Characterization of structural properties of U and Pu in model systems by advanced synchrotron based X-ray spectroscopy

This dissertation presents the investigations of different U and Pu model systems relevant for safety assessment studies of nuclear waste repositories using the X-ray based synchrotron techniques: U and Pu L3/M4,5 edges HR-XANES, L3 edge EXAFS and 3d4f RIXS as well as other complementary techniques, including XPS, XRD, SEM, TEM and UV-Vis-NIR techniques

NIEZMIENNE URZĄDZENIA PIEZOREZONANSOWE NA PODSTAWIE ADAPTACYJNYCH WIELOCZĘSTOTLIWOŚCIOWYCH SYSTEMÓW Z PROGNOZOWANYM STANDARDEM

The paper presents conceptual provisions for the construction of invariant multi-frequency piezoelectric resonance devices with a predictive reference model. The law of the optimal control of the system in real time is formulated, aimed at minimizing energy costs to ensure the trajectory of the system. The results of piezoelectric resonance system mathematical modelling in the conditions of temperature and vibration perturbations are presented.W artykule przedstawiono koncepcyjne zasady konstruowania niezmiennych wieloczęstotliwościowych piezoelektrycznych urządzeń z predykcyjnym modelem odniesienia. Zostało sformułowane prawo optymalnego sterowania systemem w czasie rzeczywistym, aby przy minimalnych stratach energii zapewnić trajektorię ruchu systemu. Przedstawiono wyniki matematycinvariant multi-frequency piezoelectric resonance system, optimal control, mathematical modelznego modelowania układu piezorezonansowego w warunkach zaburzeń temperatury i drgań

Vibrational compensation crys-tal oscillator frequency instability in multifrequency excitation mode piezoelectric resonator

Запропонований ефективний метод підвищення віброчастотної стабільності п’езорезонансних пристроїв, заснований на принципах компенсації зовнішніх віброчастотних впливів за рахунок розширення інформаційного об’єму вихідних сигналів ПРП в режимі багаточастотного збудження кварцового резонатора. Проведений аналіз спектра частотно-модульованого сигналу кварцових генераторів декаметрового і дециметрового діапазонів радіочастот в умовах синусоїдальних і випадкових широкосмугових механічних дестабілізуючих впливів. Наведено кількісні оцінки рівнів побічних складових спектра вихідного сигналу для типових параметрів кварцових генераторів, а також фазового джіттера при використанні КГ в цифрових системах. Показано, що використання запропонованого методу компенсації зовнішніх віброчастотних впливів дозволяє знизити вібромеханічну чутливість ПРП більш, ніж у 30 разів без істотних конструктивно-технологічних витрат.Introduction. The effective method of vibrational frequency stability increasing of the piezoresonant devices based on the principles of compensating of external vibrational frequency influences due to extension of information volume of PRU original signals in a mode of the QR multifrequency excitation is offered. The results. The analysis of a frequency modulated signal range of crystal oscillators of HF and UHF RF spectrums in the conditions of sinusoidal and accidental broadband mechanical destabilizing actions is carried out. The quantitative estimates of collateral spectrum components levels of an output signal for standard parameters of crystal oscillators, and also phase jitter are given using XO in digital systems. Conclusions. It is shown that use of the offered method of external vibrational frequency influences compensating allows to reduce vibrational mechanical sensitivity of PRU more than by 30 times without essential constructive and technological expenses.Предложен эффективный метод повышения виброчастотной стабильности пьезорезонансных устройств, основанный на принципах компенсации внешних виброчастотных воздействий за счет расширения информационного объема исходных сигналов ПРУ в режиме многочастотного возбуждения кварцевого резонатора. Проведен анализ спектра частотно-модулированного сигнала кварцевых генераторов декаметрового и дециметрового диапазонов радиочастот в условиях синусоидальных и случайных широкополосных механических дестабилизирующих воздействий. Приведены количественные оценки уровней побочных составляющих спектра выходного сигнала для типовых параметров кварцевых генераторов, а также фазового джиттера при использовании КГ в цифровых системах. Показано, что использование предложенного метода компенсации внешних виброчастотных воздействий позволяет снизить вибромеханическую чувствительность ПРУ более, чем в 30 раз без существенных конструктивно-технологических затрат

