Vapor Flow Patterns During a Start-Up Transient in Heat Pipes

The vapor flow patterns in heat pipes are examined during the start-up transient phase. The vapor core is modelled as a channel flow using a two dimensional compressible flow model. A nonlinear filtering technique is used as a post process to eliminate the non-physical oscillations of the flow variables. For high-input heat flux, multiple shock reflections are observed in the evaporation region. The reflections cause a reverse flow in the evaporation and circulations in the adiabatic region. Furthermore, each shock reflection causes a significant increase in the local pressure and a large pressure drop along the heat pipe

Postirradiation fiber debonding and pull-out in Sic-Sic composites *

Abstract The toughness of ceramic matrix composites is contributed by crack bridging, matrix crack deflection, fiber debonding and pull-out and other minor effects. Crack bridging relies on fibers being intact close to the crack tip, while pull-out toughening relies on the debonding and frictional characteristics of the fiber-matrix interface. The interface friction depends on the interface pressure (i.e., on misfit strains) and interface roughness. In this paper, a calculational model for postirradiation fiber debonding and pull-out toughness in Sic-Sic composites is presented. It is shown that fiber debonding and pull-out toughness in Sic-Sic composites vary significantly with neutron fluence and irradiation tem~rature, which is a direct wnsequen~ of the dependence of the misfit strain on these irradiation variables. I~t~uction Sic-Sic composites have been proposed for structural applications in fusion reactor first walls and blankets. The fracture toughness of these composites can be measured from work-of-fracture experiments, and can be theoreticafly determined by investigating the mechanisms of energy dissipation during composite failure. In addition to matrix toughness and matrix crack deflection, two other contributions are considered important in toughening SIC-SIC materials. First is the crack-tip bridging by intact fibers, which contributes a closure traction and lowers the stress intensity at the crack tip. This contribution is important in case of small cracks. The second contribution is caused by fiber debonding, fiber fracture and pull-out, which occurs at significant crack openings, thus involving * This material is based upon work supported by the US Department of Energy under award number DE-FGO3-91ER.54115. energy dissipation by interface friction. Fiber bridging, debonding and pull-out depend on the composite mismatch stresses, i.e., on misfit strains. Neutron irradiation alters the composite behavior in a complex fashion. In addition to basic property changes under irradiation, irradiation-induced swelling and creep change the mismatch stress state, which has a direct influence on the fracture strength and toughness of Sic-Sic composites. Detailed calculations of the time-evolution of mismatch stresses in Sic-Sic composites under high-temperature neutron irradiation are performed [l]. General inelastic wnstitutive equations for Sic fibers and SiC matrix, which are developed by the present authors 121 are used for that purpose. It is found that misfit strains change significantly during early irradiation, and that long-term changes depend on helium swelling and creep only, regardless of the initial thermal mismatch state. Accordingly, fiber debonding and pull-out behavior are expected to depend on the neutron fluence, In the present work, we calculate the postirradiation pull-out toughness and fiber debonding in Sic-Sic composites as functions of neutron fluence and irradiation temperature. 0022-3115/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZ 0022-3115(94)00058-

Point defect dynamics in bcc metals

We present an analysis of the time evolution of self-interstitial atom and vacancy (point defect) populations in pure bcc metals under constant irradiation flux conditions. Mean-field rate equations are developed in parallel to a kinetic Monte Carlo (kMC) model. When only considering the elementary processes of defect production, defect migration, recombination and absorption at sinks, the kMC model and rate equations are shown to be equivalent and the time evolution of the point defect populations is analyzed using simple scaling arguments. We show that the typically large mismatch of the rates of interstitial and vacancy migration in bcc metals can lead to a vacancy population that grows as the square root of time. The vacancy cluster size distribution under both irreversible and reversible attachment can be described by a simple exponential function. We also consider the effect of highly mobile interstitial clusters and apply the model with parameters appropriate for vanadium and $\alpha-$iron.Comment: to appear in Phys. Rev.

