1,721,140 research outputs found
Isotopic composition and neutronics of the Okelobondo natural reactor.
No full textThe Oklo-Okelobondo and Bangombe uranium deposits, in Gabon, Africa host Earth's only known natural nuclear fission reactors. These 2 billion year old reactors represent a unique opportunity to study used nuclear fuel over geologic periods of time. The reactors in these deposits have been studied as a means by which to constrain the source term of fission product concentrations produced during reactor operation. The source term depends on the neutronic parameters, which include reactor operation duration, neutron flux and the neutron energy spectrum. Reactor operation has been modeled using a point-source computer simulation (Oak Ridge Isotope Generation and Depletion, ORIGEN, code) for a light water reactor. Model results have been constrained using secondary ionization mass spectroscopy (SIMS) isotopic measurements of the fission products Nd and Te, as well as U in uraninite from samples collected in the Okelobondo reactor zone. Based upon the constraints on the operating conditions, the pre-reactor concentrations of Nd (150 ppm +/- 75 ppm) and Te (<1 ppm) in uraninite were estimated. Related to the burnup measured in Okelobondo samples (0.7 to 13.8 GWd/MTU), the final fission product inventories of Nd (90 to 1200 ppm) and Te (10 to 110 ppm) were calculated. By the same means, the ranges of all other fission products and actinides produced during reactor operation were calculated as a function of burnup. These results provide a source term against which the present elemental and decay abundances at the fission reactor can be compared. Furthermore, they provide new insights into the extent to which a fossil nuclear reactor can be characterized on the basis of its isotopic signatures. In addition, results from the study of two other natural systems related to the radionuclide and fission product transport are included. A detailed mineralogical characterization of the uranyl mineralogy at the Bangombe uranium deposit in Gabon, Africa was completed to improve geochemical models of the solubility-limiting phase. A study of the competing effects of radiation damage and annealing in a U-bearing crystal of zircon shows that low temperature annealing in actinide-bearing phases is significant in the annealing of radiation damage.PhDApplied SciencesEarth SciencesEnvironmental scienceGeologyHealth and Environmental SciencesNuclear engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124282/2/3137914.pd
Radionuclide retardation in crystalline rock fractures
No full textTransport and retardation of slightly sorbing sodium was studied in Syyry area SY-KR7 mica gneiss and in altered porous tonalite. Experiments were performed using dynamic fracture and crushed rock column methods and the static batch method. Flow conditions in the columns were determined using tritiated water and chloride as non-sorbing tracers. 14C-PMMA method was used to study the pore structure of matrices and the surface areas were determined by B.E.T. method. Sodium was retarded strongly in altered tonalite owing to homogeneously porous matrix structure and the composition of alteration minerals. An agreement between retardation values in batch and crushed rock column experiments as well as in fracture column experiments was good
The corrosion of uranium dioxide: An atomic-scale investigation.
No full textSpent nuclear fuel is composed of ≈95% UO2, and upon storage in geologic repository the fuel itself is a barrier to the release of radionuclides. Therefore, it is important to understand the behavior of oxygen and water with UO2 surfaces in order to determine the long-term stability of spent fuel. UO2 corrodes faster in the presence of water and oxygen as compared with oxygen alone. Fortunately, the addition of thorium to UO2 fuels increases the resistance to oxidation and decreases dissolution rates. In this thesis, atomic-scale quantum-mechanical and empirical-potential modeling methods are used to develop a mechanistic understanding of the effect of surface structure and surface chemistry on the interaction of adsorbates with UO2 and ThO2 surfaces. Oxidation rates are calculated for atomic oxygen interacting with specific UO2 surfaces, and an atomicscale model is proposed for the enhancement of UO2 oxidation rates in the presence of water. Bulk oxidation mechanisms are also explored for UO2. Surface energy calculations were performed to establish the relative reactivity of UO2 and ThO2 surfaces that are found in polycrystalline fuel pellets. Adsorption energy trends for water, oxygen, and combinations thereof were determined on the stable (111) surface and on the more reactive (110) surface of UO2 and ThO2. Oxidation rates for atomic oxygen interacting with the UO2 (111) surface are slow in comparison to the (110) surface, for which rates are nearly instantaneous. The presence of water on both surfaces significantly lowers the activation energy to oxidation, and low-spin oxygen