Ferroelastic Domain Switching in Tetragonal Zirconia Single Crystals—Microstructural Aspects

Ferroelastic domain switching is one of the possible toughening mechanisms in ceramic materials. Microstructural evidence of domain reorientation (switching) in polydomain tetragonal zirconia single crystals is observed upon the application of a unidirectional compressive stress at 1000° and 1400°C. Dark-field imaging of the three {112{ tetragonal twin variants in a [111] zone indicates that two sets of twin variants grow at the expense of the third set upon application of uniaxial compression. The diminishing variant is the one with its c axis parallel to the compression axis. Inducing cracks into polydomain tetragonal zirconia single crystals provides further evidence of domain reorientation near the crack surface. It is noted again that two sets of twin variants grow at the expense of the third set. A construction for the orientation relationship of domains and their twin boundaries is presented, and a relation between shear stress and reorientation is proposed.     

High-Resolution Transmission Electron Microscopy Studies of a Near Σ11 Grain Boundary in α-Alumina

The atomistic structure of a near Σ11 (N211) grain boundary in ultrapure α-AI₂O₃ bicrystals was determined by quantitative high-resolution transmission electron microscopy (HRTEM). High-resolution imaging revealed an atomically sharp interface with a characteristic periodic pattern at (0 1 11) ‖ (01 1 1) facets. The pattern was analyzed by comparing the HRTEM micrographs with simulated images of different structures that had been relaxed by static-lattice calculations. The best agreement with experiment was reached for the grain boundary structure with the lowest energy (1.7 J·m⁻²). Although this structure reproduces the relative translation state of the adjacent crystals to within 0.02 nm, the calculated structure was not in complete agreement with the periodic pattern at the grain boundary. The origin of this difference is probably due to a slight misalignment of the two crystals adjacent to the boundary.     

The severe accident mitigation concept and the design measures for core melt retention of the European Pressurized Reactor (EPR)

For the mitigation of severe accidents, the European Pressurized Water Reactor (EPR) has adopted and improved the defense-in-depth approaches of its predecessors, the French “N4” and the German “Konvoi” plants. Beyond the corresponding evolutionary changes, the EPR includes a new, 4th level of defense-in-depth that is aimed at limiting the consequences of a postulated severe accident with core melting. It involves a strengthening of the confinement function and the avoidance of large early releases. The latter requires the prevention of scenarios and events that can result in high loads on the containment, e.g., a failure of the Reactor Pressure Vessel (RPV) at high internal pressure. This is achieved by dedicated design measures.

The paper gives an short overview of the general concept and the strategies for: primary circuit depressurization, H₂ mitigation and the avoidance of energetic Fuel Coolant Interactions (FCIs). It then describes, in detail, the conceptual solution for the stabilization and long-term cooling of the molten core.

The EPR melt retention strategy supports itself on the use of an ex-vessel core catcher located in a compartment lateral to the pit. The related spatial and functional separation isolates the core catcher from the various loads during RPV failure and, at the same time, avoids risks resulting from an unintended initiation of the system during power operation.

Within the core catcher, the melt will be passively flooded with water from the Internal Refueling Water Storage Tank (IRWST). Due to the effective cooling of the melt from all sides a stable state will be reached within hours and complete solidification of the melt is achieved after a few days. The core catcher can optionally be supplied by the Containment Heat Removal System (CHRS). In this active mode of operation, the water levels inside spreading compartment and reactor pit rise and the pools become subcooled, so further steaming is avoided. This results in a depressurization of the containment in the long-term.     

Ein Wegweiser für Forschung und Lehre im Software-Engineering eingebetteter Systeme

Bisher wurde das Gebiet der Software für eingebettete Systeme („embedded systems”) von der Informatik zu wenig beachtet.     

On-line learning of linear functions

We present an algorithm for the on-line learning of linear functions which is optimal to within a constant factor with respect to bounds on the sum of squared errors for a worst case sequence of trials. The bounds are logarithmic in the number of variables. Furthermore, the algorithm is shown to be optimally robust with respect to noise in the data (again to within a constant factor).     

Customer network configurations and generic flow control

This paper analyses the BISDN customer network configurations as specified in CCITT Recommendation I.413 (e.g. star, bus, ring, starred bus), assumed market needs and the impact on the ATM layer functionality at the broadband user-network interface at the S_B and T_B reference points. Orderly and fair access of several terminals to a shared transmission medium requires a suitable access protocol (generic flow control). Its functionality and possible implementations are discussed.