Knight, R. M. Cassidy, B. M. Recoskie, and L. W. Green, Dynamic ion exchange chromatography for determination of number of fissions in thorium-uranium dioxide fuels. Anal Chem., 56,474,1984. 

Pajo et al. (2001a) used GD-MS to measure impurities in uranium dioxide fuel and showed that these impurities could be used to identify the original source of confiscated, vagabond nuclear materials. De las Heras et al. (2000) used GD-MS to determine neptunium in Irish Sea sediment samples. The sediment samples were compacted into a disk that was used with a tantalum secondary cathode in the glow discharge. Using a doped marine sediment standard for calibration, detection limits down to the mid pg/g level were determined. 

Standard Test Method for Determination of Impurities in Uranium Dioxide by Inductively Coupled Plasma Mass Spectrometry Standard Test Method for Gamma Energy Emission from Fission Products in Uranium Hexafluoride... 

In 1998, the plan is to include into the Handbook the results of experiments to determine the effectiveness of heterogeneous neutron absorbers of various types and sizes (e.g., boron carbide and cadmium in the form of separate rods and groups of rods). Experiments with regular cylinders filled with uranium dioxide and located in the moderator in the form of usual and heavy water mixture will also be put in the Handbook. These results are at the review stage. 

Several hundred uranium dioxide fuel assemblies make up the core of a reactor. For a reactor with an output of 1,000 MWe, a typical core contains about 75 t of low-enrichment uranium ( 3.5% U). During the operating cycle of a nuclear reactor, several competing processes determine the final radionuclide inventory in the spent fuel. These processes are... 

---