Calculation of neutron and gamma-ray energy spectra for fusion reactor shield design: Comparison with experiment-II

Measured and calculated neutron and gamma-ray energy spectra resulting from the transport of 14 MeV neutrons through a 0.30-m-thick lithium hydride slab and through a 0.05-m-thick lead slab followed by 0.30 m of lithium hydride are compared. Also reported are comparisons of the measured and calculated neutron energy spectra behind an 0.80-m-thick assembly comprised of stainless steel type 304 and borated polyethylene. The spatial dependence of the gamma-ray energy deposition rate measured using thermoluminescent detectors is compared with calculated data. The calculated data obtained using two-dimensional radiation transport methods and ENDF/B-IV cross-section data are in good agreement for all of the experimental configurations. Calculated integral neutron energy spectra agree with the measured data within 5 to 20% depending on neutron energy for the LiH and Pb plus LiH assemblies. The gamma-ray spectra agree within 20% for these slabs. The measured and calculated neutron energy spectra behind the SS-304-borated polyethylene assembly agree within 5% except at neutron energies below 5 MeV where background radiation influences the measured spectra. The gamma-ray energy deposition rates as a function of depth agree within a factor of two at all detector locations.     

Measuring implementation outcome: beyond success and failure

This paper proposes a framework for measuring the impact of a computer-based application in an organization. Recognizing the complexity of the implementation process, the framework is multi-dimensional: it seeks to measure the outcome of an implementation effort on both the people and tasks affected as separate from user's definition of “success”. The framework can be used to support either the quantitative or case study methodologies used in implementation research. Finally, the authors describe a field test of the framework to demonstrate its usefulness in an organizational setting.     

Uranium Superconductivity Redux

Since the discovery of heavy-fermion superconductivity in uranium compounds in the early 1980s, other uranium compounds have been discovered that are fully as interesting to study. However, as we look forward in the year 2000, we now have higher-purity, single crystals of the element itself. Preliminary resistivity and ac susceptibility measurements show the improved quality of the samples and thus hold the promise of understanding many aspects of its superconductivity, which have remained untouched for almost 25 years.     

Modeling the effect of statistical variations in length and diameter of randomly oriented CNTs on the properties of CNT reinforced nanocomposites

A nano-mechanical model has been developed to calculate the tensile modulus and the tensile strength of randomly oriented short carbon nanotubes (CNTs) reinforced nanocomposites, considering the statistical variations of diameter and length of the CNTs. According to this model, the entire composite is divided into several composite segments which contain CNTs of almost the same diameter and length. The tensile modulus and tensile strength of the composite are then calculated by the weighted sum of the corresponding modulus and strength of each composite segment. The existing micro-mechanical approach for modeling the short fiber composites is modified to account for the structure of the CNTs, to calculate the modulus and the strength of each segmented CNT reinforced composites. Multi-walled CNTs with and without inter-tube bridging have been considered. Statistical variations of the diameter and length of the CNTs are modeled by a normal distribution. Simulation results show that CNTs inter-tube bridging, length and diameter affect the nanocomposites modulus and strength. Simulation results have been compared with the available experimental results and the comparison concludes that the developed model can be effectively used to predict tensile modulus and tensile strength of CNTs reinforced composites.     

Modeling and Measuring All the Elements of an Ultrasonic Nondestructive Evaluation System I: Modeling Foundations

Abstract

A comprehensive model of an ultrasonic nondestructive evaluation (NDE) flaw measurement system is developed that combines models for the electrical components (pulser/receiver, cabling), electromechanical components [transducer(s)], and the acoustic/elastic processes present in the materials being inspected, including the scattering of ultrasonic waves from a flaw. This model is called the electroacoustic measurement (EAM) model. Here, in Part I, the underlying modeling foundations of the EAM model are described in detail and the use of the EAM model is demonstrated in a transducer design study. This EAM model provides a new, powerful tool for analyzing virtually any element in the measurement process. In Part II it will be shown that this power can be extended to characterizing typical commercial measurement systems through the use of models and purely electrical measurements of the system components.     

Optimality of robust disturbance‐feedback strategies

In this paper, robust disturbance‐feedback strategies for finite time‐horizon problems are studied. Linear discrete‐time systems subject to linear control, state constraints, and quadratic objective functions are considered. In addition, persistent disturbances, which enter the system additively and are contained in a polytopic set, act on the system. The synthesis of robust strategies leads in the case of the traditional robust state‐feedback and open‐loop min–max strategies to, respectively, nonconvex problems or conservatism. However, robust disturbance‐feedback problems can easily be reformulated as convex problems and solved by tractable linear matrix inequalities. Hence this approach bypasses the nonconvexity issue while maintaining the advantages of feedback strategies. As a key result, it is shown that both sources of conservatism attributed to this approach, namely, the relaxation method and the affine parametrization, can be removed at the expense of an increase in computational effort. Copyright © 2015 John Wiley & Sons, Ltd.     

Approximation of nonlinear systems with radial basis functionneural networks

A technique for approximating a continuous function of n variables with a radial basis function (RBF) neural network is presented. The method uses an n-dimensional raised-cosine type of RBF that is smooth, yet has compact support. The RBF network coefficients are low-order polynomial functions of the input. A simple computational procedure is presented which significantly reduces the network training and evaluation time. Storage space is also reduced by allowing for a nonuniform grid of points about which the RBFs are centered. The network output is shown to be continuous and have a continuous first derivative. When the network is used to approximate a nonlinear dynamic system, the resulting system is bounded-input bounded-output stable. For the special case of a linear system, the RBF network representation is exact on the domain over which it is defined, and it is optimal in terms of the number of distinct storage parameters required. Several examples are presented which illustrate the effectiveness of this technique.     

Computerized container-ship load planning: A methodology and evaluation

A heuristic is developed to plan loads for containerized cargo ships. It is designed to serve a port which uses gantry cranes (transtainers) and trucks to handle containers. The model recognizes constraints on ship stability, placing containers in a bay with the proper length, limits on stack height in under-deck bays, limits on stack weight in on-deck bays, refrigerated containers, and the need for support under a container. Provisions are made for the operator to handle overstowage of cargo for different ports and placement of oversize cargo. The model uses minimization of transtainer movement time and minimization of rehandles in the yard as an objective. The heuristic uses strategies for container placement similar to those used in manual load planning. A test of two ships and four voyages at the Port of Portland produced feasible load plans for each voyage. Transtainer movement and rehandling time varied for the four voyages; but on the average, the heuristic reduced a composite material handling measure by 4.8%.     

Singularity robust algorithm in serial manipulators

Singularity is a fundamental problem with robotic manipulators. Various methods of singularity handling have been published in the past. In this paper, a singularity robust method is presented and its implementation issues discussed. It focuses on the discontinuity problem inherent in the singularity robust techniques with removal of degenerate components. Sources of the discontinuity are discussed and the null motion is proposed as the means of providing a continuous control of the end-effector over the boundary of the singular region. The presentation of this paper covers a complete treatment of singularities, starting from identifications of the singular configurations and the associated singular directions, the handling algorithm and the discontinuity issues. The algorithm was implemented on PUMA 560. The experimental results are presented and discussed.