About 60 percent of the electricity production in Belgium originates from nuclear power plants. Belgium owns 7 nuclear pressurized water reactors, which are located in two sites: 4 reactors in Doel and 3 reactors in Tihange. Together they have a capacity of approximately 5900 MWe. All these reactors use classical uranium oxide fuel assemblies. Two of them (Doel3, Tihange2) have also accepted a limited number of mixed (uranium and plutonium) oxide fuel assemblies. These mixed fuel assemblies came from the reprocessing of spent uranium oxide fuel assemblies in La Hague (France). The reprocessing of spent fuel gives birth to vitrified high-level waste, and to different isotopes of uranium and plutonium, which can be used in the manufacture of mixed oxide fuel assemblies. Each country producing radioactive waste must find a solution to dispose them safely. The internationally accepted solution is to dispose high-level radioactive waste in a deep and stable geological layer. This seems to be the most secure and environment-friendly way to get rid of the high-level radioactive waste. One of the few stable geological layers, which could accept radioactive waste in Belgium, is the Boom clay layer. Another possible layer is the Ypresian clay layer, but it is not the reference option for the moment. The Boom clay layer is quite thin (about 100 m thick) and is not at a large depth (about 240 m below the ground surface) at the proposed disposal site, beneath the SCK CEN Nuclear Research Centre in Mol. A large number of studies have already been performed on the Boom clay layer, and on the possibility of building a high-level radioactive waste repository in this geological medium. Since 1993, the Belgian government has promulgated a moratorium on the reprocessing of spent uranium oxide fuels in La Hague. Since then, spent fuel assemblies are considered as waste, and ONDRAF/NIRAS (the Belgium Agency for Radioactive Waste and Enriched Fissile Materials) has thus to deal with them as waste. This rises a number of questions on how to deal with this new kind of waste. A solution is to directly dispose these spent fuel assemblies in containers in a repository, just like the other high-level radioactive waste. This repository would be build in the Boom clay layer at a depth of about 240 m beneath the SCK CEN. One of the questions raised by this new kind of waste is: "could the direct disposal of the spent nuclear fuel assemblies lead to nuclear criticality risks in the future?". Nuclear criticality is the ability of a system to sustain a nuclear fission chain reaction. This question was not a key issue with vitrified high-level waste because these do not include fissile uranium and plutonium isotopes, which could lead to a criticality event. The spent fuel repository will be designed in order to totally avoid the occurrence of a criticality event at the closure time. But in the future history of the repository, external events could possibly affect this. These events could maybe lead to criticality inside the repository, and this has also to be avoided. This work tries to answer this question, and to determine how to avoid a long-term criticality event inside the repository. The only complete research work answering this question has been performed in the U.S. for the Yucca Mountain repository but this design is fully different from the Belgian one studied here: for example, the waste are not only spent fuel waste, and the geological layer is volcanic tuff. The main achievements of this work are: *A first set of in-package criticality scenarios for different design options for a Belgian spent fuel repository in the Boom clay layer. *A large number of criticality calculations with different parameters (fuel type, fuel burnup, fuel enrichment, distance between the fuel assemblies, distance between the fuel rods, water fraction inside the overpack) for the different design options. *A preliminary study of the effects of the spent fuel assemblies isotopic evolution with time on the multiplication factor. *For the first time, a coupling between the in-package criticality scenarios and the criticality calculations has been performed.

Magnetism of the rare earth intermetallics present some of the most important challenges for understanding correlated electron systems . In this thesis I distil this immense challenge, to understanding the unusual magnetic properties of the rare earth intermetallic PrPtAl. At first glance, PrPtAl appears to be a typical local f moment system, where the electronic states of Pr3+ are composed of nine singlet states, split by the crystal electric field for the J = 4 spin-orbit state in low crystal symmetry (orthorhombic, Pnma). The absence of a magnetic ground state would naively lead us to expect PrPtAl to be a simple paramagnet, but the results from this thesis show that the material is more complex, ordering magnetically at 5.7 K in spite of its singlet ground state. This thesis investigates the emergence of magnetic order in PrPtAl. For this purpose, the properties of PrPtAl were measured using high quality single crystals grown using the Czochralski technique. These crystals were used to measure: bulk properties at the Centre for Science at Extreme Conditions (CSEC, University of Edinburgh) and to perform neutron and x-ray magnetic scattering experiments at central facilities within Europe (ISIS, ESRF) and North America (NCNR). The results of this thesis conclusively show that PrPtAl does not directly realise ferromagnetism, but initially orders into two modulated magnetic states between 5.7-5.2 K and 5.2-4.7 K. These states cannot be explained using a simple local moment picture, but appear to be driven by a complex interaction between local moments and conduction electrons, in a possible quantum order-by-disorder type mechanism.

