Rietveld_l-1995: Re: Rietveld-software For Mac

• Rietveld L-1995 Re Rietveld-software For Mac Os X

Epsilon-phase alloy precipitates have been observed with varied compositions and sizes in spent nuclear fuels, such as UO 2. The presence of the inclusions, along with other oxide precipitates, gas bubbles, and irradiation-induced structural defects, can significantly degrade the physical properties of the fuel. To predict fuel performance, a fundamental study of the precipitation processes is needed.

This study uses ceria (CeO 2) as a surrogate for UO 2. Polycrystalline CeO 2 films doped with Mo, Ru, Rh, Pd, and Re (surrogate for Tc) were grown at 823 K using pulsed laser deposition, irradiated at 673 K with He + ions, and subsequently annealed at higher temperatures. A number of methods, including transmission electron microscopy and atom probe tomography, were applied to characterize the samples. The results indicate that there is a uniform distribution of the doped metals in the as-grown CeO 2 film. Pd particles of ∼3 nm in size appear near the dislocation edges after He + ion irradiation to ∼13 dpa.

Thermal annealing at 1073 K in air leads to formation of precipitates of Mo and Pd near the grain boundaries. Further annealing at 1373 K produces 70 nm sized precipitates consisting of nanograins at cavities. Nuclear fuels, such as UO 2, are designed to perform under harsh conditions in a nuclear reactor, including high dose and high temperature. During reactor operation, the composition of the fuel gradually changes as the chain fission reactions starts from fissile nuclides ( 235U). A typical burnup of 40 MWd/kg U results in the conversion of 4% of the uranium to approximately 3% fission products.

The resulting fission fragments exhibit a bimodal distribution with many hundreds of fission products having half-lives of only days to weeks. More than 20 fission products can be detected in the fuel even at a moderate burnup. However, there are only a few dominating families of fission product phases, including oxide solid solutions, perovskite oxide known as gray phase, metallic Mo–Tc–Ru–Rh–Pd white phase with Mo and Ru being the main components, and fission gas bubbles.

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The metallic phases exist due to their high content of noble metals with low oxygen affinity. In addition to a small fraction of body-centered cubic (bcc) β-Mo(Tc, Ru) phase and face-centered cubic (fcc) α-Pd(Ru, Rh) phase, the metallic particles are found primarily in the hexagonal close-packed (hcp) ε-Ru(Mo, Tc, Rh, Pd) phase with a broad variation in component concentrations. The metallic ε-phase has a melting point between 2073 and 2273 K and possesses high resistance to corrosion.

Its composition depends on fission yield, oxygen potential, temperature gradient and fuel burnup. Pores and grain boundaries (GBs) are preferred sinks for the metallic precipitates through the diffusion process. The diffusion is more significant at the higher temperature zone near the central part of the fuel pellet, leading to the formation of larger metallic precipitates.

For normally operated light water reactor (LWR) fuels, the central temperature rarely exceeds 1473 K and the ε particles are typically less than 1 μm in diameter. The microstructure of the irradiated fuel is complex due to various factors, including the presence of various fission product precipitates, β-decay induced changes in charge imbalance, increase in oxygen potential, temperature gradient, irradiation-enhanced GB migration, and lattice disorder produced by elastic collisions with high-energy neutrons and energetic daughter nuclide recoils in the damage cascade process. There is a long history of effort devoted to characterization of spent nuclear fuels. Based on the knowledge available in 1967, the chemical states of fission products were categorized into soluble fission products, metallic and nonmetallic fission products. More extensive analyses of irradiated fuels later led to a specific classification of four groups: fission gas (Kr and Xe) and volatile isotopes (Br and I), metallic precipitates consisting of Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb and Te, oxides with Rb, Cs, Ba, Zr, Nb, Mo, and Te, and dissolved oxides containing Sr, Zr, Nb, and 8 rare earth elements. A detailed microscopy study revealed that the GBs of spent fuels were decorated with fine gas bubbles and larger ε particles of ∼30 nm in diameter, while 5–10 nm diameter particle-bubble aggregates were uniformly distributed in the interior of the irradiated UO 2 grains. The average composition of the ε particles was determined to be 40% Mo, 30% Ru, 10% Tc, 15% Pd, and 5% Rh with lattice parameters a = b = 0.273 nm and c/a = 1.61.

