Calculation and Experimental Determination of the Neutronics Characteristics of the IRT-T Research Reactor

Atomic Energy - The results of calculations and experimental determination of the neutronics characteristics of the IRT-T research reactor are presented. The IRT-T reactor is a pool reactor with...     

Graphical models for optimal power flow

Optimal power flow (OPF) is the central optimization problem in electric power grids. Although solved routinely in the course of power grid operations, it is known to be strongly NP-hard in general, and weakly NP-hard over tree networks. In this paper, we formulate the optimal power flow problem over tree networks as an inference problem over a tree-structured graphical model where the nodal variables are low-dimensional vectors. We adapt the standard dynamic programming algorithm for inference over a tree-structured graphical model to the OPF problem. Combining this with an interval discretization of the nodal variables, we develop an approximation algorithm for the OPF problem. Further, we use techniques from constraint programming (CP) to perform interval computations and adaptive bound propagation to obtain practically efficient algorithms. Compared to previous algorithms that solve OPF with optimality guarantees using convex relaxations, our approach is able to work for arbitrary tree-structured distribution networks and handle mixed-integer optimization problems. Further, it can be implemented in a distributed message-passing fashion that is scalable and is suitable for “smart grid” applications like control of distributed energy resources. Numerical evaluations on several benchmark networks show that practical OPF problems can be solved effectively using this approach.     

Networks originating from the multiple cracking of different scales in rocks and swelling soils

The objective of this work is not a fracture prediction or prevention. We are interested in modeling the crack network geometry in rocks and swelling soils and in the application of the model to rock volume fragmentation or preferential flow in swelling soils. Natural and explosion-induced rock fragmentation is important in geophysics and mining. Preferential flow in swelling soils is important in agricultural and environmental engineering. The presentation gives a brief review of the authors' work in this area. A concentration criterion of crack connection and effective independency of cracks in the case of multiple cracking are a basis for the modeling of a crack network. This basis enables one to introduce a condition of fragment formation at crack connection and a number of relevant concepts (an average cracking – an average crack number of x dimension in volume; crack connection probability of x dimension; fragment formation probability; average and maximum fragment dimensions; crack connectedness – a ratio connected to the total number of cracks; and crack network tortuosity), as well as to suggest quantitative relations between the concepts. In the frame of an application, the average fragment dimension and crack connectedness (or the maximum fragment dimension and fragment formation probability) can depend on the spatial coordinates and parameters specific for the application. The simplest application relates to the block-dimension distribution of a rock mass for statistically homogeneous conditions. The second application relates to the granulometric composition of a blasted rock mass in quarries. In this case the specific parameters are the preliminary rock disturbance (including natural cracking), charge construction, blasting scheme and others. The third application relates to the shrinkage crack network geometry in swelling clay soils, the spatial coordinate being the soil depth. The specific parameters are an upper layer thickness (of a few tens of centimeters) of intensive cracking and the maximum crack depth (its boundary being the depth of the water table level). Crack width, depth, spacing, volume, and tortuosity of the crack network are estimated by using the shrinkage curve of the soil and a water content profile. The fourth application relates to the hydraulic properties of capillary crack networks in swelling soils as compared to those of the soil matrix. For all the applications considered, comparison between the model prediction and available data shows good agreement.     

PMD-induced fluctuations of bit-error rate in optical fiber systems

This paper presents a method that allows evaluating the performance of an optical fiber system where bit errors result from a complex interplay of spontaneous noise generated in optical amplifiers and birefringent disorder of the transmission fiber. We demonstrate that in the presence of temporal fluctuations of birefringence characteristics, the bit-error rate (BER) itself is insufficient for characterizing system performance. Adequate characterization requires introducing the probability distribution function (PDF) of the BER obtained by averaging over many realizations of birefringent disorder. Our theoretical analysis shows that this PDF has an extended tail indicating the importance of anomalously large values of BER. We present the results of comprehensive analysis of the following issues: 1) The dependence of the PDF tail shape on detection details, such as filtering and regular temporal shift adjustment; 2) the changes in the PDF of BER that occur when the first- or higher order polarization mode dispersion (PMD) compensation techniques are applied; 3) an alternative PMD compensation method capable of providing more efficient suppression of extreme outages.