This dissertation employs the theory of self-organizing criticality (SOC) into the study of Chinese cities. SOC was proposed at the end of the 1980s to explain system complexity by combining both self-organizing and critical behaviors. SOC has been broadly used in explaining phenomena in physical and social sciences. However, few attempts have been made to connect urban studies with SOC because of the extreme complexity of urban phenomena. This study develops a generalized SOC to study Chinese cities at both the inter-urban and the intra-urban levels. At the inter-urban level, this study finds that the rank size distribution of Chinese cities has followed Zipf's law since 1984. In addition, the rank size dynamics of Chinese cities experienced a spatiotemporal shift. Before 1996, city rank increases in a few small- and middle-sized cities because of favorable economic policies offered by the central government. After 1996, a majority of the Chinese cities began to be involved in this rank size shuffling. Cities with increasing ranks present clustered distribution, mainly along the south and east coastal areas. Part of the reason is that the market economy mechanism has transcended policy factors in determining the city competitiveness. At the intra-urban level, the study shows that Shenzhen's urban physical development is currently facing physical environmental thresholds, shifting the development strategies spatiotemporally from fringe and isolated growth to fringe and infill growth. The resulted urban patches show power law relationship both in the area-perimeter distributions and the magnitude-frequency distributions. In summary, this research proves the applicability of the generalized SOC in urban studies. At both the inter-urban and the intra-urban levels, the Chinese cities present the characteristics of SOC. Given a stable condition of power law, shifts occur in the inside dynamics of China's urban system and Shenzhen city. This study is one of the few empirical urban studies based on SOC. The study contributes to the literature on SOC theory and provides theoretical breakthroughs in studying Chinese cities. Finally, this study has potential implications on urban policies and urban development strategies.

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For any graph parameter, the removal of a vertex from a graph can increase the parameter, decrease the parameter, or leave the parameter unchanged. This dissertation focuses on the case where the removal of a vertex decreases the parameter for the cases of independent domination and total domination. A graph is said to be independent domination vertex-critical, or i-critical, if the removal of any vertex decreases the independent domination number. Likewise, a graph is said to be total domination vertex-critical if the removal of any vertex decreases the total domination number. Following these notions, a graph is independent domination bicritical, or i-bicritical, if the removal of any two vertices decreases the independent domination number, and a graph is total domination bicritical if the removal of any two vertices decreases the total domination number. Additionally, a graph is called strong independent domination bicritical, or strong i-bicritical, if the removal of any two independent vertices decreases the independent domination number by two. Construction results for i-critical graphs, i-bicritical graphs, strong i-bicritical graphs, total domination critical graphs, and total domination bicritical graphs are studied. Many known constructions are extended to provide necessary and sufficient conditions to build critical and bicritical graphs. New constructions are also presented, with a concentration on i-critical graphs. One particular construction shows that for any graph G, there exists an i-critical, i-bicritical, and strong i-bicritical graph H such that G is an induced subgraph of H. Structural properties of i-critical graphs, i-bicritical graphs, total domination critical graphs, and total domination bicritical graphs are investigated, particularly for the connectedness and edge-connectedness of critical and bicritical graphs. The coalescence construction, which has appeared in earlier literature, constructs a graph with a cut-vertex and this construction is studied in great detail for i-critical graphs, i-bicritical graphs, total domination critical graphs, and total domination bicritical graphs. It is also shown that strong i-bicritical graphs are 2-connected and thus the coalescence construction is not useful in this case. Domination vertex-critical graphs (graphs where the removal of any vertex decreases the domination number) have been studied in the literature. A well-known result gives an upper bound on the diameter of such graphs. Here similar techniques are used to provide upper bounds on the diameter for i-critical graphs, strong i-bicritical graphs, and total domination critical graphs. The upper bound for the diameter of i-critical graphs trivially gives an upper bound for the diameter of i-bicritical graphs. For a graph G, the gamma-graph of G is the graph where the vertex set is the collection of minimum dominating sets of G. Adjacency between two minimum dominating sets in the gamma-graph occurs if from one minimum dominating set a vertex can be removed and replaced with a vertex to arrive at the other minimum dominating set. One can think of adjacency between minimum dominating sets in the gamma-graph as a swap of two vertices between minimum dominating sets. In the single vertex replacement adjacency model these two vertices can be any vertices in the minimum dominating sets, and in the slide adjacency model these two vertices must be adjacent in G. (Hence the gamma-graph obtained from the slide adjacency model is a subgraph of the gamma-graph obtained in the single vertex replacement adjacency model.) Results for both adjacency models are presented concerning the maximum degree, the diameter, and the order of the gamma-graph when G is a tree. / Graduate / 0405 / michaedwards@gmail.com

