Characterization of chromia scales formed in supercritical carbon dioxide

Abstract

Initial experimental work at 700°–800 °C is in progress to develop a lifetime model for supercritical CO₂ (sCO₂) compatibility for a 30-year lifetime of a >700 °C concentrated solar power system. Nickel-based alloys 282, 740H and 625 and Fe-based alloy 25 are being evaluated in 500-h cycles at 1 and 300 bar, and 10-h cycles in 1 bar industrial grade CO₂. The alloys showed similar low rates of oxidation in 1 and 300 bar CO₂ in 500-h cycles at 750 °C. However, in 10-h cycles, alloy 25 showed accelerated attack at 700° and 750 °C. Transmission electron microscopy scale cross-sections on alloy 25 after 1000 h at 700 °C in sCO₂ and in air only showed a small row of carbides beneath the scale in the former environment. Similar characterisation was performed on alloys 625 and 282 after sCO₂ exposure at 750 °C.     

Nanohertz frequency determination for the gravity probe B high frequency superconducting quantum interference device signal

In this paper, we present a method to measure the frequency and the frequency change rate of a digital signal. This method consists of three consecutive algorithms: frequency interpolation, phase differencing, and a third algorithm specifically designed and tested by the authors. The succession of these three algorithms allowed a 5 parts in 10(10) resolution in frequency determination. The algorithm developed by the authors can be applied to a sampled scalar signal such that a model linking the harmonics of its main frequency to the underlying physical phenomenon is available. This method was developed in the framework of the gravity probe B (GP-B) mission. It was applied to the high frequency (HF) component of GP-B's superconducting quantum interference device signal, whose main frequency f(z) is close to the spin frequency of the gyroscopes used in the experiment. A 30 nHz resolution in signal frequency and a 0.1 pHz/s resolution in its decay rate were achieved out of a succession of 1.86 s-long stretches of signal sampled at 2200 Hz. This paper describes the underlying theory of the frequency measurement method as well as its application to GP-B's HF science signal.     

Expression and Functional Contribution of Different Organic Cation Transporters to the Cellular Uptake of Doxorubicin into Human Breast Cancer and Cardiac Tissue

Doxorubicin is a frequently used anticancer drug to treat many types of tumors, such as breast cancer or bronchial carcinoma. The clinical use of doxorubicin is limited by its poorly predictable cardiotoxicity, the reasons of which are so far not fully understood. The drug is a substrate of several efflux transporters such as P-gp or BCRP and was recently reported to be a substrate of cation uptake transporters. To evaluate the potential role of transporter proteins in the accumulation of doxorubicin at its site of action (e.g., mammary carcinoma cells) or adverse effects (e.g., heart muscle cells), we studied the expression of important uptake and efflux transporters in human breast cancer and cardiac tissue, and investigated the affinity of doxorubicin to the identified transporters. The cellular uptake studies on doxorubicin were performed with OATP1A2*1, OATP1A2*2, and OATP1A2*3-overexpressing HEK293 cells, as well as OCT1-, OCT2-, and OCT3- overexpressing MDCKII cells. To assess the contribution of transporters to the cytotoxic effect of doxorubicin, we determined the cell viability in the presence and absence of transporter inhibitors in different cell lines. Several transporters, including P-gp, BCRP, OCT1, OCT3, and OATP1A2 were expressed in human heart and/or breast cancer tissue. Doxorubicin could be identified as a substrate of OCT1, OCT2, OCT3, and OATP1A2. The cellular uptake into cells expressing genetic OATP1A2 variants was markedly reduced and correlated well with the increased cellular viability. Inhibition of OATP1A2 (naringin) and OCT transporters (1-methyl-4-phenylpyridinium) resulted in a significant decrease of doxorubicin-mediated cytotoxicity in cell lines expressing the respective transporters. Similarly, the excipient Cremophor EL significantly inhibited the OCT1-3- and OATP1A2-mediated cellular uptake and attenuated the cytotoxicity of doxorubicin. In conclusion, genetic and environmental-related variability in the expression and function of these transporters may contribute to the substantial variability seen in terms of doxorubicin efficacy and toxicity.     

Microstructural development in irradiated U-7Mo/6061 Al alloy matrix dispersion fuel

A U-7Mo alloy/6061 Al alloy matrix mini-dispersion fuel plate was irradiated in the Advanced Test Reactor and then examined using optical metallography and scanning electron microscopy to characterize the developed microstructure. Results were compared to the microstructure of the as-fabricated dispersion fuel to identify changes that occurred during irradiation. The layer that formed on the surface of the fuel U-7Mo particles during fuel plate fabrication exhibits stable irradiation performance as a result of the ∼0.88 wt% Si present in the fuel meat matrix. During irradiation, the pre-formed interaction layer changed very little in thickness and composition. The overall irradiation performance of the fuel plate to moderate power and burnup was considered excellent.     

