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Approved for public release; distribution is unlimited / Observations of the temporal evolution of ripples are analyzed in terms of geometry, migration, crest orientation, and their predicted geometry by models using wave orbital velocities. Two weeks of bedform data were obtained in the surf zone during the RIPEX/SBE in April, 2001. Bed sediment consists of medium- to coarse-grained sand (D50=0.43mm). Models capture temporal trends in ripple geometry, but regression analyses show that they do not handle the range in forcing characteristics and geometries in the surf zone well. Transport models of bedload and total load formulated under uni-directional flows qualitatively capture the temporal evolution of observed transport by ripples, suggesting that under low to moderate forcing, bed load and suspended load occur mostly within the bed-following bottom boundary layer, andare measurable by ripple migration alone. Models predict large transport rates when flat beds were observed, so that at higher forcing ripples cannot be used to measure total sediment transport. Using a two-dimensional probability density function (PDF) of vector displacement peaks, a new ripple analysis model is proposed, incorporating a hierarchy of forcing complexity that includes such physical processes as directional spreading, axis rotation, orbital asymmetry, superimposed currents and infragravity wave velocities. The two-dimensional PDF's are compared with concurrent three-dimensional bed maps and are found to assist in describing ripple sizes, types, orientations, and migration velocities. / Lieutenant, United States Navy

Graduation date: 1998

At the brink of nuclear waste repository crises, viable alternatives for the long term radiotoxic wastes are seriously being considered worldwide. Minor actinides serve as one of these targeted wastes. Partitioning and transmutation in fission reactors is one possible incineration option and could potentially serve as a source of nuclear fuel required for sustainability of energy resources. The objective of this research was to evaluate the neutronic performance of the pebble-bed Very High Temperature Reactor (VHTR) configurations with various fuel loadings. The configuration adjustments and design sensitivity studies specifically targeted the achievability of spectral variations. The development of several realistic full-core 3D models and validation of all modeling techniques used was a major part of this research effort. In addition, investigating design sensitivities helped identify the parameters of primary interest. The full-core 3D models representing the prototype and large scale cores were created for use with SCALE 5.0 and SCALE 5.1 code systems. Initially the models required the external calculation of a Dancoff correction factor; however, the recent release of SCALE 5.1 encompassed inherent double heterogeneity modeling capabilities. The full core 3D models with multi-heterogeneity treatments are in agreement with available pebble-bed High Temperature Test Reactor data and were validated through benchmark studies. Analyses of configurations with various fuel loadings have indicated promising performance and safety characteristics. It was found that through small configuration adjustments, the pebble-bed design can be tweaked to produce desirable spectral shifts. The future operation of Generation IV nuclear energy systems would be greatly facilitated by the utilization of minor actinides as a fuel component. This would offer development of new fuel cycles, and support sustainability of a fuel source.

Thesis (M.S. in Meteorology and Physical Oceanography)--Naval Postgraduate School, March 2003. / Thesis advisor(s): Timothy P. Stanton, Edward B. Thornton. Includes bibliographical references (p. 79-83). Also available online.

In samples from 4095 m and 398 m in the Porcupine Seabight foraminiferans constitute 58.4 - 79.3~ of the total~meiofauna. Nematodes followed by harpacticoid copepods are the most abundant meiofaunal metazoan taxa. Foraminiferal densities (967 - 1331 individuals 10 cm-2 ) and diversities (68 - 95 species; Shannon- Wiener index = 3.21 - 4.15) are higher than recorded in most previous studies. This is attributed to the detailed sorting procedure adopted which also provides information on the sizestructure and vertical distribution of foraminiferal assemblages in the top 5 cm of sediment. Most specimens were smaller than 150 ~m and some were infaunal. Komokiaceans only occurred at the deeper site. Allogromiids, hormosinaceans and unilocular agglutinating forms had increased relative abundances at the deeper site while the Rotaliina were relativelY,less common. The composition of these assemblages is discussed in relation to changes in the sediment structure, as observed in resin impregnated sediment cores. The morphology and fine structure of two large agglutinating foraminiferans, Rhizammina al&aeformis Brady and Pelosina aff. aborescens Pearcy, was examined using light microscopy, SEM and TEM. In R. al&aeformis the test is a long, branching tube which sometimes forms a tangled mass on the sediment surface. It contains a protoplasmic strand and extracellular mass of stercomata. The protoplasm is deeply invaginated to produce a system of labyrinthine spaces communicating with the tube lumen and possibly providing sites for extracellular digestion. The stercomata are enveloped in an organic sheet-like system and contain clay minerals. In f. aff. arborescens the tree-like test projects above the sediment surface. Protoplasm occurs in the central lumen. in fine branching canals which ramify through the thick. muddy. test wall. as well as in the distal branching complex. In TEM the protoplasm has a highly vacuolated appearance. There are no stercomata. Differences between the two species seem to reflect different feeding strategies; R. alKaeform1s 1s probably a deposit feeder and f. aff. arbor esc ens a suspension feeder.

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