Analyses with improved accuracy made it possible to determine the compositions and lattice parameters of submicron- and nanometer-sized ε particles extracted from a spent fuel. More recently, the nanostructures of ε particles from a dissolved high-burnup spent fuel were examined using an aberration-corrected scanning transmission electron microscope (STEM). The results suggest that individual 10–300 nm sized ε particles consist of 1–3 nm crystallites in an amorphous matrix, and the smaller particles were found to be more Ru-rich. In addition, using Re as a surrogate for Tc in spent fuel, successful synthesis of the ε phase alloys as a potential waste form to immobilize 99Tc for permanent disposal was also reported. 99Tc is an isotope with a long half-life of 2.13 × 10 5 years and a high solubility under oxidizing conditions. A case study of samples from Oklo natural fission reactors suggested release of Mo and most of the Tc from the ε phase. The presence of the ε particles, along with other oxide precipitates, gas bubbles, and irradiation-induced structural defects, can significantly degrade the physical properties of the fuel, including thermal conductivity, swelling, creep, and melting point.

In order to predict fuel performance, it is important to investigate the critical parameters, including dose and temperature, under which precipitation occurs in the UO 2 matrix. Ion irradiation combined with thermal annealing can play a significant role in this effort. It allows for simulation of different doses at different stages of particle precipitation as well as different temperatures for different locations in the pellets. Irradiated structures may be simulated within hours to days as opposed to weeks to months or longer in a nuclear reactor to achieve a required dose. In addition, there is no radiological activation in the ion irradiated material, allowing for immediate release of samples for characterization. The new results could improve our understanding of the precipitation process as well as provide experimentally determined parameters needed for validation of model calculations. To date, there have been few reports on modeling metallic ε phases.

In this study, cubic phase CeO 2 is selected as a surrogate for UO 2. The two materials have the same cubic crystal structure (space group 225, Laue grope 11) with nearly identical lattice parameters (0.5411 and 0.5471 nm for CeO 2 and UO 2, respectively). They also have similar physical properties, such as melting temperature (2873 K for CeO 2 and 3143 K for UO 2) and thermal diffusivity. A similar precipitation process in CeO 2 and UO 2 is generally expected. In this study, formation of nanoparticles in doped CeO 2 is reported as a function of ion irradiation and thermal annealing.

The samples used in this study were prepared using pulsed laser deposition (PLD) in a custom designed off-axis system (PVD Products Inc., Wilmington, Maryland). The commercial target was a 2″ diameter and 0.125″ thick disk synthesized by Plasmaterials, Inc. (Livermore, California) with a customized composition of 95 wt% CeO 2, 2 wt% Mo, 0.75 wt% Pd, 0.25 wt% Rh, 1.5 wt% Ru, and 0.5 wt% Re. The metal dopants have weight percentages that are typical in the five-metal ε-phase particles found in spent fuels. The overall purity of the CeO 2 target material was 99.9% with impurities on the level of ppm or less except for some elements (e.g., Si) in the tens of ppm. The substrate was polycrystalline yttrium stabilized zirconia (YSZ) with a purity of 99% with Si as a major impurity, obtained from Marketech International (Port Townsend, Washington).

Utilization of the polycrystalline substrate was intended to grow polycrystalline CeO 2 films that contain GBs similar to real fuel. The lattice mismatch between CeO 2 and cubic ZrO 2 is ∼5% along their corresponding axial directions. Instead of commonly used O 2 gas during the growth of oxides, Ar gas was used to filter out possible large clusters in the plume while preventing oxidation of the dopant metals. The YSZ substrate was first heated in 10 –2 Torr oxygen environment in the deposition chamber to minimize any carbon or hydrocarbon on the surface (surface cleaning).

The chamber was then evacuated to 10 –8 Torr, and the Ar gas was flushed into the chamber to reach 10 –2 Torr. Deposition was performed at the substrate temperature of 823 K with a pulsed KrF excimer laser (248 nm) operating at 5 Hz (Coherent COMPexPro 102). The laser beam was focused onto the target with an energy density of ∼2.4 J cm –2. The typical growth rate was 0.2 nm/s. The doped CeO 2 films on YSZ were irradiated at normal incidence with 90 keV He + ions to a fluence of 4 × 10 17 He +/cm 2 at 673 K. The irradiation was performed over an area of 10 mm × 10 mm, covering the entire film surface.

A uniform irradiation over the large area was achieved using a magnetic beam rastering system. SRIM (Stopping and Range of Ions in Matter) simulation was performed to obtain the depth profiles of the displaced atoms and implanted He in CeO 2 with a bulk density ρ = 7.215 g/cm 3. The surface sputtering/monolayer collision step mode was chosen to eliminate artifacts into the sample damage by the energetic light ion in the near surface region. In the simulation, the threshold displacement energies of E d(Ce) = 56 eV and E d(O) = 27 eV were adopted. For 90 keV He + ion implantation at normal incidence, the peak disordering rate is 0.32 atomic displacements per atom (dpa) per 10 16 He +/cm 2 at 324 nm and the He profile is peaked at 372 nm with a maximum of 0.57 at.% He per 10 16 He +/cm 2, as shown in.