Probing Covalency in the UO3 Polymorphs by U M4 edge HR- XANES

Local atomic and electronic structure investigations of uranium trioxide (UO3) crystalline phases performed by the U M4 edge HR-XANES technique is presented. The experimental U M4 edge HR-XANES spectra of α-UO3, β-UO3 and γ-UO3 polymorphic phases are compared with spectra of uranate (CaU2O7) and uranyl (UO3•1-2(H2O)) compounds. We describe a finger print approach valuable for characterization of variations of U-O axial bond lengths. Theoretical calculations of spectra using full-multiple-scattering theory (FEFF9.6 code) are performed. We have tested and selected input parameters, which provide best agreement between experimental and calculated spectra

Multi-criteria model for selection of optical linear terminals based on FUZZY TOPSIS method

Optical networks are an integral part of modern telecommunication systems. Huge traffic and ever-increasing requirements for data transmission capacity and quality encourage the wider use of modern optical technologies in telecommunications. The rational choice of equipment for the design of optical networks is an urgent task at present. The subject of this study is the process of selecting optical line terminals, which is associated with the evaluation of possible options by a set of indicators. The OLT (optical line terminal) in PON technology implements the function of organizing subscriber lines and is also a node equipment - an L4 switch that combines the functions of routing (IP), traffic fragmentation (VLAN), switching (MAC), quality of service (QoS), and some necessary network service functions. The optimal structuring of PON access networks demands a judicious selection of software and hardware, guided by their tactical and technical characteristics, in view of the substantial information load they entail. One of the effective approaches to solving such problems is the use of MCDM (Multiple Criteria Decision Making) methods. Purpose: to construct a TOPSIS model of the optimal choice of optical linear terminals in the conditions of unclear information for different cases of aggregation of evaluations of decision makers. Task: to formalize the process of selecting optical linear terminals; develop a multi-criteria mathematical model for the effective selection of optical line terminals. Methods used in the study: The fuzzy TOPSIS method, four methods of aggregating the opinions of decision makers. The following results were obtained. Alternatives and criteria for their evaluation are defined. On the basis of interviews with decision makers, evaluations of the degree of importance of the criteria and alternatives to the criteria were determined. Linguistic changes were used to describe decision-makers’ evaluations, which were interpreted as triangular fuzzy numbers. During this study, four methods of aggregating the evaluations of decision makers were used. A fuzzy TOPSIS model for selecting an optical line terminal is constructed. The ranking of the selected optical line terminals is obtained and the best alternative is determined. The results of modeling with different methods of aggregating assessments of decision makers are compared. Conclusions. The use of the Fuzzy TOPSIS method for optimal selection of optical linear terminals is proposed. The influence of the methods of aggregation of evaluations of decision-makers is analyzed

U redox state and speciation of U in contact with magnetite nanoparticles : High resolution XANES, EXAFS, XPS and TEM study