L-Drawings of Directed Graphs

We introduce L-drawings, a novel paradigm for representing directed graphs aiming at combining the readability features of orthogonal drawings with the expressive power of matrix representations. In an L-drawing, vertices have exclusive $x$- and $y$-coordinates and edges consist of two segments, one exiting the source vertically and one entering the destination horizontally. We study the problem of computing L-drawings using minimum ink. We prove its NP-completeness and provide a heuristics based on a polynomial-time algorithm that adds a vertex to a drawing using the minimum additional ink. We performed an experimental analysis of the heuristics which confirms its effectiveness.Comment: 11 pages, 7 figure

Evidence for Irradiation Triggered Nonuniform Defect Distribution In Multiharmonic Magnetic Susceptibility of Neutron Irradiated YBa2Cu3O7-x

Multiharmonic ac-magnetic susceptibility \ch11,\chi2,chi3, of neutron irradiated Li-doped YBa2Cu3O7-x has revealed a nonmonotonic dependence of all harmonics on the neutron fluence. The irradiation has a strongly depressive influence on the intergrain connection suggesting an increase of the effective thickness of the intergranular Josephson junction at aneutron fluence of 0.98x10$^{17}$ cm$_{-2}$. Less damaged are the intragrain properties. A spectacular enhancement of the superconducting intragranular properties reflected in the characteristics of all harmonics was observed at highest fluence \Phi = 9.98x10$^{17}$ cm$_{-2}$. We assume that this effect results from the development of a space inhomogeneous distribution with alternating defectless and defect rich regions.Comment: 24 pages, 9 figures, accepted to J. Supercon

Revisited experimental comparison of node-link and matrix representations

Visualizing network data is applicable in domains such as biology, engineering, and social sciences. We report the results of a study comparing the effectiveness of the two primary techniques for showing network data: node-link diagrams and adjacency matrices. Specifically, an evaluation with a large number of online participants revealed statistically significant differences between the two visualizations. Our work adds to existing research in several ways. First, we explore a broad spectrum of network tasks, many of which had not been previously evaluated. Second, our study uses a large dataset, typical of many real-life networks not explored by previous studies. Third, we leverage crowdsourcing to evaluate many tasks with many participants

Development of advanced blanket performance under irradiation and system integration through JUPITER-II project

a b s t r a c t The Japan-USA collaborative program, JUPITER-II, has made significant progress in a research program titled "The irradiation performance and system integration of advanced blanket" through a six-year plan for [2001][2002][2003][2004][2005][2006]. The scientific concept of this program is to study the elemental technology in macroscopic system integration for advanced fusion blankets based on an understanding of the relevant mechanics at the microscopic level. The program has four main research emphases: (1) Flibe molten salt system: Flibe handling, reduction-oxidation control by Be and Flibe tritium chemistry; thermofluid flow simulation experiment and numerical analysis. (2) Vanadium /Li system: MHD ceramics coating of vanadium alloys and compatibility with Li; neutron irradiation experiment in Li capsule and radiation creep. (3) SiC/He system: Fabrication of advanced composites and property evaluation; thermomechanics of SiC system with solid breeding materials; neutron irradiation experiment in He capsule at high temperatures. (4) Blanket system modeling: Design-based integration modeling of Flibe system and V/Li system; multiscale materials system modeling including He effects. This paper describes the perspective of the program including the historical background, the organization and facilities, and the task objectives. Important recent results are reviewed

Updated Reference Design of a Liquid Metal Cooled Tandem Mirror Fusion Breeder

Detailed studies of key techinical issues for liquid metal cooled fusion breeder (fusion-fission hybrid blankets) have been performed during the period 1983-4. Based upon the results of these studies, the 1982 reference liquid metal cooled tandem mirror fusion breeder blanket design was updated and is described. The updated reference blankets provides increased breeding and lower technological risk in comparison with the original reference blanket. In addition to the blanket design revisions, a plant concept, cost, and fuel cycle economics assessment is provided. The fusion breeder continues to promise an economical source of fissile fuel for the indefinite future