The ion fluence of 4 × 10 17 He +/cm 2 applied in this study corresponds to a maximum of ∼13 dpa and ∼23 atom% He at their respective peak maxima. Furnace annealing was performed for the irradiated films at 1073 and 1373 K for 10 h each in air. The thermal treatments had a ramp-up time of 2 h and a ramp-down rate of 2 °C/min in the high temperature range above ∼673 K and smaller rates at lower temperatures to allow sufficient time for the samples to cool down without abrupt quenching. After the combination of He irradiation and thermal annealing, six samples were available for characterization, including as-grown, as irradiated, annealed at 1073 and 1373 K with irradiation between depths of 200 and 400 nm, and annealed at 1073 and 1373 K without irradiation at depths larger than 500 nm. Additional unirradiated sample was also annealed at 1073 K for 10 h in air. The samples were characterized using a number of methods.

Both symmetric X-ray diffraction (XRD) and grazing-angle incidence XRD (GIXRD) were performed using a Philips X’Pert multipurpose diffractometer (MPD) with a fixed Cu anode (λ Kα = 0.154 nm) operating at 45 kV and 40 mA. The data was analyzed using Jade software (Materials Data, Inc., Livermore, California) and X-ray powder diffraction database PDF 4+. Both scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were performed on a carbon-coated sample surface using a JEOL JSM-7600FESEM. The EBSD data was collected at a specimen tilting angle of 70° with electron energy of 20 keV.

The scan was conducted over an area of 56.3 μm × 38.8 μm for the CeO 2 film and 34.1 μm × 28.0 μm for the YSZ substrate, both with a 75 nm step size. Indexing was accomplished using structural parameters for cubic zirconia, monoclinic zirconia, and cubic CeO 2. Time-of-flight secondary ion mass spectrometry (ToF-SIMS, IONTOF, GmbH, Germany) was employed to map the metal dopants in the sputtering target as well as to detect the doped elements in the CeO 2 film with the sputter beams of 1.0 keV O 2 + molecular ions over an area of 300 μm × 300 μm and of 20 keV Ar n+ ions over 200 μm × 200 μm, respectively. The analytical beam was 25 keV Bi + over 50 μm × 50 μm in both cases.

Shows an SEM image in plan view of a YSZ substrate (left side) and an as-grown film (right side). In addition to some dust particles on the surface (artifacts), extensive surface pitting is observed on both the substrate and film surfaces with varied sizes mostly on the order of 1 μm with a few as large as 10 μm. The observed pitting is a result of the film growth that follows the surface contour of the sintered YSZ substrate. The sharp border, distinguished by the varied contrast between the substrate and film at the center of is a result of a shadow mask on the sample surface during film growth near the edge of the substrate. A typical GIXRD pattern at a fixed incident angle ω = 2° for an as-grown film on YSZ substrate is shown in.

The grazing-angle geometry was chosen to maximize the diffraction intensity from the film. From, cubic CeO 2 with space group Fm3̅ m (225) is identified as a single phase (PDF#00-034–-0394(RDB), a = 0.5411 nm, ρ = 7.215 g/cm 3) in the film. All major diffraction peaks from the cubic CeO 2 are observed, suggesting that the film is polycrystalline in nature.

In spite of the grazing-angle geometry, cubic ZrO 2 (PDF#00-049-1642(RDB), a = 0.5128 nm, ρ = 6.069 g/cm 3), and monoclinic ZrO 2 (PDF#00-037-1484(RDB), a = 0.5313 nm, b = 0.5213 nm, c = 0.5147 nm, ρ = 5.817 g/cm 3) from the YSZ substrate also appear, as indicated in. The existence of the dual-phase nature of the substrate has been confirmed by symmetric X-ray diffraction (XRD) from a blank as-received YSZ substrate (Figure S2 in the ). Other crystalline phases, including those potentially from metal dopants, are not visible from the diffraction pattern. Since the CeO 2 (111) peak is overlapped with the monoclinic ZrO 2 (1̅11) peak, the well-resolved CeO 2 (220) peak at 2θ = 47.78° is chosen to estimate the average crystallite size in the film assuming that the lattice strain effect is small. The value is determined to be 23.6 nm based on the Scherrer formula.

Rietveld L-1995 Re Rietveld-software For Mac Os X

It should be noted that the average crystallite size should not be larger than the mean grain size (defined below) because the former is the average interplanar distance between the adjacent planar defects that discontinue the lattice periodicity in a crystalline grain. A large average crystallite size indicates a low defect concentration in the individual grains.