424 research outputs found
PADAMOT : project overview report
Background and relevance to radioactive waste management
International consensus confirms that placing radioactive wastes and spent nuclear fuel deep
underground in a geological repository is the generally preferred option for their long-term
management and disposal. This strategy provides a number of advantages compared to leaving it
on or near the Earth’s surface. These advantages come about because, for a well chosen site, the
geosphere can provide:
• a physical barrier that can negate or buffer against the effects of surface dominated natural
disruptive processes such as deep weathering, glaciation, river and marine erosion or
flooding, asteroid/comet impact and earthquake shaking etc.
• long and slow groundwater return pathways from the facility to the biosphere along which
retardation, dilution and dispersion processes may operate to reduce radionuclide
concentration in the groundwater.
• a stable, and benign geochemical environment to maximise the longevity of the engineered
barriers such as the waste containers and backfill in the facility.
• a natural radiation shield around the wastes.
• a mechanically stable environment in which the facility can be constructed and will
afterwards be protected.
• an environment which reduces the likelihood of the repository being disturbed by inadvertent
human intrusion such as land use changes, construction projects, drilling, quarrying and
mining etc.
• protection against the effects of deliberate human activities such as vandalism, terrorism and
war etc.
However, safety considerations for storing and disposing of long-lived radioactive wastes must
take into account various scenarios that might affect the ability of the geosphere to provide the
functionality listed above. Therefore, in order to provide confidence in the ability of a repository
to perform within the deep geological setting at a particular site, a demonstration of geosphere
“stability” needs to be made. Stability is defined here to be the capacity of a geological and
hydrogeological system to minimise the impact of external influences on the repository
environment, or at least to account for them in a manner that would allow their impacts to be
evaluated and accounted for in any safety assessments.
A repository should be sited where the deep geosphere is a stable host in which the engineered
containment can continue to perform according to design and in which the surrounding
hydrogeological, geomechanical and geochemical environment will continue to operate as a
natural barrier to radionuclide movement towards the biosphere. However, over the long periods
of time during which long-lived radioactive wastes will pose a hazard, environmental change at
the surface has the potential to disrupt the stability of the geosphere and therefore the causes of
environmental change and their potential consequences need to be evaluated.
As noted above, environmental change can include processes such as deep weathering,
glaciation, river and marine erosion. It can also lead to changes in groundwater boundary
conditions through alternating recharge/discharge relationships. One of the key drivers for
environmental change is climate variability. The question then arises, how can geosphere stability be assessed with respect to changes in climate? Key issues raised in connection with
this are:
• What evidence is there that 'going underground' eliminates the extreme conditions that
storage on the surface would be subjected to in the long term?
• How can the additional stability and safety of the deep geosphere be demonstrated with
evidence from the natural system?
As a corollary to this, the capacity of repository sites deep underground in stable rock masses to
mitigate potential impacts of future climate change on groundwater conditions therefore needs to
be tested and demonstrated. To date, generic scenarios for groundwater evolution relating to
climate change are currently weakly constrained by data and process understanding. Hence, the
possibility of site-specific changes of groundwater conditions in the future can only be assessed
and demonstrated by studying groundwater evolution in the past. Stability of groundwater
conditions in the past is an indication of future stability, though both the climatic and geological
contexts must be taken into account in making such an assertion
Density of mechanisms within the flexibility window of zeolites
By treating idealized zeolite frameworks as periodic mechanical trusses, we
show that the number of flexible folding mechanisms in zeolite frameworks is
strongly peaked at the minimum density end of their flexibility window. 25 of
the 197 known zeolite frameworks exhibit an extensive flexibility, where the
number of unique mechanisms increases linearly with the volume when long
wavelength mechanisms are included. Extensively flexible frameworks therefore
have a maximum in configurational entropy, as large crystals, at their lowest
density. Most real zeolites do not exhibit extensive flexibility, suggesting
that surface and edge mechanisms are important, likely during the nucleation
and growth stage. The prevalence of flexibility in real zeolites suggests that,
in addition to low framework energy, it is an important criterion when
searching large databases of hypothetical zeolites for potentially useful
realizable structures.Comment: 11 pages, 3 figure
An analytical upper bound on the number of loci required for all splits of a species tree to appear in a set of gene trees
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Probiotics, prebiotics, and the host microbiome: the science of translation
Recent advances in our understanding of the community structure and function of the human microbiome have implications for the potential role of probiotics and prebiotics in promoting human health. A group of experts recently met to review the latest advances in microbiota/microbiome research and discuss the implications for development of probiotics and prebiotics, primarily as they relate to effects mediated via the intestine. The goals of the meeting were to share recent advances in research on the microbiota, microbiome, probiotics, and prebiotics, and to discuss these findings in the contexts of regulatory barriers, evolving healthcare environments, and potential effects on a variety of health topics, including the development of obesity and diabetes; the long-term consequences of exposure to antibiotics early in life to the gastrointestinal (GI) microbiota; lactose intolerance; and the relationship between the GI microbiota and the central nervous system, with implications for depression, cognition, satiety, and mental health for people living in developed and developing countries. This report provides an overview of these discussions
