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AbstractMetallic uranium is a fundamental raw material for producing nuclear fuel elements for research reactors and irradiation targets for producing 99Mo, as U3Si2, UMo alloy, UAlx, and uranium thin foils. Magnesiothermic reduction of UF4is a possible route in the nuclear fuel cycle for producing uranium as a metal ingot. The main concern about the reducing scale to produce low-enriched (metallic) uranium (LEU) (around 1 kg) is the relatively low yield compared to calciothermic reduction. Nevertheless, the magnesiothermic reduction has the advantages of having lower cost and being a safer method for dealing with uranium processing. The magnesiothermic process, as a batch, is closed inside a sealed crucible. In the present study, in order to have a qualitative idea of the kinetics during the ignition moment, the slag projected over the lateral inner face of the crucible was used to sketch the general magnesiothermic evolution. The methods used were metallographic observation and X-ray diffraction followed by Rietveld refinement. The results of these analyses led to the conception of a general reaction development during the short time between the ignition of the reducing reaction and final settlement of the products. Relevant information from this study led to the conclusion that uranium is not primarily present in the lateral slag projection over the crucible during the reaction, and the temperature level may reach 1500°C or more, after the ignition.

Electrodeposition process of natural uranyl many has several deposit possibilities, since the process variables and uranium natural isotopes and secular equilibrium may induce the formation of several uranium compounds. One of these deposit possibilities is the strong beta-emitter thorium-234. The present work showed that pulsed polarization and 2-propanol natural uranyl solution (50 mM [U]) induced the formation of intense beta-emission in the samples, with activity beta-alpha ratio reaching levels of 300, which is much higher than the secular equilibrium ratio around 1. The natural decay of the deposited samples showed that the beta-emitter as most probable element to be thorium-234 within a pertinent regression exponential decay curve adjustment (R2=0.998) giving a typical experimentation value for half-life as 22.88 days. This value was found lower than the typical 24.10 days for thorium-234 (specific activity of 85.69x1013 Bq/g), probably due to activity contribution of uranium-235 daughter, thorium-231 (specific activity of 19.67x1015 Bq/g), which beta-decays having half-life of 25.6 h. Further experimentation with pulsed polarization of the cathode (0 to -15V; 12.5 Hz - duty time = 87.5% and process time varying until 3600s) also showed that thorium-234 depends on solution acidity, solution usage and deposition time. XRD spectra suggests that the deposit would be probably a combined structure of uranium-thorium hydride, which decomposes with time.

Electrodeposition of uranium is a common practice to create samples for alpha spectrometry and it could be an alternative way to produce irradiation LEU targets to fabricate radiopharmaceuticals as 99Mo used for cancer diagnosis. Usually electrodeposition of uranium uses ionic or aqueous solutions to produce uranium deposits in less acidic electrolytes (pH>2.5). During uranium electrodeposition, there is a high competition with H2 evolution, once cathodic potentials are very high. In less acidic electrolyte the electrodeposition is uranyl hydroxyl and uranium oxides compounds, formed directly from uranyl (U-VI) structure. A reliable regression equation (R2=0.836) for alpha emission activity of uranium deposition was obtained, based on cell temperature and electrodeposition time. The deposition has oxide/hydroxide nature, acting as insulator during electrochemical process. The maximum level of deposited uranium, in terms of alpha activity, was around 34 Bq.cm-2 (-1.8 VAg/AgCl, 2000 s, 60degC). In this condition, the inferred maximum amount of uranium was ~5.4 mg [U] /cm2, which might be interesting to build probe samples to simulate irradiation targets.

A regular way to produce samples for alpha-spectrometry and other nuclear emissions could be by means of electrodeposition. Uranium electrodeposition may also be used to fabricate irradiation targets for radiopharmaceutical production. In this work, the electrodeposition of natural uranium was made by pulsed potential in galvanostatic state over nickel substrate, having a cathodic area of 2 cm2. The samples have been exposed to UO2(NO3)2 acidic concentrated solution diluted 1:20 in 2-propanol, having final concentration at 50 mM [U] with pH<1. The pulsed potential varied between two levels, from 0 V to [?]10 V and from 0 V to [?]15 V, inside a tubular electrochemical cell made of quartz. The pulsed electrodeposition had a rectangular potential wave with duty cycle of 75% and a frequency of 25 Hz, with a constant duration of 1400 s. The higher alpha emission activity that was obtained from the oxide/hydroxide electrodeposit reached the level of 30 Bq/cm2 for a cathodic potential polarization at 0V/-15V during 1400 s of pulsed electrodeposition. Explanation of uranium electrodeposition at lower pHs was based on potential migration of polarity at the cathode during pulsing potential, allowing production of OH- and nascent oxygen in anodic microregions.

Direct irradiation on targets inside nuclear research or multiple purpose reactors is a common route to produce 99Mo-99mTc radioisotopes. The electroplating of low enriched uranium over nickel substrate might be a potential alternative to produce targets of 235U. This work makes use of pulse electroplating either to create a nickel substrate and also to electrodeposit UO2(NO3)2 solution diluted in isopropyl alcohol with final concentration of 10 mol/L [U]. The pulse electroplating employed 1 to 3 repetitions of 15 kilocycles, with duty cycle of 80% at 17 Hz at room temperature. The deposit presented an amorphous consistency. The amount of deposited uranium, measured by means of a-counting, was equivalent to around 2000 ug/cm2 after 2700s of pulse electrodeposition. MEV/EDS technique and alpha-spectroscopy revealed that U-deposited material was natural uranium grade substance entrapped inside nickel substrate.

The direct incorporation of Gd2O3 powder into UO2 powder by dry mechanical blending is the most attractive process for producing UO2-Gd2O3 nuclear fuel. However, previous experimental results by our group indicated that pore formation due to the Kirkendall effect delays densification and, consequently, diminishes the final density of this type of nuclear fuel. Considering this mechanism as responsible for the poor sintering behavior of UO2-Gd2O3 fuel prepared by the mechanical blending method, it was possible to propose, discuss and, in certain cases, preliminarily test feasible adjustments in fabrication procedures that would minimize, or even totally compensate, the negative effects of pore formation due to the Kirkendall effect. This work presents these considerations.