Model polyamide thin films were prepared through a controlled interfacial polymerization route known as molecular layer by layer (mLbL). Films were synthesized directly onto quartz crystals and subjected to halogenated aqueous environments that are known to cause degradation of the amide network. A quartz crystal microbalance (QCM) was used as the detection platform to ascertain mass loss due to degradation in real time. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements were also performed at various stages of the degradation sequence to elucidate the chemical and morphological changes at the surfaces respectively. Appropriate strategies for accurately comparing material degradation resistance are proposed along with modifications to the crosslinked polyamide chemistry to produce more halogen tolerant polymeric surfaces.

Fiber optic temperature sensors are used in a variety of harsh environment applications. We have explored use of such temperature sensors in commercial gas turbines to measure the temperature at various regions of interest within the turbine system. More specifically, fiber optic temperature rakes were designed and installed on a commercial gas turbine under full load conditions. This work will focus on failure mechanisms observed at multiple length scales that impact the performance of high temperature optical fiber sensors. It was found that Au-coated silica fibers, which are a standard in the industry, undergo various failure modes when subjected to combinations of high temperature and high vibration. More specifically, the Au coating became soft/ductile as the temperature is increased. We also observed that the Au coating was not well bonded to the silica fiber, as expected since there were no adhesion layers present. These effects led to significant damage of the fiber optic under high vibrations. We also found that vibrations from the gas turbine coupled into fundamental modes of the fiber optic probe assembly, which were analyzed by detailed dynamic mechanical analysis. This led to the fiber impacting the internal wall of the probe assembly, which caused further damage and failure of the fiber and the Au coating. The silica fibers returned from the field also exhibited significant twisting throughout most of their length. This suggests the fibers reached temperatures above their strain point (about 1000 C for pure silica glass), which is explained by either a) the strain point had been significantly reduced by the presence of the Ge dopant, or b) the temperature was higher than expected in the gas turbine exhaust region. It was also hypothesized that complex anelastic effects may play a role under the high temperature, high vibration environment experienced by the probes. Detailed structural analysis of the fiber optic temperature sensors by scanning electron microscopy, ToF-SIMS, and X-ray microscopy will be presented to corroborate the above simulations and proposed damage mechanisms. Finally, we note that the fiber Bragg gratings (FBG) present within the temperature probes provided promising temperature data, and were in fact not damaged/erased by the high temperature environment.

Accurately predicting fission gas releases (FGR) from high burn-up fuels during off-normal conditions, such as a loss-of-coolant accident (LOCA), is an important, major challenge. A significant step forward would be to identify and assess the basic mechanisms causing this FGR. A helpful way of better understanding these basic mechanisms is to separate these effects and to perform irradiations on materials simulating the nuclear fuel. Mesoporous or dense CeO2 and UO2 samples (with thin film geometry) were selected for these studies as materials representative of irradiated fuels. A basic mechanism to obtain a better understanding of FGR is described and a new methodology using thin film samples is developed to test the validity of this mechanism.

Thorium has long been considered a possible source of fuel for use in power generating nuclear reactors. While much attention and interest have focused on thorium oxide, metallic thorium alloys were investigated in the past as fuel candidates for fast and thermal breeder reactors. In this work, thorium’s two solid allotropes (face centered cubic α and body centered cubic β) are modelled using Density Functional Theory (DFT). The Vienna Ab Initio Simulation Package (VASP) is used to determine structural and elastic properties, as well as the density of states. The Voigt-Reuss-Hill approximations are used to predict the phases’ bulk moduli, shear moduli, Young’s moduli, and Poisson’s ratios. Four exchange-correlation potentials are used and compared. These are the LDA, the PBE-GGA, the RPBE, and the PBEsol. All potentials showed relatively good accuracy when predicting the lattice constant. The RPBE was the most accurate, slightly over predicting by < 0.2%. The RPBE and the PBE-GGA were the most accurate in predicting elastic properties, performing almost equally, while slightly over predicting most values.

Periodic mesoporous materials possess high surface to volume ratio and nano-scale sized pores, making them potential candidates for heterogeneous catalysis, ion exchange, gas sensing and other applications. In this study, we use in situ small angle x-ray scattering (SAXS) and molecular dynamics (MD) simulations to investigate the mechanical and hydrothermal stability properties of periodic mesoporous SBA-15 silica and SBA-15 type aluminosilica (Al-SBA-15) to extreme conditions. The mesoporous SBA-15 silica and Al-SBA-15 aluminosilica possess amorphous frameworks and have similar pore size distribution (pore size ∼9-10 nm). The in situ SAXS measurements were made at the B1 beamline, at the Cornell High Energy Synchrotron Source (CHESS). The mesoporous SBA-15 silica and Al-SBA-15 aluminosilica specimens were loaded in a diamond anvil cell (DAC) for pressure measurements, and, separately, with water in the DAC for hydrothermal measurements to high P-T conditions (to 255 °C and ∼ 114 MPa). Analyses of the pressure-dependent SAXS data show that the mesoporous Al-SBA-15 aluminosilica is substantially more mechanically stable than the SBA-15 silica. Hydrothermal measurements show a small net swelling of the framework at elevated P-T conditions, due to dissolution of water into the pore walls. Under elevated P-T conditions, the Al-SBA-15 aluminosilica shows significantly greater hydrothermal stability than the SBA-15 silica. Our MD simulations show that the bulk modulus value of periodic mesoporous SBA-15 silica varies exponentially with percentage porosity. Molecular dynamics simulations are being made in order to better understand how the pore architecture and the chemical composition of the host structure govern the stability properties of the mesoporous materials.

