Effects of Applying the Implicit Particle Fuel Model for Pebble-Bed Reactors

In the pebble-bed high temperature gas-cooled reactor, there exist randomly located TRISO coated fuel particles in the pebbles and randomly located pebbles in the core, which is known as the double stochastic heterogeneity. In the previous research, the regular lattice pattern was used to approximately simulate the pebble unit cells because the difficulties in modeling the randomly located TRISO geometric. This work aimed at to quantify the stochastic effect of high-temperature gas cooled pebble-bed reactor unit cells, and in view of the strong ability to carry out the accurate simulation of random media, the implicit particle fuel model of Monte Carlo method is applied to analyze to the difference between regular distribution and random distribution. Infinite multiplication factors of the pebble-bed reactor unite cells were calculated by the implicit particle fuel model and simple cube regular lattice pattern at different TRISO packing factor from 0.5%–50%. The results showed that the simple cube regular lattice pattern underestimates the infinite multiplication factors for most packing fractions, but overrates the infinite multiplication factors when the packing fraction is very low.

Feature Technology

Throughout the course of the development of CAD, CAPP, and CAM systems, unambiguous representation of a design’s geometry and topology remain an essential part of the task. Since the mid-1990’s, the technology has matured enough to enable such a representation. While geometry and topology provides a basic description of a design part, direct use of it for creation of the part and other applications, can be cumbersome. Take creation of a simple plane with four straight edges as an example. For a B-rep model to fully define the plane, four points are to be created first to be used as four vertices. They are used to define four edges, which are connected one after another to form a closed loop. Finally, a flat surface is fitted onto the loop to form the plane. When a cube is to be designed, the above process needs to be repeated five more times for the other five faces though some of the vertices and edges may be re-used. In addition, the directions of the solid have to be defined through each face. Clearly, this is not a trivial task. Users would find it helpful if the creation of geometry and topology is hidden behind them and only some meaningful parameters of the solid are provided. In the case of a simple cube, length, width, and depth would be the parameters. Hence, the concept of features (i.e. cube or block in the above example) emerged, as did the associated technologies. The same applies for other domains, such as manufacturing and engineering analysis. This chapter aims to give a succinct introduction to various feature technologies such as feature defintions, feature taxonomy, feature representation schemes, and feature-based methodologies. Several important issues are highlighted. These include the application-dependent nature of features, and surface features versus volumetric features.

Epitaxial Growth of CoGa on (100)GaAs by Metal-Organic Molecular Beam Epitaxy

ABSTRACTEpitaxial layers of the intermetallic β-CoGa cubic phase were grown at low temperature on (100)GaAs by metal-organic molecular beam epitaxy (MOMBE) using GaEt3 and CpCo(CO)2 as vapor sources. The film composition and the lattice mismatch on (100)GaAs may be adjusted by controlling the molecular beam pressure ratio. The growth on a Co-saturated GaAs surface leads to the formation of bi-phased CoGa-CoAs films whereas epitaxial single-phased β-CoGa layers are grown on a Ga-terminated GaAs surface with the simple cube on cube orientation [100](001 )CoGa//[100](001)GaAs. Annealing experiments under inert atmosphere have shown that MOMBE CoGa films are thermally stable on GaAs until ca. 823 K. Ohmic and Schottky CoGa/GaAs contacts have been made depending on the doping of the substrate by this process.

A peculiar occurrence of Magnetite in Upper Bunter Sands

The specimen of Upper Bunter Sands, in which this magnetite occurs, was collected at Hinksford (Staffordshire), near Stourbridge. It is a white, very loosely coherent sand of medium grain ; and contains, as was found by separating the constituents in mercury-potassium iodide, turbid orthoclase, microcline, fragments of the micro-crystalline ground-mass of acid lavas or intrusive rocks, quartz, staurolite, tourmaline, garnet, zircon, rutile, muscovite, a little haematite, and abundant magnetite.The grains of magnetite are very minute, averaging 0.067 mm., but nevertheless they present, with very few exceptions, a perfect crystal outline, that of a simple cube, or, in a few rare cases, that of a regular octahedron.