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Molten salt reactors - safety options galore
Safety features and attributes of molten salt reactors (MSR) are described. The unique features of fluid fuel reactors of on-line continuous processing and the ability for so-called external cooling result in simple and safe designs with low excess reactivity, low fission product inventory, and small source term. These, in turn, make a criticality accident unlikely and reduce the severity of a loss of coolant to where they are no longer severe accidents. A melt down is not an accident for a reactor that uses molten fuel. The molten salts are stable, non-reactive and efficient heat transfer media that operate at high temperatures at low pressures and are highly compatible with selected structural materials. All these features reduce the accident plethora. Freeze valves can be used for added safety. An ultimate safe reactor (U.S.R) is described with safety features that are passive, inherent and non-tamperable (PINT)
High-speed Civil Transport Aircraft Emissions
Estimates are given for the emissions from a proposed high speed civil transport (HSCT). This advanced technology supersonic aircraft would fly in the lower stratosphere at a speed of roughly Mach 1.6 to 3.2 (470 to 950 m/sec or 920 to 1850 knots). Because it would fly in the stratosphere at an altitude in the range of 15 to 23 km commensurate with its design speed, its exhaust effluents could perturb the chemical balance in the upper atmosphere. The first step in determining the nature and magnitude of any chemical changes in the atmosphere resulting from these proposed aircraft is to identify and quantify the chemically important species they emit. Relevant earlier work is summarized, dating back to the Climatic Impact Assessment Program of the early 1970s and current propulsion research efforts. Estimates are provided of the chemical composition of an HSCT's exhaust, and these emission indices are presented. Other aircraft emissions that are not due to combustion processes are also summarized; these emissions are found to be much smaller than the exhaust emissions. Future advances in propulsion technology, in experimental measurement techniques, and in understanding upper atmospheric chemistry may affect these estimates of the amounts of trace exhaust species or their relative importance
Grid Size Effects With Smeared Cracking in Finite Element Analysis of Reinforced Concrete
The effects of modeling parameters on the response of finite element representations of reinforced concrete members are examined. Convergence of load-deflection curves and cracking patterns is studied. Nonlinear behavior is limited to cracking of the concrete and yielding of the reinforcement. The ''smeared" crack representation is governed by a limiting tensile stress criterion. Concrete is treated as linear elastic in compression. Reinforcement has a bilinear stress-strain curve. Constant strain bar elements and rectangular isoparametric elements model the steel and the concrete, respectively. Analyses are performed for flexural members with span-to-depth ratios of 12-1, 5-l and 2-1, under both a uniformly distributed load and a concentrated load at midspan, using a minimum of three variations in grid refinement. Load-deflection curves exhibit convergence with grid refinement. Concrete tensile s tr eng th has a negligible influence on response for the members studied. Load increment size affects the response only in unstable regions of the load deflection curve and does not affect the stiff ness or the post-yield response
Geometry of River Networks II: Distributions of Component Size and Number
The structure of a river network may be seen as a discrete set of nested
sub-networks built out of individual stream segments. These network components
are assigned an integral stream order via a hierarchical and discrete ordering
method. Exponential relationships, known as Horton's laws, between stream order
and ensemble-averaged quantities pertaining to network components are observed.
We extend these observations to incorporate fluctuations and all higher moments
by developing functional relationships between distributions. The relationships
determined are drawn from a combination of theoretical analysis, analysis of
real river networks including the Mississippi, Amazon and Nile, and numerical
simulations on a model of directed, random networks. Underlying distributions
of stream segment lengths are identified as exponential. Combinations of these
distributions form single-humped distributions with exponential tails, the sums
of which are in turn shown to give power law distributions of stream lengths.
Distributions of basin area and stream segment frequency are also addressed.
The calculations identify a single length-scale as a measure of size
fluctuations in network components. This article is the second in a series of
three addressing the geometry of river networks.Comment: 16 pages, 13 figures, 4 tables, Revtex4, submitted to PR
Geometry of River Networks I: Scaling, Fluctuations, and Deviations
This article is the first in a series of three papers investigating the
detailed geometry of river networks. Large-scale river networks mark an
important class of two-dimensional branching networks, being not only of
intrinsic interest but also a pervasive natural phenomenon. In the description
of river network structure, scaling laws are uniformly observed. Reported
values of scaling exponents vary suggesting that no unique set of scaling
exponents exists. To improve this current understanding of scaling in river
networks and to provide a fuller description of branching network structure, we
report here a theoretical and empirical study of fluctuations about and
deviations from scaling. We examine data for continent-scale river networks
such as the Mississippi and the Amazon and draw inspiration from a simple model
of directed, random networks. We center our investigations on the scaling of
the length of sub-basin's dominant stream with its area, a characterization of
basin shape known as Hack's law. We generalize this relationship to a joint
probability density and show that fluctuations about scaling are substantial.