The evolution of reproductive isolation in a simultaneous hermaphrodite, the freshwater snail Physa
<p>Abstract</p> <p>Background</p> <p>The cosmopolitan freshwater snail <it>Physa acuta </it>has recently found widespread use as a model organism for the study of mating systems and reproductive allocation. Mitochondrial DNA phylogenies suggest that <it>Physa carolinae</it>, recently described from the American southeast, is a sister species of <it>P. acuta</it>. The divergence of the <it>acuta/carolinae </it>ancestor from the more widespread <it>P. pomilia </it>appears to be somewhat older, and the split between a hypothetical <it>acuta/carolinae/pomilia </it>ancestor and <it>P. gyrina </it>appears older still.</p> <p>Results</p> <p>Here we report the results of no-choice mating experiments yielding no evidence of hybridization between <it>gyrina </it>and any of four other populations (<it>pomilia, carolinae</it>, Philadelphia <it>acuta</it>, or Charleston <it>acuta</it>), nor between <it>pomilia </it>and <it>carolinae</it>. Crosses between <it>pomilia </it>and both <it>acuta </it>populations yielded sterile F1 progeny with reduced viability, while crosses between <it>carolinae </it>and both <it>acuta </it>populations yielded sterile F1 hybrids of normal viability. A set of mate-choice tests also revealed significant sexual isolation between <it>gyrina </it>and all four of our other <it>Physa </it>populations, between <it>pomilia </it>and <it>carolinae</it>, and between <it>pomilia </it>and Charleston <it>acuta</it>, but not between <it>pomilia </it>and the <it>acuta </it>population from Philadelphia, nor between <it>carolinae </it>and either <it>acuta </it>population. These observations are consistent with the origin of hybrid sterility prior to hybrid inviability, and a hypothesis that speciation between <it>pomilia </it>and <it>acuta </it>may have been reinforced by selection for prezygotic reproductive isolation in sympatry.</p> <p>Conclusions</p> <p>We propose a two-factor model for the evolution of postzygotic reproductive incompatibility in this set of five <it>Physa </it>populations consistent with the Dobzhansky-Muller model of speciation, and a second two-factor model for the evolution of sexual incompatibility. Under these models, species trees may be said to correspond with gene trees in American populations of the freshwater snail, <it>Physa</it>.</p
Phenomenology of the Lense-Thirring effect in the Solar System
Recent years have seen increasing efforts to directly measure some aspects of
the general relativistic gravitomagnetic interaction in several astronomical
scenarios in the solar system. After briefly overviewing the concept of
gravitomagnetism from a theoretical point of view, we review the performed or
proposed attempts to detect the Lense-Thirring effect affecting the orbital
motions of natural and artificial bodies in the gravitational fields of the
Sun, Earth, Mars and Jupiter. In particular, we will focus on the evaluation of
the impact of several sources of systematic uncertainties of dynamical origin
to realistically elucidate the present and future perspectives in directly
measuring such an elusive relativistic effect.Comment: LaTex, 51 pages, 14 figures, 22 tables. Invited review, to appear in
Astrophysics and Space Science (ApSS). Some uncited references in the text
now correctly quoted. One reference added. A footnote adde
Addressing Inter-Gene Heterogeneity in Maximum Likelihood Phylogenomic Analysis: Yeasts Revisited
Phylogenomic approaches to the resolution of inter-species relationships have become well established in recent years. Often these involve concatenation of many orthologous genes found in the respective genomes followed by analysis using standard phylogenetic models. Genome-scale data promise increased resolution by minimising sampling error, yet are associated with well-known but often inappropriately addressed caveats arising through data heterogeneity and model violation. These can lead to the reconstruction of highly-supported but incorrect topologies. With the aim of obtaining a species tree for 18 species within the ascomycetous yeasts, we have investigated the use of appropriate evolutionary models to address inter-gene heterogeneities and the scalability and validity of supermatrix analysis as the phylogenetic problem becomes more difficult and the number of genes analysed approaches truly phylogenomic dimensions. We have extended a widely-known early phylogenomic study of yeasts by adding additional species to increase diversity and augmenting the number of genes under analysis. We have investigated sophisticated maximum likelihood analyses, considering not only a concatenated version of the data but also partitioned models where each gene constitutes a partition and parameters are free to vary between the different partitions (thereby accounting for variation in the evolutionary processes at different loci). We find considerable increases in likelihood using these complex models, arguing for the need for appropriate models when analyzing phylogenomic data. Using these methods, we were able to reconstruct a well-supported tree for 18 ascomycetous yeasts spanning about 250 million years of evolution
Computational Biology Methods and Their Application to the Comparative Genomics of Endocellular Symbiotic Bacteria of Insects
Comparative genomics has become a real tantalizing challenge in the postgenomic era. This fact has been mostly magnified by the plethora of new genomes becoming available in a daily bases. The overwhelming list of new genomes to compare has pushed the field of bioinformatics and computational biology forward toward the design and development of methods capable of identifying patterns in a sea of swamping data noise. Despite many advances made in such endeavor, the ever-lasting annoying exceptions to the general patterns remain to pose difficulties in generalizing methods for comparative genomics. In this review, we discuss the different tools devised to undertake the challenge of comparative genomics and some of the exceptions that compromise the generality of such methods. We focus on endosymbiotic bacteria of insects because of their genomic dynamics peculiarities when compared to free-living organisms
Protein Evolution by Molecular Tinkering: Diversification of the Nuclear Receptor Superfamily from a Ligand-Dependent Ancestor
Phylogenetic reconstruction of the structure and function of the ancestor of the nuclear receptor protein family reveals how functional diversity evolves by subtle tinkering with an ancestral template
Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes
This article discusses mechanistic studies of ethylene hydrophenylation catalyzed by bipyridyl Pt(II) complexes
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