Transmission electron microscopy characterization of irradiated U–7Mo/Al–2Si dispersion fuel

The plate-type dispersion fuels, with the atomized U(Mo) fuel particles dispersed in the Al or Al alloy matrix, are being developed for use in research and test reactors worldwide. It is found that the irradiation performance of a plate-type dispersion fuel depends on the radiation stability of the various phases in a fuel plate. Transmission electron microscopy was performed on a sample (peak fuel mid-plane temperature ～109 °C and fission density ～4.5 × 10²⁷ f m^?3) taken from an irradiated U–7Mo dispersion fuel plate with Al–2Si alloy matrix to investigate the role of Si addition in the matrix on the radiation stability of the phase(s) in the U–7Mo fuel/matrix interaction layer. A similar interaction layer that forms in irradiated U–7Mo dispersion fuels with pure Al matrix has been found to exhibit poor irradiation stability, likely as a result of poor fission gas retention. The interaction layer for both U–7Mo/Al–2Si and U–7Mo/Al fuels is observed to be amorphous. However, unlike the latter, the amorphous layer for the former was found to effectively retain fission gases in areas with high Si concentration. When the Si concentration becomes relatively low, the fission gas bubbles agglomerate into fewer large pores. Within the U–7Mo fuel particles, a bubble superlattice ordered as fcc structure and oriented parallel to the bcc metal lattice was observed where the average bubble size and the superlattice constant are 3.5 nm and 11.5 nm, respectively. The estimated fission gas inventory in the bubble superlattice correlates well with the fission density in the fuel.     

Diffusion Barrier Selection from Refractory Metals (Zr,Mo and Nb) Via Interdiffusion Investigation for U-Mo RERTR Fuel Alloy

U-Mo alloys are being developed as low enrichment monolithic fuel under the Reduced Enrichment for Research and Test Reactor (RERTR) program. Diffusional interactions between the U-Mo fuel alloy and Al-alloy cladding within the monolithic fuel plate construct necessitate incorporation of a barrier layer. Fundamentally, a diffusion barrier candidate must have good thermal conductivity, high melting point, minimal metallurgical interaction, and good irradiation performance. Refractory metals, Zr, Mo, and Nb are considered based on their physical properties, and the diffusion behavior must be carefully examined first with U-Mo fuel alloy. Solid-to-solid U-10 wt.%Mo versus Mo, Zr, or Nb diffusion couples were assembled and annealed at 600, 700, 800, 900 and 1000 °C for various times. The interdiffusion microstructures and chemical composition were examined via scanning electron microscopy and electron probe microanalysis, respectively. For all three systems, the growth rate of interdiffusion zone were calculated at 1000, 900 and 800 °C under the assumption of parabolic growth, and calculated for lower temperature of 700, 600 and 500 °C according to Arrhenius relationship. The growth rate was determined to be about 10³ times slower for Zr, 10⁵ times slower for Mo and 10⁶ times slower for Nb, than the growth rates reported for the interaction between the U-Mo fuel alloy and pure Al or Al-Si cladding alloys. Zr, however was selected as the barrier metal due to a concern for thermo-mechanical behavior of UMo/Nb interface observed from diffusion couples, and for ductile-to-brittle transition of Mo near room temperature.     

Phase characteristics of a number of U-Pu-Am-Np-Zr metallic alloys for use as fast reactor fuels

Metallic fuel alloys consisting of uranium, plutonium, and zirconium with minor additions of americium and neptunium are under evaluation for potential use to transmute long-lived transuranic actinide isotopes in fast reactors. A series of test designs for the Advanced Fuel Cycle Initiative (AFCI) have been irradiated in the Advanced Test Reactor (ATR), designated as the AFC-1 and AFC-2 designs. Metal fuel compositions in these designs have included varying amounts of U, Pu, Zr, and minor actinides (Am, Np). Investigations into the phase behavior and relationships based on the alloy constituents have been conducted using X-ray diffraction and differential thermal analysis. Results of these investigations, along with proposed relationships between observed behavior and alloy composition, are provided. In general, observed behaviors can be predicted by a ternary U-Pu-Zr phase diagram, with transition temperatures being most dependent on U content. Furthermore, the enthalpy associated with transitions is strongly dependent on the as-cast microstructural characteristics.     

Staphylococcus epidermidis sepsis in the intensive care nursery: a characterization of risk associations in infants < 1,000 g

We undertook to determine Staphylococcus epidermidis colonization patterns and risks of sepsis in a cohort of 82 consecutive intensive care nursery admissions (birth weight 1,285 +/- 57 g), with 24 infants weighing < 1,000 g at birth. Colonization was determined by skin and stool cultures collected at three time points. Multiple neonatal variables were classified into three intervals preceding the time of sample collection including the occurrence of S. epidermidis sepsis. 16 infants (20%) developed S. epidermidis sepsis. 81% of these episodes occurred in infants < 1,000 g. Skin colonization was nearly universal at all sampling points. Rectal colonization was 63.6% initially (10 +/- 0.4 days), then declined to 32% by the third sample (37 +/- 0.4 days). Neither prevalence of skin nor rectal colonization influenced the incidence of sepsis significantly. Statistically significant risk associations for sepsis for the entire intensive care nursery population included: low birth weight, gestational age, presence of a central line, and delayed feeding. For infants < 1,000 g the occurrence of sepsis during the second study time period (54% of the episodes) was associated with preceding steroid exposure. During the third study time period, birth weight and delayed attainment of full enteral feeds showed a statistically significant association with sepsis. We conclude that infants < 1,000 g are at an increased risk of S. epidermidis sepsis. Extreme immaturity, steroid therapy, and prolonged hyperalimentation are all significant risk associations.