The effects of an aggressive environment on the subcritical crack growth of a continuous-fiber ceramic composite

Time-dependent crack growth measurements of ceramic composites in aggressive environments are being conducted on materials consisting of CVI SiC reinforced with Nicalon fibers (SiC/SiC{sub f}) having C and BN fiber-matrix interfaces. Crack velocities are determined as a function of applied stress intensity. Results have been obtained for crack velocity-stress intensity relationships in pure Ar and in Ar plus 2000 ppM O{sub 2} atmospheres at 1100{degrees}C. A 2D micromechanics model is used to represent the time-dependence of observed crack bridging events and is able to rationalize the observed phenomena

Characterization of U-Mo Foils for AFIP-7

Twelve AFIP in-process foil samples, fabricated by either Y-12 or LANL, were shipped from LANL to PNNL for potential characterization using optical and scanning electron microscopy techniques. Of these twelve, nine different conditions were examined to one degree or another using both techniques. For this report a complete description of the results are provided for one archive foil from each source of material, and one unirradiated piece of a foil of each source that was irradiated in the Advanced Test Reactor. Additional data from two other LANL conditions are summarized in very brief form in an appendix. The characterization revealed that all four characterized conditions contained a cold worked microstructure to different degrees. The Y-12 foils exhibited a higher degree of cold working compared to the LANL foils, as evidenced by the highly elongated and obscure U-Mo grain structure present in each foil. The longitudinal orientations for both of the Y-12 foils possesses a highly laminar appearance with such a distorted grain structure that it was very difficult to even offer a range of grain sizes. The U-Mo grain structure of the LANL foils, by comparison, consisted of a more easily discernible grain structure with a mix of equiaxed and elongated grains. Both materials have an inhomogenous grain structure in that all of the characterized foils possess abnormally coarse grains

Structural Ceramic Composites for Nuclear Applications

A research program has been established to investigate fiber reinforced ceramic composites to be used as control rod components within a Very High Temperature Reactor. Two candidate systems have been identified, carbon fiber reinforced carbon (Cf/C) and silicon carbide fiber reinforced silicon carbide (SiCf/SiC) composites. Initial irradiation stability studies to determine the maximum dose for each composite type have been initiated within the High Flux Isotope Reactor at Oak Ridge National Laboratory. Test samples exposed to 10 dpa irradiation dose have been completed with future samples to dose levels of 20 and 30 dpa scheduled for completion in following years. Mechanical and environmental testing is being conducted concurrently at the Idaho National Laboratory and at Pacific Northwest National Laboratory. High temperature test equipment, testing methodologies, and test samples for high temperature (up to 1600º C) tensile strength and long duration creep studies have been established. Specific attention was paid to the architectural fiber preform design as well as the materials used in construction of the composites. Actual testing of both tubular and flat, "dog-bone" shaped tensile composite specimens will begin next year. Since there is no precedence for using ceramic composites within a nuclear reactor, ASTM standard test procedures will be established from these mechanical and environmental tests. Close collaborations between the U.S. national laboratories and international collaborators (i.e. France and Japan) are being forged to establish both national and international test standards to be used to qualify ceramic composites for nuclear reactor applications

Materials characterization of cermet anodes tested in a pilot cell

Cermet anodes were evaluated as nonconsumable substitutes for carbon anodes using a pilot-scale reduction cell at the Reynolds Manufacturing Technology Laboratory. After pilot cell testing, tile anodes were subjected to extensive materials characterization and physical properties measurements at the Pacific Northwest Laboratory. Significant changes in the composition of the cermet anodes were observed including the growth of a reaction layer and penetration of electrolyte deep into the cermet matrix. Fracture strength and toughness were measured as a function of temperature and the ductile-brittle transition wasreduced by 500C following pilot cell testing. These results imply difficulties with anode material and control of operating conditions in the pilot cell, and suggest that additional development work be performed before the cermet anodes are used in commercial reduction cells. The results also highlight specific fabrication and operational considerations that should be addressed in future testing

Characterization of U-Mo Foils for AFIP-7

Twelve AFIP in-process foil samples, fabricated by either Y-12 or LANL, were shipped from LANL to PNNL for potential characterization using optical and scanning electron microscopy techniques. Of these twelve, nine different conditions were examined to one degree or another using both techniques. For this report a complete description of the results are provided for one archive foil from each source of material, and one unirradiated piece of a foil of each source that was irradiated in the Advanced Test Reactor. Additional data from two other LANL conditions are summarized in very brief form in an appendix. The characterization revealed that all four characterized conditions contained a cold worked microstructure to different degrees. The Y-12 foils exhibited a higher degree of cold working compared to the LANL foils, as evidenced by the highly elongated and obscure U-Mo grain structure present in each foil. The longitudinal orientations for both of the Y-12 foils possesses a highly laminar appearance with such a distorted grain structure that it was very difficult to even offer a range of grain sizes. The U-Mo grain structure of the LANL foils, by comparison, consisted of a more easily discernible grain structure with a mix of equiaxed and elongated grains. Both materials have an inhomogenous grain structure in that all of the characterized foils possess abnormally coarse grains

Materials Degradation and Detection (MD2): Deep Dive Final Report

An effort is underway at Pacific Northwest National Laboratory (PNNL) to develop a fundamental and general framework to foster the science and technology needed to support real-time monitoring of early degradation in materials used in the production of nuclear power. The development of such a capability would represent a timely solution to the mounting issues operators face with materials degradation in nuclear power plants. The envisioned framework consists of three primary and interconnected “thrust” areas including 1) microstructural science, 2) behavior assessment, and 3) monitoring and predictive capabilities. A brief state-of-the-art assessment for each of these core technology areas is discussed in the paper

Financial Literacy in the US: A Robustness Check of the Lusardi-Mitchell Questions

This study tested the reliability of financial literacy measures used in previous studies based on a set of items referred to as the "Lusardi-Mitchell questions". Using a sample of 514 American adults that completed a questionnaire with fifty financial literacy items, the authors compared (1) the results from a set of multivariate regression analyses that used the Lusardi-Mitchell questions to investigate the explanatory power of financial literacy on different financial behaviors, with (2) results that used alternative financial literacy measures that differ by the topic of the items or the number of the items. The study also addressed the reverse-causality issue in an attempt to develop a measure of financial literacy that is not biased by the learning-by-doing effect that could justify the hypothesis that individual financial knowledge is the consequence (and not the cause) of financial behaviors. Results suggest that the Lusardi-Mitchell questions provide a measure of financial literacy that is close to the results obtained by the use of more sophisticated measures. In addition, the results supported the hypothesis that the main relationship between financial literacy and financial behavior sees the former affect the latter, denying the reverse causality hypothesis

The effects of an aggressive environment on the subcritical crack growth of a continuous-fiber ceramic composite

Time-dependent crack growth measurements of ceramic composites in aggressive environments are being conducted on materials consisting of CVI SiC reinforced with Nicalon fibers (SiC/SiC[sub f]) having C and BN fiber-matrix interfaces. Crack velocities are determined as a function of applied stress intensity. Results have been obtained for crack velocity-stress intensity relationships in pure Ar and in Ar plus 2000 ppM O[sub 2] atmospheres at 1100[degrees]C. A 2D micromechanics model is used to represent the time-dependence of observed crack bridging events and is able to rationalize the observed phenomena