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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Reply to: How much does corrosion of nuclear waste matrices matter

We thank Mallants and Chapman for their comments on our recent work and are mostly in agreement. However, while the applicability of the results to different environmental conditions requires more attention, we maintain that this study could be relevant even to repositories in saturated reduced environments. Oxidants other than oxygen might be present in anoxic groundwater repository environments and could also trigger the corrosion mechanism we described. .

Guo, Xiaolei↗
DOE OSTI2020

Milestone 2.7: Evaluation of Techniques for the Measurement of Molecular Hydrogen Gas in Helium Matrices

Published data by Task 2 (Oxyhydroxide Layer Radiolytic Gas Generation Resolution) for Technical Considerations and Challenges for Extended (>50 yrs) Dry Storage of Aluminum Cladded Spent Nuclear Fuel (ASNF), demonstrated that radiolytic molecular hydrogen (H2) production from gamma irradiated aluminum alloy 1100 (Al-1100) coupons exhibited significant dependence on backfill gaseous environment conditions: air, due to the presence of oxygen, completely inhibited H2 production; nitrogen promoted H2 production; and argon yielded more H2 than nitrogen environments. The concern here is that helium has been proposed as the backfill gas for extended storage and is more inert than argon, which may translate into significantly more H2 production than current Task 3 argon-based models predict. However, the measurement of H2 in helium media was not possible using the previously establish gas chromatography (GC) flame ionization detector, due to similarities in thermal conductivity. This milestone was initiated to evaluate an alternative GC approach that employed a mercuric oxide (HgO) reduction gas detector (RGD). Using the HgO RGD setup, H2 was successfully calibrated in the presence of a helium carrier gas, and then subsequently measured in control and irradiated crush-tube sample vials, consistent with previous sample measurements. Overall, the HgO RGD approach was found to be sufficient for future helium environment irradiations.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2013

Minor actinide transmutation in thorium and uranium matrices in heavy water moderated reactors

The irradiation of Th{sup 232} breeds fewer of the problematic minor actinides (Np, Am, Cm) than the irradiation of U{sup 238}. This characteristic makes thorium an attractive potential matrix for the transmutation of these minor actinides, as these species can be transmuted without the creation of new actinides as is the case with a uranium fuel matrix. Minor actinides are the main contributors to long term decay heat and radiotoxicity of spent fuel, so reducing their concentration can greatly increase the capacity of a long term deep geological repository. Mixing minor actinides with thorium, three times more common in the Earth's crust than natural uranium, has the additional advantage of improving the sustainability of the fuel cycle. In this work, lattice cell calculations have been performed to determine the results of transmuting minor actinides from light water reactor spent fuel in a thorium matrix. 15-year-cooled group-extracted transuranic elements (Np, Pu, Am, Cm) from light water reactor (LWR) spent fuel were used as the fissile component in a thorium-based fuel in a heavy water moderated reactor (HWR). The minor actinide (MA) transmutation rates, spent fuel activity, decay heat and radiotoxicity, are compared with those obtained when the MA were mixed instead with natural uranium and taken to the same burnup. Each bundle contained a central pin containing a burnable neutron absorber whose initial concentration was adjusted to have the same reactivity response (in units of the delayed neutron fraction β) for coolant voiding as standard NU fuel. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2018

Estimation of Commodity Origin-Destination Matrices and Flows: The Network Tomography of U.S. Wheat Shipments

