Reliability

This special volume of Statistical Sciences presents some innovative, if not provocative, ideas in the area of reliability, or perhaps more appropriately named, integrated system assessment. In this age of exponential growth in science, engineering and technology, the capability to evaluate the performance, reliability and safety of complex systems presents new challenges. Today's methodology must respond to the ever-increasing demands for such evaluations to provide key information for decision and policy makers at all levels of government and industry--problems ranging from international security to space exploration. We, the co-editors of this volume and the authors, believe that scientific progress in reliability assessment requires the development of processes, methods and tools that combine diverse information types (e.g., experiments, computer simulations, expert knowledge) from diverse sources (e.g., scientists, engineers, business developers, technology integrators, decision makers) to assess quantitative performance metrics that can aid decision making under uncertainty. These are highly interdisciplinary problems. The principal role of statistical sciences is to bring statistical rigor, thinking and methodology to these problems.Comment: Published at http://dx.doi.org/10.1214/088342306000000664 in the Statistical Science (http://www.imstat.org/sts/) by the Institute of Mathematical Statistics (http://www.imstat.org

Transport of Sellafield-derived C-14 from the Irish Sea through the North Channel

Since the early 1950s, the Sellafield nuclear fuel reprocessing plant in Northwest England has released radio-carbon into the Irish Sea in a mainly inorganic form as part of its authorized liquid effluent discharge. In contrast to the trend in which the activities of most radionuclides in the Sellafield liquid effluent have decreased substantially, C-14 discharges have increased since 1994-95. This has largely been due to a policy change favoring marine discharges over atmospheric discharges. C-14 is radiologically important due to its long half life, mobility in the environment, and propensity for entering the food chain. Current models for radionuclide dispersal in the Irish Sea are based on a reversible equilibrium distribution coefficient (k(d)), an approach which has been shown to be inadequate for C-14. Development of predictive models for the fate of Sellafield-derived C-14 requires a thorough understanding of the biogeochemical fluxes between different carbon reservoirs and the processes controlling the net flux of C-14 out of the Irish Sea, through he North Channel. In this study., both an empirical and a halving time approach indicate that close to 100% of the C-14 that is discharged from Sellafield is dispersed beyond the Irish Sea on a time-scale of months in the form of DIC, with little transfer to the PIC, POC, and DOC fractions, indicating that the "dilute and disperse" mechanism is operating satisfactorily. This is consistent with previous research that indicated little transfer of C-14 to Irish Sea sediments, While significant C-14 enhancements have been observed in the biota of the Irish Sea, this observation is not necessarily in conflict with either of the above as the total biomass has to be taken into account in any calculations of C-14 retention within the Irish Sea

Sources of anthropogenic C-14 to the North Sea

The Sellafield nuclear fuel reprocessing plant on the northwest coast of England is the largest source of anthropogenic radiocarbon to the UK coastal environment. In a mid-1990s study of C-14 distribution around the UK coast, the pattern of dilution with increasing distance from Sellafield appeared to be perturbed by anomalously high C-14 activities in marine biota in the coastal environment of northeast England. This present study was undertaken during 1998 and 1999 to determine whether this C-14 enhancement was due to Sellafield or the nuclear power plants on the east coast. Seawater, seaweed (Fucus sp.), and mussel (Mytilus edulis) samples that were collected from the vicinity of the Torness and Hartlepool advanced gas-cooled reactor (AGR) nuclear power stations were all enhanced above the contemporary regional background activity derived from natural production and atmospheric nuclear weapons testing. We used previously published dilution factors and transfer times for Tc-99 between Sellafield and various points on the UK coast to determine likely Sellafield- derived C-14 contributions to the activities at the nuclear power plant sites. The results suggest that the activities observed at Torness, which are only marginally enhanced above the natural background activity, are possibly due to discharges from Sellafield; however, the significant C-14 enhancements at Hartlepool are not Sellafield-derived. Furthermore, since both reactors have the same fundamental design, the low activities at the Torness AGR imply that the activities at Hartlepool are not from the AGR, suggesting that there is an input of C-14 to the marine environment in the vicinity of Hartlepool which is probably non-nuclear-power related. However, there is no other authorized site in the area that could account for the observed C-14 enrichments; therefore, further research is required to ascertain the source of this C-14

Systematics and evolutionary paleoecology of crinoids from the St. Louis Limestone (Mississippian, Meramecian) of the Illinois Basin