Long-term storage of high-level radioactive waste is associated with potential radioecological hazards. One chemical element of high interest is uranium (U), which can mainly exists as a mobile U(VI) (oxidizing conditions) and sparingly soluble U(IV) (reducing conditions) species. It is expected that the main inorganic reducing agent for U(VI) in the environment are ferrous species in magnetite, formed on the steel canisters surface as an intermediate iron (Fe) corrosion product [1]. Results obtained from laboratory experiments for the interaction of U(VI) with magnetite nanoparticles point to partial reduction of U(VI) [2] or the formation of ~3 nm uranium dioxide (UO₂) particles on the surface layer [3]. The evidence for U(VI) reduction to intermediate U(V) state was found with no direct evidence of U(IV), which is in contradiction with thermodynamic calculations [4]. Continuous interaction and related phase dissolution/recrystallization processes can also lead to U redox changes and structural U incorporation into Fe oxides, resulting in U immobilization [5]. U redox state and speciation analyses are still very challenging due to simultaneous formation of several different species in such mineral systems. New advanced spectroscopic methods for characterization of such systems will provide more precise results from speciation studies. The main goal of our investigation is to assess the U M4 edge high energy resolution X-ray absorption near edge structure (HR-XANES) spectroscopy technique for detection of U(V) possibly co-existing with U(IV) and U(VI) under reducing conditions on/in Fe containing minerals. The U M4 edge HR-XANES has an advantage compared to the conventional U L3 edge XANES, as the measured spectra are less dominated by corehole lifetime broadening effects and therefore have narrower spectral features [6-8]. This technique facilitates the detection of minor contribution of one oxidation state in mixtures. We have investigated the U redox states and speciation in a set of samples where U coprecipitated with magnetite nanoparticles (~ 20 nm) with U concentrations varying in the 1000-10000 ppm range (1000, 3000, 6000 and 10000 ppm). In addition to U M4 edge HR-XANES, U L3 edge extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques have been applied. The studied system models the interaction of U(VI) with magnetite in aqueous solution, important for the understanding of the retarding effect of Fe corrosion products on U in the context of deep geological spent nuclear fuel disposal. These spectroscopic results can be compared with thermodynamic calculations and geochemical models describing this interaction. After 10 days U interaction with magnetite U M4 edge HR-XANES results indicate the formation of U(IV), U(V) and U(VI) mixtures in varying ratios, depending on the initial U loading. Going from 10000 to 3000 ppm, the U(VI) content decreases continuously and is no longer found in the 1000 ppm sample. At the same time the U(IV) and U(V) fractions increase. U(V) is stabilized as the main U redox state in the 1000 ppm sample along with a smaller U(IV) contribution. After 20 days of contact time XPS data show the predominance of U(IV) and U(V) species in the 6000 ppm sample. However, mostly U(V) and some U(IV) is found for the 1000 ppm sample. For all samples aged for 240 days U L3 XANES and EXAFS strongly suggest the formation of a UO₂ phase, UO₂ is the dominating species in the 10000 ppm sample with U-O bond distance 2.33. Å as determined by EXAFS. UO₂ crystalline clusters with about 5 nm size formed on the surface of the magnetite nanoparticles are also found by TEM in the 10000 and 3000 ppm samples. The major and minor contributions of U(V) and U(IV), respectively, for the 1000 ppm sample after 240 days confirm the assumption that the U redox kinetics has completed within less than 10 days at this U concentration. EXAFS analyses reveal U(V)-Fe interaction in the second U coordination sphere, which substantially increases from the 10000 to 1000 ppm sample and is the dominating species in the 1000 ppm sample

Aqueous U(VI) interaction with magnetic nanoparticles in a mixed flow reactor system: HR-XANES study

The redox variations and changes in local atomic environment of uranium (U) interacted with the magnetite nanoparticles were studied in a proof of principle experiment by the U L3 and M4 edges high energy resolution X-ray absorption near edge structure (HR-XANES) technique. We designed and applied a mixed flow reactor (MFR) set-up to maintain dynamic flow conditions during U-magnetite interactions. Formation of hydrolyzed, bi- and poly-nuclear U species were excluded by slow continuous injection of U(VI) (10-6 M) and pH control integrated in the MFR set-up. The applied U HR-XANES technique is more sensitive to minor changes in the U redox states and bonding compared to the conventional XANES method. Major U(VI) contribution in uranyl type of bonding is found in the magnetite nanoparticles after three days operation time of the MFR. Indications for shortening of the U-Oaxial bond length for the magnetite compared to the maghemite system are present too