This study proceeds from the belief that an examination of railroads and railcar flows provides important insights into economic geography and to the economics of transportation. The flows of wheat in the United States are examined using rail carload waybill survey data in order to model the network routes likely taken from origin to destination, a characterization of the flows themselves, and modeling the flows through the comparison of standard gravity models with two relatively new gravity estimation methodologies suggested in the network analysis literature. The analysis covers Wheat flows through the U.S. for the years 2013 through 2015. Rail flows are highly conducive to the emergence of high volume corridors, a primary hypothetical relationship described by Taaffe, Morrill and Gould (1963). Hubs are expected to develop in market centers with sufficient traffic, as in a Christaller (1966) system, where market centers above a certain size have terminal facilities, or serve as primary interchange points. Centers of different sizes may also have more than one load center thereby concentrating and attracting more flows (o’Kelly, 1998). Implications are for commodity production and processing locations to develop concentrations of flows and the development of identifiable commodity-specific components of the national rail network. As the railroad nodes are structured in relation to the railroad network, the origin and destination nodes take the form of an irregular lattice (Besag 1974). One implication of this irregular lattice network is the probable presence of network autocorrelation in the flows: the constraints of network connectivity assure flows are not open between the terminals, but travel to and from distinct neighbors on the network. In addition, the network terminal structure can be expected to follow a hierarchical central place ordering. Evidence of such an ordering may be determined by examining the clustering and concentration of flows at nodes and, by association, particular network links serving those nodes (Fujita and Thisse 2002; Hesse and Rodrigue 2004; Nierat 1997). The presence of hierarchies implies economies of scale are being realized which leads to a centralizing or agglomerating effect (Fujita and Thisse, 2002 and Brakman et al, 2001). Thus the transportation sector will also develop an associated central place hierarchy with its own associated network of flows and flow concentrations.

Peterson, Steven↗
DOE OSTI2015

From deep TLS validation to ensembles of atomic models built from elemental motions

Procedures are described for extracting the vibration and libration parameters corresponding to a given set of TLS matrices and their simultaneous validation. Knowledge of these parameters allows the generation of structural ensembles corresponding to these matrices. The translation–libration–screw model first introduced by Cruickshank, Schomaker and Trueblood describes the concerted motions of atomic groups. Using TLS models can improve the agreement between calculated and experimental diffraction data. Because the T, L and S matrices describe a combination of atomic vibrations and librations, TLS models can also potentially shed light on molecular mechanisms involving correlated motions. However, this use of TLS models in mechanistic studies is hampered by the difficulties in translating the results of refinement into molecular movement or a structural ensemble. To convert the matrices into a constituent molecular movement, the matrix elements must satisfy several conditions. Refining the T, L and S matrix elements as independent parameters without taking these conditions into account may result in matrices that do not represent concerted molecular movements. Here, a mathematical framework and the computational tools to analyze TLS matrices, resulting in either explicit decomposition into descriptions of the underlying motions or a report of broken conditions, are described. The description of valid underlying motions can then be output as a structural ensemble. All methods are implemented as part of the PHENIX project.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

The Targeted Synthesis of Single Site Vanadyl Species on the Surface and in the Framework of Silicate Building Block Materials

A new synthetic methodology for the targeted preparation of single site, atomically dispersed vanadyl groups in silicate matrices is described. This methodology requires functionalized silicate building blocks Si{sub 8}O{sub 20}(OSnMe{sub 3}){sub 8} that become linked together through vanadyl ({triple_bond}V=O) groups in the matrix. A sequential addition strategy is illustrated which allows the targeting of specific connectivities for the vanadyl group to the silicate building block matrix (i.e. the number of V-O-Si bonds linking the vanadyl unit). Silicate matrices containing exclusively 3-connected (OV(OSi{sub cube}){sub 3}), 2-connected (OV(OR)(OSi{sub cube}){sub 2}) or 1-connected (OVCl{sub 2}(OSi{sub cube})) vanadyl sites are described and characterized via a wide variety spectroscopic and physical techniques (gravimetric analysis, EXAFS, AA and solid state NMR ({sup 51}V, {sup 29}Si, and {sup 17}O)). We demonstrate how the combination of gravimetric, solid state NMR (SSNMR) and EXAFS data can be used to uniquely define the vanadyl sites in these matrices. Furthermore, the use of {sup 17}O SSNMR (1D and MQMAS) is illustrated as an indirect spectroscopic probe to follow changes in the ligands bound to vanadium atom within the vanadyl groups in these matrices.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Sparse Cholesky factorization for solving nonlinear PDEs via Gaussian processes

