The goal of a network interdiction problem is to model competitive decision-making between two parties with opposing goals. The simplest interdiction problem is a bilevel model consisting of an 'adversary' and an interdictor. In this setting, the interdictor first expends resources to optimally disrupt the network operations of the adversary. The adversary subsequently optimizes in the residual interdicted network. In particular, this dissertation considers an interdiction problem in which the interdictor places radiation detectors on a transportation network in order to minimize the probability that a smuggler of nuclear material can avoid detection. A particular area of interest in stochastic network interdiction problems (SNIPs) is the application of so-called prioritized decision-making. The motivation for this framework is as follows: In many real-world settings, decisions must be made now under uncertain resource levels, e.g., interdiction budgets, available man-hours, or any other resource depending on the problem setting. Applying this idea to the stochastic network interdiction setting, the solution to the prioritized SNIP (PrSNIP) is a rank-ordered list of locations to interdict, ranked from highest to lowest importance. It is well known in the operations research literature that stochastic integer programs are among the most difficult optimization problems to solve. Even for modest levels of uncertainty, commercial integer programming solvers can have difficulty solving models such as PrSNIP. However, metaheuristic and large-scale mathematical programming algorithms are often effective in solving instances from this class of difficult optimization problems. The goal of this doctoral research is to investigate different methods for modeling and solving SNIPs (optimization) and PrSNIPs (prioritization via optimization). We develop a number of different prioritized and unprioritized models, as well as exact and heuristic algorithms for solving each problem type. The mathematical programming algorithms that we consider are based on row and column generation techniques, and our heuristic approach uses adaptive tabu search to quickly find near-optimal solutions. Finally, we develop a group of hybrid algorithms that combine various elements of both classes of algorithms. / text

published_or_final_version / Physics / Master / Master of Philosophy

No description available.

In order to evaluate the safety of radioactive waste disposal underground it is important to understand the way in which radioactive material is transported through the rock to the surface. If the rock is fractured the usual models may not be applicable. In this work we look at three aspects of fracture networks: connectivity, flow and transport. These are studied numerically by generating fracture networks in a computer and modelling the processes which occur. Connectivity relates to percolation theory, and critical densities for fracture systems are found in two and three dimensions. The permeability of two-dimensional networks is studied. The way that permeability depends on fracture density, network size and spread of fracture length can be predicted using a cut lattice model. Transport through the fracture network by convection through the fractures and mixing at the intersections is studied. The Fickian dispersion equation does not describe the resulting hydrodynamic dispersion. Extensions to the techniques to three dimensions and to include other processes are discussed.

In this study, two methods of analysing activity concentrations of natural radionuclides (U, Th and K) in soil are critically compared. These are the Window Analysis (WA) and Full Spectrum Analysis (FSA). In the usual WA method, the activity concentrations are determined from the net counts of the windows set around individual &gamma / -ray peaks associated with the decay of U, Th and K. In the FSA method, the full energy spectrum is considered and the measured spectrum is described as the sum of the three standard spectra (associated with U, Th and K, respectively), each multiplied by an unknown concentration. The concentrations are determined from the FSA and correspond to the activity concentrations of U, Th and K in the soil. The standard spectra derived from separate calibration measurements using the HPGe detector, represents the response of the HPGe to a Marinelli sample beaker containing an activity concentration of 1 Bq/kg.

Thesis (doctoral)--Universiẗat Göttingen, 2004. / Includes bibliographical references (p. xiv-xxxv).

Univ., Diss.--Göttingen, 2005.

No description available.

>Magister Scientiae - MSc / Radon is a naturally occurring radioactive element in the 238U decay series that is found in high concentrations in certain geological formations such as Caves. Exposure to high concentrations of radon has been positively linked to the incidence of lung cancer. This study used Electret ion chambers and the RAD7 continuous radon monitor to measure radon concentrations in the Cango Caves in the Western Cape Province, South Africa. Measurements were taken during summer i.e. February 2004 and March 2005. The results for the radon activity concentrations range from the minimum of about 800 Bq.m-3 to a maximum of 2600 Bq.m-3. The two techniques give very similar results, though the Electret ion chamber results appear to be consistently higher by a few percent where measurements were taken at the same locations. A mathematical model has been developed to investigate the radon concentrations in the Cave. Diffusion and ventilation have been considered as mechanisms for explaining the distribution of radon concentrations. The ventilation rate in the Cave has been estimated under certain assumptions, and it is found to be about 7 ×10−6 s−1 for the Van Zyl hall which is the first large chamber in the Cave. The radon concentration increases as one goes deeper into the Cave, but then becomes fairly constant for the deeper parts. The annual effective dose that the guides are exposed to in the Cave as a result of the radon concentrations, depends strongly on the time that they spend in the Cave and in which, halls they spend most of their time in the Cave. The initial results indicate an annual effective dose of 4-10 mSv, but this needs to be further investigated. / South Africa

Magister Scientiae - MSc / In this study, two methods of analysing activity concentrations of natural radionuclides (U, Th and K) in soil are critically compared. These are the Window Analysis (WA) and Full Spectrum Analysis (FSA). In the usual WA method, the activity concentrations are determined from the net counts of the windows set around individual γ-ray peaks associated with the decay of U, Th and K. In the FSA method, the full energy spectrum is considered and the measured spectrum is described as the sum of the three standard spectra (associated with U, Th and K, respectively), each multiplied by an unknown concentration. The concentrations are determined from the FSA and correspond to the activity concentrations of U, Th and K in the soil. The standard spectra derived from separate calibration measurements using the HPGe detector, represents the response of the HPGe to a Marinelli sample beaker containing an activity concentration of 1 Bq/kg. / South Africa