We find strong deviations from scaling at small scales which can be explained
by the existence of linear network structure. At intermediate scales, we find
slow drifts in exponent values indicating that scaling is only approximately
obeyed and that universality remains indeterminate. At large scales, we observe
a breakdown in scaling due to decreasing sample space and correlations with
overall basin shape. The extent of approximate scaling is significantly
restricted by these deviations and will not be improved by increases in network
resolution.Comment: 16 pages, 13 figures, Revtex4, submitted to PR
Can uptake length in strams be determined by nutrient addition experiments? Results from an interbiome comparison study
Nutrient uptake length is an important parnmeter tor quantifying nutrient cycling in streams. Although nutrient tracer additions are the preierred method for measuring uptake length under ambient nutrient concentrations, short-term nutrient addition experiments have more irequently been used to estimate uptake length in streams. Theoretical analysis of the relationship between uptake length determined by nutrient addition experiments (Sw\u27) and uptake length determined by tracer additions (Sw)predicted that Sw\u27 should be consistently longer than 5, , and that the overestimate of uptake length by Sw( should be related to the level of nutrient addition above ambient concentrations and the degree of nutrient limitation. To test these predictions, we used data irom an interbiorne study of NH,- uptake length in which 15NH,- tracer and short-term NH,-a ddition experiments were performed in 10 streams using a uniform experimental approach. The experimental results largely contirmed the theoretical predictions: sw\u27 was consistently longer than Sw and Sw\u27:Sw ratios were directly related to the level of NH,- addition and to indicatvrs of N limitation. The experimentally derived Sw\u27:Sw, ratios were used with the theoretical results to infer the N limitation status of each stream. Together, the theoretical and experimental results showed the tracer experiments should be used whenever possible to determine nutrient uptake length in streams. Nutrient addition experiments may be useful for comparing uptake lengths between different streams or cliiferent times in the same stream. however, provided that nutrient additions are kept as low as possible and of similar miagnitude
Stochastic analysis of surface roughness
For the characterization of surface height profiles we present a new
stochastic approach which is based on the theory of Markov processes. With this
analysis we achieve a characterization of the complexity of the surface
roughness by means of a Fokker-Planck or Langevin equation, providing the
complete stochastic information of multiscale joint probabilities. The method
was applied to different road surface profiles which were measured with high
resolution. Evidence of Markov properties is shown. Estimations for the
parameters of the Fokker-Planck equation are based on pure, parameter free data
analysis
Stress transmission in granular matter
The transmission of forces through a disordered granular system is studied by
means of a geometrical-topological approach that reduces the granular packing
into a set of layers. This layered structure constitutes the skeleton through
which the force chains set up. Given the granular packing, and the region where
the force is applied, such a skeleton is uniquely defined. Within this
framework, we write an equation for the transmission of the vertical forces
that can be solved recursively layer by layer. We find that a special class of
analytical solutions for this equation are L\'evi-stable distributions. We
discuss the link between criticality and fragility and we show how the
disordered packing naturally induces the formation of force-chains and arches.
We point out that critical regimes, with power law distributions, are
associated with the roughness of the topological layers. Whereas, fragility is
associated with local changes in the force network induced by local granular
rearrangements or by changes in the applied force. The results are compared
with recent experimental observations in particulate matter and with computer
simulations.Comment: 14 pages, Latex, 5 EPS figure
Factors affecting ammonium uptake in streams - an inter-biome perspective
The Lotic Intersite Nitrogen experiment (LINX) was a coordinated study of the relationships between North American biomes and factors governing ammonium uptake in streams. Our objective was to relate inter-biome variability of ammonium uptake to physical, chemical and biological processes. 2. Data were collected from 11 streams ranging from arctic to tropical and from desert to rainforest. Measurements at each site included physical, hydraulic and chemical characteristics, biological parameters, whole-stream metabolism and ammonium uptake. Ammonium uptake was measured by injection of \u275~-ammonium and downstream measurements of 15N-ammonium concentration. 3. We found no general, statistically significant relationships that explained the variability in ammonium uptake among sites. However, this approach does not account for the multiple mechanisms of ammonium uptake in streams. When we estimated biological demand for inorganic nitrogen based on our measurements of in-stream metabolism, we found good correspondence between calculated nitrogen demand and measured assimilative nitrogen uptake. 4. Nitrogen uptake varied little among sites, reflecting metabolic compensation in streams in a variety of distinctly different biomes (autotrophic production is high where allochthonous inputs are relatively low and vice versa). 5. Both autotrophic and heterotrophic metabolism require nitrogen and these biotic processes dominate inorganic nitrogen retention in streams. Factors that affect the relative balance of autotrophic and heterotrophic metabolism indirectly control inorganic nitrogen uptake
Gradient Clogging in Depth Filtration
We investigate clogging in depth filtration, in which a dirty fluid is
``cleaned'' by the trapping of dirt particles within the pore space during flow
through a porous medium. This leads to a gradient percolation process which
exhibits a power law distribution for the density of trapped particles at
downstream distance x from the input. To achieve a non-pathological clogging
(percolation) threshold, the system length L should scale no faster than a
power of ln w, where w is the width. Non-trivial behavior for the permeability
arises only in this extreme anisotropic geometry.Comment: 4 pages, 3 figures, RevTe
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