Very little attention has been paid to crinoids of the St. Louis Limestone since the 19th century. Now that the stratigraphic boundaries of the formation are well defined and a collection of crinoids specific to the St. Louis has become available, updated systematics and evolutionary paleoecology is possible.;Previous work recognized a significant transition in the macroevolution of crinoids that occurred during the Mississippian Period. The diversity of crinoid taxa changed substantially from faunas dominated by camerates and primitive cladids to those dominated by advanced cladids. Much of that change took place during the Meramecian Stage, which includes the St. Louis Limestone. The preceding Salem Limestone contained a prevalence of camerates whereas the following superjacent Ste. Genevieve Limestone contained a more advanced cladids. Questions that are considered include: what changes in morphology of crinoids accompanied that transition; what, if any, environmental change occurred that influenced the major transition in crinoids; and what was the precise change in the distribution of crinoid taxa throughout the deposition of the St. Louis Limestone? The approach to providing a plausible description and explanation includes an evaluation of the geologic setting, a re-evaluation of crinoid systematics, and an investigation of the evolutionary paleoecology of the St. Louis Limestone crinoids.;Lithology of the St. Louis Limestone is compared to the Salem Limestone below and the Ste. Genevieve limestone above. A change in environment occurred within the St. Louis. The Salem and Ste. Genevieve were deposited in a deeper water setting, whereas St. Louis deposition took place in a low-energy, mostly lagoonal setting. This is reflected in microscopic study by the prevalence of mudstone and wackestone in the St. Louis while the Salem and St. Genevieve are granistones.;Systematic evaluation confirmations 17 species of crinoids previously assigned to the St. Louis. Three crinoid species that were identified within other formations, but not in the St. Louis, were added. One new species, Armenocrinus howelli was identified. Three species were synonymized within the St. Louis. Eight species previously listed as belonging to the St. Louis were found to be incorrectly assigned.;Crinoids had been adapting in their overall morphology prior to the Meramecian. Various changes in feeding structures, predatory defenses and mobility occurred from the Ordovician through the early Meramecian. Those adaptations allowed crinoids with the most advantageous morphology to survive adverse environmental conditions. In fact, two events took place during the Meramecian that would have impacted the survival of crinoids. Durophagous predators increased in numbers and a prolonged regression reduced habitat area during the deposition of the St. Louis. Those two factors impacted crinoids in a negative way. That evolutionary bottleneck resulted in the better adapted advanced cladids radiating, whereas the previously dominant camerates saw a substantial decrease in diversity

Fabrication process development of SiC/superalloy composite sheet for exhaust system components

A chemical compatibility study was conducted between SiC filament and the following P/M matrix alloys: Waspaloy, Hastelloy-X, NiCrAlY, Ha-188, S-57, FeCrAlY, and Incoloy 800. None of the couples demonstrated sufficient chemical compatibility to withstand the minimum HIP consolidation temperatures (996 C) or intended application temperature of the composite (982 C). However, Waspaloy, Haynes 188, and Hastelloy-X were the least reactive with SiC of the candidate alloys. Chemical vapor deposited tungsten was shown to be an effective diffusion barrier between the superalloy matrix and SiC filament providing a defect-free coating of sufficient thickness. However, the coating breaks down when the tungsten is converted into intermetallic compounds by interdiffusion with matrix constituents. Waspaloy was demonstrated to be the most effective matrix alloy candidate in contact with the CVD tungsten barrier because of its relatively low growth rate constant of the intermediate compound and the lack of formation of Kirkendall voids at the matrix-barrier interface. Fabrication methods were developed for producing panels of uniaxial and angle ply composites utilizing CVD tungsten coated filament

Dynamic effects of electromagnetic wave on a damped two-level atom

We studied the dynamic effects of an electromagnetic(EM) wave with circular polarization on a two-level damped atom. The results demonstrate interesting ac Stark split of energy levels of damped atom. The split levels have different energies and lifetimes, both of which depend on the interaction and the damping rate of atom. When the frequency of the EM wave is tuned to satisfy the resonance condition in the strong coupling limit, the transition probability exhibits Rabi oscillation. Momentum transfer between atom and EM wave shows similar properties as the transition probability under resonance condition. For a damped atom interacting with EM field, there exists no longer stable state. More importantly, if the angular frequency of the EM wave is tuned the same as the atomic transition frequency and its amplitude is adjusted appropriately according to the damping coefficients, we can prepare a particular 'Dressed State' of the coupled system between atom and EM field and can keep the system coherently in this 'Dressed state' for a very long time. This opens another way to prepare coherent atomic states.Comment: latex, 2 figure

The probability density function tail of the Kardar-Parisi-Zhang equation in the strongly non-linear regime

An analytical derivation of the probability density function (PDF) tail describing the strongly correlated interface growth governed by the nonlinear Kardar-Parisi-Zhang equation is provided. The PDF tail exactly coincides with a Tracy-Widom distribution i.e. a PDF tail proportional to $\exp( - c w_2^{3/2})$, where $w_2$ is the the width of the interface. The PDF tail is computed by the instanton method in the strongly non-linear regime within the Martin-Siggia-Rose framework using a careful treatment of the non-linear interactions. In addition, the effect of spatial dimensions on the PDF tail scaling is discussed. This gives a novel approach to understand the rightmost PDF tail of the interface width distribution and the analysis suggests that there is no upper critical dimension.Comment: 17 pages, 2 figure