TlBr crystals have superior radiation detection properties; however, their properties degrade in the range of hours to weeks when an operating electrical field is applied. To account for this rapid degradation using the widely-accepted vacancy migration mechanism, the vacancy concentration must be orders of magnitude higher than any conventional estimates. The present work has incorporated a new analytical variable charge model in molecular dynamics (MD) simulations to examine the structural changes of materials under electrical fields. Our simulations indicate that dislocations in TlBr move under electrical fields. This discovery can lead to new understanding of TlBr aging mechanisms under external fields.

Under high burnup UO2 fuel pellets can experience high burnup structure (HBS) at the rim also known as rim effect. The HBS is exceptionally porous with fine grain sizes. HBS increases the swelling further than it would have achieved at a larger grain size. A theoretical swelling model is used in conjunction with a grain subdivision simulation to calculate the swelling of UO2. In UO2 the nucleation sites are at vacancies and the bubbles are concentrated at grain boundaries. Vacancies are created due to irradiation and gas diffusion is dependent on vacancy migration. In addition to intragranular bubbles, there are intergranular bubbles at the grain boundaries. Over time as intragranular bubbles and gas atoms accumulate on the grain boundaries, the intergranular bubbles grow and cover the grain faces. Eventually they grow into voids and interconnect along the grain boundaries, which can lead to fission gas release when the interconnection reaches the surface. This is known as the saturation point. While the swelling model used does not originally incorporate a changing grain size, the simulation allows for more accurate swelling calculations by introducing a fractional HBS based on the temperature and burnup of the pellet. The fractional HBS is introduced with a varying grain size. Our simulations determine the level of swelling and saturation as a function of burnup by combining an independent model and simulation to obtain a more comprehensive model.

Oxidation behavior of alumina forming ferritic stainless steel (FeCrAl type stainless steels) grades APM and APMT, which are candidate alloys for fuel cladding, was studied using thermogravimetric analysis under dry air condition, and compared to that of ZIRLO®. In addition to the dry air condition, we also studied the high temperature oxidation behavior of APM and APMT under 100% steam condition in order to compare the effect of environment on the oxidation behavior of these alloys. APM and APMT showed an excellent oxidation resistance at high temperatures compared to ZIRLO® under dry air condition due to a stable Al2O3 oxide scale formed at the surface. Under steam condition, the oxidation rate of APM and APMT was found to be higher compared to that under the dry air condition.

We show that alloying with rare earth metals (REMs) can dramatically improve the machineability of a range of titanium alloys, even though the REM is not incorporated in the alloy matrix. The mechanism for this is that under cutting, shear bands are formed within which the nano-precipitates of REM are shear mixed. This lowers the melting point such that the mechanism of deformation changes from dislocation mechanism to localised amorphisation and shear softening. The material then fractures along the thin, amorphous shear-band. Outside the shear band, the REM remains as precipitates. The new alloys have similar mechanical properties and biocompatibility to conventional materials.

A new approach for adjusting molecular dynamics results on UO2 thermal conductivity to include phonon-spin scattering has been used to improve calculations on U x Pu1−x O2 and U x Th1−x O2. We demonstrate that by including spin scattering a strong asymmetry as a function of uranium actinide fraction, x, is obtained. Greater degradation is shown for U x Th1−x O2 than U x Pu1−x O2. Minimum thermal conductivities are predicted at U0.97Pu0.03O2 and U0.58Th0.42O2, although the degradation in U x Pu1−x O2 is negligible relative to pure UO2.

A Kinetic Monte Carlo simulation, using a modified version of the SPPARKS code, of simple defects and complex vacancy clusters was run on a bcc lattice. In this simulation the complexity of void formation was varied by introducing a detachment rate for individual vacancies leaving the void and either treating this value as constant for all size voids or having this value be dependent on the size of the void. Molecular Dynamics simulations were used to determine the binding energies of vacancies for voids of varying size. The simulation was then run over long time periods to determine the number of defects in the simulation under irradiation conditions. It was found that the additional complexity of size dependent void detachment rates had little effect on the defect concentrations and thus a constant barrier should be sufficient for simulations of voids in bcc metals.

In support of a recent surge in research to develop an accident tolerant reactor, accident tolerant fuels and cladding candidates are being investigated. Relative motion between the fuel rods and fuel assembly spacer grids can lead to excessive fuel rod wear and, in some cases, to fuel rod failure. Based on industry data, grid-to-rod-fretting (GTRF) has been the number one cause of fuel failures within the U.S. pressurized water reactor (PWR) fleet, accounting for more than 70% of all PWR leaking fuel assemblies. APMT, an Fe-Cr-Al steel alloy, is being examined for the I2S-LWR project as a possible alternative to conventional fuel cladding in a nuclear reactor due to its favorable performance under LOCA conditions. Tests were performed to examine the reliability of the cladding candidate under simulated fretting conditions of a pressurized water reactor (PWR). The contact is simulated with a rectangular and a cylindrical specimen over a line contact area. A combination of SEM analysis and wear & work rate calculations are performed on the samples to determine their performance and wear under fretting. While APMT can perform favorably in loss of coolant accident scenarios, it also needs to perform well when compared to Zircaloy-4 with respect to fretting wear.