Metal Oxidation Kinetics and the Transition from Thin to Thick Films

We report an investigation of growth kinetics and transition from thin to thick films during metal oxidation. In the thin film limit (< 20 nm), Cabrera and Mott's theory is usually adopted by explicitly considering ionic drift through the oxide in response to electric fields, where the growth kinetics follow an inverse logarithmic law. It is generally accepted that Wagner's theory, involving self-diffusion, is valid only in the limit of thick film regime and leads to parabolic growth kinetics. Theory presented here unifies the two models and provides a complete description of oxidation including the transition from thin to thick film. The range of validity of Cabrera and Mott's theory and Wagner's theory can be well defined in terms of the Debye-Huckel screening length. The transition from drift-dominated ionic transport for thin film to diffusion-dominated transport for thick film is found to strictly follow the direct logarithmic law that is frequently observed in many experiments

Microstructural and microchemical mechanisms controlling intergranular stress corrosion cracking in light-water-reactor systems

This review paper examines mechanisms controlling IGSCC in selected LWR components. Emphasis is placed on identifying material microstructures and microchemistries which promote susceptibility to premature failure. Two important examples are evaluated in some detail: stainless steel pipe cracking and primary-side SCC of alloy 600 steam generator tubing. In each case, grain boundary segregation and precipitation phenomena in these materials are reviewed and assessed relative to the mechanisms of IGSCC. This paper summarizes materials presented at the 1993 International Summer School on the Fundamentals of Radiation Damage held at the University of Illinois. A more comprehensive overview of SCC mechanisms and LWR examples was provided at the school, but will not be included in this article. Microstructural and microchemical aspects controlling IGSCC described here serve as a lead-in to the following paper focussing on how irradiation influences SCC resistance of reactor core components.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31289/1/0000195.pd

Effects of irradiation on intergranular stress corrosion cracking

Intergranular stress corrosion cracking (IASCC) is a pervasive and generic problem in current light water reactor and advanced reactor designs that can lead to widespread component failure. IASCC is believed to be due to either to changes in the grain boundary composition, the microstructure or the water chemistry and corrosion potential. Of greatest interest are the changes in composition and microstructure since IASCC exhibits a well-defined, although not invariant, dose threshold. Changes in grain boundary composition are a result of radiation-induced segregation (RIS) and result in enrichment of nickel, depletion of chromium as well as changes in the impurity element compositions at the grain boundary. Although the basic theory of RIS is believed to be understood, quantitative descriptions of observed changes are not yet possible and hinder the correlation between RIS and IASCC. Changes in the microstructure are intimately linked to the strength and ductility of the irradiated alloy and strong correlations between IASCC and irradiated yield strength have been found. However, a fundamental understanding of the deformation mechanisms and the way in which deformation is coupled to IG cracking in alloys irradiated under LWR conditions (250-360[deg]C, 1-5 dpa) is lacking. Finally, although radiation is known to affect IGSCC through changes in water chemistry and corrosion potential, it is not a necessary condition. Overshadowing and slowing progress on this important problem is a lack of well-defined-data from properly irradiated and properly characterized materials, due principally to inherent experimental and financial difficulties. As such, the specific mechanism(s) of IASCC remain unknown.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31287/1/0000193.pd

Non-Coding RNA Sequencing of Equine Endometrium During Maternal Recognition of Pregnancy.

Maternal recognition of pregnancy (MRP) in the mare is not well defined. In a non-pregnant mare, prostaglandin F2α (PGF) is released on day 14 post-ovulation (PO) to cause luteal regression, resulting in loss of progesterone production. Equine MRP occurs prior to day 14 to halt PGF production. Studies have failed to identify a gene candidate for MRP, so attention has turned to small, non-coding RNAs. The objective of this study was to evaluate small RNA (&lt;200 nucleotides) content in endometrium during MRP. Mares were used in a cross-over design with each having a pregnant and non-mated cycle. Each mare was randomly assigned to collection day 11 or 13 PO (n = 3/day) and endometrial biopsies were obtained. Total RNA was isolated and sequencing libraries were prepared using a small RNA library preparation kit and sequenced on a HiSeq 2000. EquCab3 was used as the reference genome and DESeq2 was used for statistical analysis. On day 11, 419 ncRNAs, representing miRNA, snRNA, snoRNA, scaRNA, and vaultRNA, were different between pregnancy statuses, but none on day 13. Equine endometrial ncRNAs with unknown structure and function were also identified. This study is the first to describe ncRNA transcriptome in equine endometrium. Identifying targets of these ncRNAs could lead to determining MRP

