Qualitative Reasoning about Relative Directions : Computational Complexity and Practical Algorithm

Qualitative spatial reasoning (QSR) enables cognitive agents to reason about space using abstract symbols. Among several aspects of space (e.g., topology, direction, distance) directional information is useful for agents navigating in space. Observers typically describe their environment by specifying the relative directions in which they see other objects or other people from their point of view. As such, qualitative reasoning about relative directions, i.e., determining whether a given statement involving relative directions is true, can be advantageously used for applications, for example, robot navigation, computer-aided design and geographical information systems. Unfortunately, despite the apparent importance of reasoning about relative directions, QSR-research so far could not provide efficient decision procedures for qualitative reasoning about relative directions. Accordingly, the question about how to devise an efficient decision procedure for qualitative reasoning about relative directions has meanwhile turned to the question about whether an efficient decision procedure exists at all. Answering the latter existential question, which requires a formal analysis of relative directions from a computational complexity point of view, has remained an open problem in the field of QSR. The present thesis solves the open problem by proving that there is no efficient decision procedure for qualitative reasoning about relative directions, even if only left or right relations are involved. This is surprising as it contradicts the early premise of QSR believed by many researchers in and outside the field, that is, abstracting from an infinite domain to a finite set of relations naturally leads to efficient reasoning. As a consequence of this rather negative result, efficient reasoning with any of the well-known relative direction calculi (OPRAm, DCC, DRA, LR) is impossible. Indeed, the present thesis shows that all the relative direction calculi belong to one and the same class of ∃R-complete problems, which are the problems that can be reduced to the NP-hard decision problem of the existential theory of the reals, and vice versa. Nevertheless, in practice, many interesting computationally hard AI problems can be tackled by means of approximative algorithms and heuristics. In the same vein, the present thesis shows that qualitative reasoning about relative directions can also be tackled with approximative algorithms. In the thesis we develop the qualitative calculus SVm which allows for a practical algorithm for qualitative reasoning about relative directions. SVm also provides an effective semi-decision procedure for the OPRAm calculus, the most versatile one among the relative direction calculi. In this thesis we substantiate the usefulness of SVm by applying it in the marine navigation domain

Effect of Gd-alloyed Neutron Absorber on Thermal Performance of a Spent Fuel Cask

Department of Nuclear EngineeringA spent fuel cask must be designed to provide safety functions. In particular, in order to maintain the sub-criticality safety, different neutron absorbers have been used for the spent nuclear fuel management system. Although BORAL is the most used neutron absorber, several problems have been reported and it has some potential problems for long term storage of spent nuclear fuels. Recently, a Gd-alloyed duplex stainless steel demonstrated the possibility of fabrication and under development for an advanced neutron absorber and structural material as well. In this study, the effect of Gd alloyed neutron absorber on thermal performance of a spent fuel cask has been studied. The thermal properties of Gd-alloyed DSS was measured using the specimens provided by KITEC. The effect of Gd-alloyed duplex stainless steel adoption was analyzed for the reference casks, KSC-1 and KORAD-21. The analysis method was verified by comparing the analysis results with the reported values. Their system with Gd-alloyed DSS should remove decay heat with passive cooling. The KSC-1 and KORAD-21 casks were modeled with a 2D axis-symmetry condition and 3D symmetry condition, respectively, using ANSYS FLUENT v17.0. Based on the verified method, thermal performance of KORAD-21 which adopts Gd-alloyed DSS was analyzed. The maximum fuel cladding temperature with Gd-alloyed DSS exceeded allowable temperature of 400 ??? and it could affect the fuel integrity. Therefore, basket wall thickness and disk thickness were optimized to enhance thermal performance. When the basket wall thickness was reduced, the gap between the basket surface and disk square holes was consequently increased. The increased gap enhanced the upward flow of helium and it improved the decay heat removal. Additionally, disk thickness was optimized to 60mm from 20mm. The increased heat-conducting surface enhanced the conduction heat transfer. As a result, UNIST-version design of the KORAD-21 cask was developed with the 5.0mm basket wall thickness and 60mm disk thickness. Thermal performance of UNIST design cask satisfied thermal requirements in normal operation.ope

Metastatic carcinoma of the oral region : an analysis of 21 cases

Metastatic carcinoma to the jaws and oral region are very rare, representing less than 1% of all oral tumors. Unfortunately, oral metastasis is usually manifestation of an advanced stage of primary cancer, and indicates widespread disease and poor prognosis. In this retrospective study, a total of 2039 patients with history of oral malignant tumor between 1980 and 2012 at Seoul National University Dental Hospital were evaluated. We analyzed the dental and medical records, and histopathological database of 2039 patients to assess the prevalence of oral metastasis of carcinoma in terms of sex and age, as well as, the most common origin of primary cancer, and prevalent site and histopathological type of metastatic carcinoma. Among 2039 patients, 21 (1.03%) were finally diagnosed with metastatic carcinoma of the jaws and oral region. Among the 21 patients, only 11 had a working diagnosis as oral metastasis upon clinical evaluation before performing a biopsy. The mean age at the time of diagnosis with a metastatic carcinoma was 56.86, and there was a male preponderance. Metastatic carcinoma was more frequent in the jaws than in the soft tissue, especially in the mandible compared to the maxilla. The most frequent primary site was the lungs, followed by the liver and breasts. The predominant histopathological types were hepatocellular carcinoma and adenocarcinoma. Patient outcomes indicated a poor prognosis with the time from the appearance of the metastasis to death was only 12 months. According to these cases, oral metastases of carcinoma were exceedingly rare in Koreans. It can allow the clinicians take into account the possible presence of metastases and lead to early diagnosis