is found to be a necessary precursor to uranium oxidation. The semi-conducting nature of UO2 enhances oxidation in the presence of water, a phenomenon not observed on the insulating ThO2 surface. Bulk oxidation mechanisms for UO2 were also explored and two U5+ ions are found to be more stable that one U6+ ion upon the addition of O2- to the UO2 structure to form U4O9. The presence of U5+ versus U6+ has implications for oxygen diffusion rates in bulk UO2. These results provide insight into the properties that make nuclear fuel more corrosion resistant and the atomic-scale mechanisms that control oxidation and corrosion.PhDEarth SciencesEnvironmental scienceGeologyHealth and Environmental SciencesPhysical chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126821/2/3276294.pd
Dissolution of UO<sub>2</sub> at various parametric conditions::A comparison between calculated and experimental results
No full textThe solubilities of uranium measured in deionized water, in sodium bicarbonate solutions as a function of carbonate concentration and in two types of synthetic groundwaters have been compared with those predicted using the reaction pathway program, PHREEQE. All the measurements were carried out under both air-saturated, oxidizing and anoxic, reducing conditions.The experimental solubility values of uranium under oxidizing conditions are, in general, lower when compared to the corresponding theoretical ones calculated by PHREEQE. A critical factor is the choice of solubility constant for the solid phase. The reason for the lower solubility values may also be the mechanism of dissolution leading for example either to a situation where low dissolution rate is a limiting factor, or to formation of some solid phase of uranium with lower solubility. The experimental solubility values under reducing conditions appear to be in good agreement with the theoretical solubility values
Corrosion of uranium oxide and thermodynamic properties of solid solutions in the zircon group.
No full textCorrosion resistance and the ability to incorporate and retain impurities are important properties in determining the usefulness of a material for applications as varied as designing a nuclear waste form or calculating the temperature or time at which a particular rock formed. This thesis applies both experimental and computational techniques in order to gain insight into the corrosion and substitution behavior of actinide oxides and orthosilicates. Experimental studies of uraninite corrosion in miniature waste packages show that oxidation of UO2, a proxy for spent nuclear fuel (SNF), is minimal in the presence of a relatively large amount of corroding steel over laboratory time-scales. The oxidative dissolution of spent nuclear fuel in an oxidizing repository with a steel canister may be broken into three phases: (1) before breach, when no water is available and the corrosion rate is negligible, (2) immediately after breach, when the steel begins to corrode, conditions inside of the canister are more reducing, and SNF does not corrode significantly, and (3) long after breach, SNF is exposed to the open air and oxidized. The common accessory mineral zircon (ZrSiO4) is considered a host phase for Pu and other elements using a combination of quantum-mechanical and Monte-Carlo simulations. Results show that while hafnon (HfSiO4) and zircon form a nearly ideal solid solution, the maximum amount of Pu, U, Th, and Ce that can be incorporated at thermodynamic equilibrium is extremely limited and can be ranked in the following order: (Zr,Hf)SiO 4 > (Th,U)SiO4 > (Zr,Pu)SiO4 > (Zr,Ce)SiO 4> (Hf,Pu)SiO4 > (Zr,U)SiO4 > (Zr,Th)SiO 4. Calculations on Ti incorporation in zircon for use as a geothermometer confirm that Ti prefers to occupy the Si site at pressures below 1 GPa. However, at higher pressures, particularly above 3.5 GPa, substitution into the Zr site is preferable. Results suggest a pressure correction of 100°C/GPa at 750°C, twice as large as a previous estimate. The Ti-in-zircon geothermometer must be recalibrated if it is to be used on ultra-high pressure samples.PhDEarth SciencesGeochemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/127116/2/3382179.pd
Diffusion Measurements on Crystalline Rock Matrix
No full textAbstractA new gas flow technique is introduced such that experiments on very long samples are possible. This new technique together with increased accuracy of the measurements, allows the observation of power law tails in the break-through curves. Dispersion in these experiments can be controlled in great detail, and therefore the power law tails can be used to determine very accurately the parameters relevant in matrix diffusion. Results for rock and metal samples are shown, and they are fitted with model calculations which include both dispersion and matrix diffusion. The introduced technique, which is designed for ordinary drill cores, is suitable for scanning a large number of samples in a very short time.</jats:p
Nanoscale and atomistic processes in minerals.