Quantitative analysis of radiation-induced grain-boundary segregation measurements

Radiation-induced and precipitation-induced grain-boundary segregation profiles are routinely measured by scanning-transmission electron microscopy using energy-dispersive X-ray spectroscopy (STEM-EDS). However, radiation-induced grain-boundary segregation (RIS) profiles achieved at low and moderate temperatures are exceedingly narrow, typically less than 10 nm full width at half maximum. Since the instrumental spatial resolution can be a significant fraction of this value, the determination of grain boundary compositions poses a formidable challenge. STEM-EDS and Auger electron spectroscopy (AES) measurements are reported, performed on controlled-purity alloys of type 304L stainless steel irradiated with 3.4 MeV protons to 1 displacement per atom at 400[deg]C. Because of statistical noise and the practical lower limit on the step size in STEM, deconvolution of the measured data does not yield physical results. An alternative analysis of STEM data is presented. Numerical calculations of RIS profiles are convoluted with the instrumental broadening function and modified iteratively to fit the data, yielding a "best estimate" profile. This "best estimate" is convoluted with the Auger intensity profile to yield a simulated AES measurement, which is compared with the actual AES measurement to provide an independent test of the validity of the "best estimate". For impurities with a narrow segregation profile and an Auger electron escape depth of one monolayer, a combination of STEM and AES data allows a determination of the width of the segregated layer. It is found that, in an ultrahigh-purity alloy doped with P, the latter is essentially contained in a single monolayer.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31453/1/0000374.pd

Nondestructive and Destructive Examination Studies on Removed-from-Service Control Rod Drive Mechanism Penetrations

Studies conducted at the Pacific Northwest National Laboratory (PNNL) in Richland, Washington, focused on assessing the effectiveness of nondestructive examination (NDE) techniques for inspecting control rod drive mechanism (CRDM) nozzles and J-groove weldments. The primary objectives of this work are to provide information to the U.S. Nuclear Regulatory Commission (NRC) on the effectiveness of NDE methods as related to the in-service inspection of CRDM nozzles and J-groove weldments and to enhance the knowledge base of primary water stress corrosion cracking (PWSCC) through destructive characterization of the CRDM assemblies. Two CRDM assemblies were removed from service, decontaminated, and then used in a series of NDE and destructive examination (DE) measurements; this report addresses the following questions: 1) What did each NDE technique detect? 2) What did each NDE technique miss? 3) How accurately did each NDE technique characterize the detected flaws? 4) Why did the NDE techniques perform or not perform? Two CRDM assemblies including the CRDM nozzle, the J-groove weld, buttering, and a portion of the ferritic head material were selected for this study. This report focuses on a CRDM assembly that contained suspected PWSCC, based on in-service inspection data and through-wall leakage. The NDE measurements used to examine the CRDM assembly followed standard industry techniques for conducting in-service inspections of CRDM nozzles and the crown of the J-groove welds and buttering. These techniques included eddy current testing (ET), time-of-flight diffraction ultrasound, and penetrant testing. In addition, laboratory-based NDE methods were employed to conduct inspections of the CRDM assembly with particular emphasis on inspecting the J-groove weld and buttering. These techniques included volumetric ultrasonic inspection of the J-groove weld metal and visual testing via replicant material of the J-groove weld. The results from these NDE studies were used to guide the development of the destructive characterization plan. The NDE studies found several crack-like indications. The NDE and DE studies determined that one of these was a through-weld, radially oriented PWSCC crack in the wetted surface of the J-groove weld, located at the transition point between the weld and the buttering. The crack was 6 mm long on the surface and quickly grew to 25 mm long at a depth of 8 mm, covering the length of the weld between the penetration tube and the carbon steel. The NDE studies found that only ET was able to detect the through-weld crack. The crack was oriented poorly for the ultrasonic testing and was too tight for accurate dye penetrant testing or visual testing. The ET voltage response of the through-wall crack was 30% of the response from a deep electrical discharge machined notch. Destructive examination showed the crack is PWSCC and that it initiated on the wetted surface, grew and expanded through the weld metal, and exited into the annulus. The crack was branched and discontinuous along its length