In the areas of Service Failure Recovery and Expectancy-Disconfirmation, the extant literature contains studies of predictive expectations conducted in a consumer services setting, which show how a customer believes a seller would respond during a service failure situation. However, a focus on the expectations of enterprise managers and purchasing decision-makers for how a provider should respond in such a situation has not been explored. In addition, the literature contains studies that support the influence of service criticality and failure severity on recovery satisfaction, but the mechanisms by which these variables impact recovery satisfaction has not been extensively discussed. In order to contribute to this discussion, the current study adds to the current Recovery Disconfirmation model by illustrating how service criticality and failure severity influences customers’ normative recovery expectations, which in turn affect customer disconfirmation and recovery satisfaction. This research contributes to the marketing and information technology literature by applying the expectancy disconfirmation model to managers and purchasing decision-makers in an enterprise Internet services setting. Practical implications of this research include helping information technology services providers to understand how customers establish expectations of their provider, and how to design recovery responses to optimize customer satisfaction after a service failure.

In a lead-cooled fast reactor (LFR), small bubbles (in the order of one mm or less) may enter the coolant from a leaking steam generator. If such a leakage is undetected the small bubbles may eventually coalesce into a larger bubble in local stagnation zones under the active core. If such a bubble or void releases and passes through the core, it could drive the reactor into prompt criticality. It is therefore desirable to be able to detect the initial stages of such void formation. In this thesis, a methodology to detect such leaks is presented together with a study on void-induced reactivity effects in various LFR's. The methodology developed is based on information from two fission chambers positioned radially outside the core. The fissile content of the fission chambers consist either of 235U or 242Pu making them sensitive to different parts of the neutron spectrum. It is shown that the information from the fission chambers can be used to obtain an early indication of the presence of a small leak within typically a month. Furthermore, it is shown that for all but the smallest LFR’s, prompt criticality due to voids passing the core cannot be excluded. One conclusion is that the methodology may form an attractive complement to the general monitoring system of future LFR’s but, as is noted, it has potential for further developments.

Thesis (M.S.)--Ohio State University, 2009. / Title from first page of PDF file. Includes vita. Includes bibliographical references (p. 133-134).

Predicting events in metastable systems is an important but challenging problem. It can help society forecast, prevent, or prepare for upcoming catastrophes. However, many metastable systems in nature operate near a critical point and are empirically unpredictable. We developed machine learning predictors, applied them to the prediction of nucleation events in the metastable Ising model, near and far from the spinodal critical point. We observed decreasing predictability as the critical point is approached, and found that this unpredictability is due to the vanishing density difference between the nucleating droplet and the background. We also developed a tensor representation of Lennard-Jones con gurations using the symmetry order parameters of the particles and use this representation to predict nucleation in a dense Lennard Jones liquid. Finally, we investigated the noise-induced critical point in two variations of the OFC model - a coupled OFC model and a OFC model with multiplicative noise. In both variations, we found a critical phase boundary that separates the ergodic and non-ergodic phase and the termination point of the phase boundary, which is consistent with a higher-order phase transition.