No full textOne of the main questions concerning the occurrence and prevalence of nanoscale phases in Earth systems is the role that temperature, particle size, and the nature and composition of the mineral host play. The study of the gold-in-pyrite nanoparticle-host system reveals that the occurrence of gold nanoparticles is highly dependent on the arsenic content of pyrite. This compositional dependence allows one to determine the occurrence of nanoparticles of native gold versus solid solution gold in arsenian pyrite samples, and therefore, evaluate the saturation state of gold with respect to native gold in the aqueous solutions that precipitated the pyrite samples. In addition, first-principles simulations of the thermodynamic mixing properties of arsenic into pyrite (and marcasite) show that the amount of arsenic that pyrite can accommodate in solid solution is limited. Our simulations predict that for arsenic contents of up to∼6 wt%, homogeneous arsenian pyrite decomposes into a heterogeneous mixture of arsenic-iron-sulfur nanoscale phases. Apart from, e.g., the gold-content dependence on the arsenic content of pyrite, the stability of nanoscale phases in geologic systems is strongly dependent on particle size and temperature. Results from heating experiments under high-resolution observation show that when incorporated in an arsenian pyrite host, gold nanoparticles become unstable unless the nanoparticle size-distribution is coarsened by Ostwald ripening. In this context, our observations reveal that gold nanoparticles remain stable until they start to coarsen into larger particles above ∼370°C. Therefore, nanoparticulate metals, usually thought to be prevalent at low-temperatures (T c-axis, Above this temperature, helium atoms start to diffuse from one channel parallel c-axis to the next, in a direction perpendicular to the c-axis. Considering the limited data available for this system, results suggest that the strong anisotropic nature of helium diffusion at low temperatures must be considered in future diffusivity experiments, as well as in bulk modeling of relevance to zircon (U-Th)/He thermochronology.PhDEarth SciencesGeochemistryGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126272/2/3238068.pd
The long-term stability of becquerelite
No full textUranium-series disequilibria data, in conjunction with petrographic analyses, indicate that the uranyl oxide hydrate becquerelite can persist for hundreds of thousands of years, possibly longer. Becquerelite probably forms continuously as ground water compositions permit and is resistant to U leaching by ground water. On the time scale of interest for the geologic disposal of spent UO2 nuclear fuel, becquerelite is a long-lived sink for uranium in oxidizing, U and Ca-bearing ground waters. Such long-term stability also supports recent solubility experiments that indicate natural becquerelite has a lower solubility product than that determined for synthetic becquerelites
U series concentration distributions around rock fractures:Qualitative indication of matrix diffusion
No full textAbstractConcentrations of U series nuclides (238U, 234U, 230Th, 226Ra and 231Pa) across secondary U accumulations in rock matrix around water-carrying fractures were examined to find records of radionuclide movement. The accumulations, loaded in rock pores, were separated without dissolving the host rock. The concentration profiles observed, form patterns which point to both “old” and “young” U accumulations and, moreover, post accumulation movement of certain radionuclides. In the case of the young U accumulation, shown to have occurred between 10,000 and 33,000 years ago, the apparent diffusivity of Da=6.10-15 m2/s for U could be calculated.</jats:p
Radiation and thermal effects on zeolites, smectites and crystalline silicotitanates.
No full textLong-term radiation and thermal effects on materials in the near-field of a nuclear waste repository have been evaluated using accelerated laboratory experiments by energetic electron or ion beam irradiations. The materials studied include: zeolites, smectites, and crystalline silicotitanates (CST). In situ transmission electron microscopy (TEM) during irradiation by 200 keV electrons has shown that all of the studied materials are susceptible to radiation-induced amorphization. At room temperature, complete amorphization was observed after ionizing doses of 1010∼10 12 Gy or displacement doses of ∼0.1 dpa. The critical amorphization dose increased with temperature for CST. A peak amorphization dose was found at ∼400°C for the smectites. A new experimental approach was developed using a combination of proton irradiation, TEM, and electron microprobe analysis techniques to overcome the difficulties in studying the chemical properties in the radiation-damaged region. A clear correlation between the structural damage and changes in ion-exchange and desorption capacities has been established for zeolite-Y, which maybe useful for predicting the long-term behavior of the near-field materials in a nuclear waste repository. Radiation-induced decrease in the release rate of radionuclides has been observed for the first time in the damaged materials, indicating that the structural damage in near-field materials may be beneficial for retarding the release of radionuclides from geologic disposal repository into biosphere. Possible mechanisms for radiation-induced changes in structure, ion exchange and desorption capacities have been proposed.PhDApplied SciencesNuclear engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/125389/2/3016857.pd
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