In recent years, there has been widespread adoption of machine learning-based approaches to automate the solving of partial differential equations (PDEs). Among these approaches, Gaussian processes (GPs) and kernel methods have garnered considerable interest due to their flexibility, robust theoretical guarantees, and close ties to traditional methods. They can transform the solving of general nonlinear PDEs into solving quadratic optimization problems with nonlinear, PDE-induced constraints. However, the complexity bottleneck lies in computing with dense kernel matrices obtained from pointwise evaluations of the covariance kernel, and its partial derivatives, a result of the PDE constraint and for which fast algorithms are scarce. The primary goal of this paper is to provide a near-linear complexity algorithm for working with such kernel matrices. We present a sparse Cholesky factorization algorithm for these matrices based on the near-sparsity of the Cholesky factor under a novel ordering of pointwise and derivative measurements. The near-sparsity is rigorously justified by directly connecting the factor to GP regression and exponential decay of basis functions in numerical homogenization. We then employ the Vecchia approximation of GPs, which is optimal in the Kullback-Leibler divergence, to compute the approximate factor. This enables us to compute ϵ-approximate inverse Cholesky factors of the kernel matrices with complexity O(N log d (N/ϵ)) in space and O(N log 2d (N/ϵ)) in time. We integrate sparse Cholesky factorizations into optimization algorithms to obtain fast solvers of the nonlinear PDE. We numerically illustrate our algorithm’s near-linear space/time complexity for a broad class of nonlinear PDEs such as the nonlinear elliptic, Burgers, and Monge-Ampère equations. In summary, we provide a fast, scalable, and accurate method for solving general PDEs with GPs and kernel methods.

97 MATHEMATICS AND COMPUTING↗
NASA NTRS1996

DCTune Perceptual Optimization of Compressed Dental X-Rays

In current dental practice, x-rays of completed dental work are often sent to the insurer for verification. It is faster and cheaper to transmit instead digital scans of the x-rays. Further economies result if the images are sent in compressed form. DCTune is a technology for optimizing DCT (digital communication technology) quantization matrices to yield maximum perceptual quality for a given bit-rate, or minimum bit-rate for a given perceptual quality. Perceptual optimization of DCT color quantization matrices. In addition, the technology provides a means of setting the perceptual quality of compressed imagery in a systematic way. The purpose of this research was, with respect to dental x-rays, 1) to verify the advantage of DCTune over standard JPEG (Joint Photographic Experts Group), 2) to verify the quality control feature of DCTune, and 3) to discover regularities in the optimized matrices of a set of images. We optimized matrices for a total of 20 images at two resolutions (150 and 300 dpi) and four bit-rates (0.25, 0.5, 0.75, 1.0 bits/pixel), and examined structural regularities in the resulting matrices. We also conducted psychophysical studies (1) to discover the DCTune quality level at which the images became 'visually lossless,' and (2) to rate the relative quality of DCTune and standard JPEG images at various bitrates. Results include: (1) At both resolutions, DCTune quality is a linear function of bit-rate. (2) DCTune quantization matrices for all images at all bitrates and resolutions are modeled well by an inverse Gaussian, with parameters of amplitude and width. (3) As bit-rate is varied, optimal values of both amplitude and width covary in an approximately linear fashion. (4) Both amplitude and width vary in systematic and orderly fashion with either bit-rate or DCTune quality; simple mathematical functions serve to describe these relationships. (5) In going from 150 to 300 dpi, amplitude parameters are substantially lower and widths larger at corresponding bit-rates or qualities. (6) Visually lossless compression occurs at a DCTune quality value of about 1. (7) At 0.25 bits/pixel, comparative ratings give DCTune a substantial advantage over standard JPEG. As visually lossless bit-rates are approached, this advantage of necessity diminishes. We have concluded that DCTune optimized quantization matrices provide better visual quality than standard JPEG. Meaningful quality levels may be specified by means of the DCTune metric. Optimized matrices are very similar across the class of dental x-rays, suggesting the possibility of a 'class-optimal' matrix. DCTune technology appears to provide some value in the context of compressed dental x-rays.

Watson, Andrew B.↗