1,329 research outputs found
The Influence of the Packing Factor on the Fuel Temperature Hot Spots in a Particle-Bed GCFR
In the recent past the so-called GCFR has been again a subject of study by the international scientific community. This type of reactors, although still in a preliminary stage of development, is a very interesting perspective because combines the positive characteristics common to all the fast reactors with those of the reactors cooled by helium. Up to now, almost all the analyses on the GCFR thermodynamic aspects have been performed starting from a "global" point of view: generally the core has been modelled as a porous medium and only the global parameters have been taken into account. The local effects have been included inadhoccorrective peak factors. The analyses carried out in the present research will be devoted to the characterization of the local effects, on a microscopic scale. In order to have reliable "global" nuclear and thermal-fluid-dynamic data, the performed analyses will be based on simulations previously performed using the RELAP5-3D code, assuming as input parameters the ETDR core ones. For each considered case, the variation ranges of the evaluated parameters have been estimated on the basis of the "best" and the "worst" cases. To summarize the obtained results, even in transient conditions, the variations of the considered input parameters are less significant for the local output values if compared to those due to the assumed packing factor. As a consequence, in a more general core calculation, the obtained local temperature (and velocity) values will have to be corrected by a proper factor that would have to take into account the results of this research
Probing equilibrium glass flow up to exapoise viscosities
Glasses are out-of-equilibrium systems aging under the crystallization
threat. During ordinary glass formation, the atomic diffusion slows down
rendering its experimental investigation impractically long, to the extent that
a timescale divergence is taken for granted by many. We circumvent here these
limitations, taking advantage of a wide family of glasses rapidly obtained by
physical vapor deposition directly into the solid state, endowed with different
"ages" rivaling those reached by standard cooling and waiting for millennia.
Isothermally probing the mechanical response of each of these glasses, we infer
a correspondence with viscosity along the equilibrium line, up to exapoise
values. We find a dependence of the elastic modulus on the glass age, which,
traced back to temperature steepness index of the viscosity, tears down one of
the cornerstones of several glass transition theories: the dynamical
divergence. Critically, our results suggest that the conventional wisdom
picture of a glass ceasing to flow at finite temperature could be wrong.Comment: 4 figures and 1 supplementary figur
Ultrafast hot electron dynamics in plasmonic nanostructures: Experiments, modelling, design
Metallic nanostructures exhibit localized surface plasmons (LSPs), which offer unprecedented opportunities for advanced photonic materials and devices. Following resonant photoexcitation, LSPs quickly dephase, giving rise to a distribution of energetic ‘hot’ electrons in the metal. These out-of-equilibrium carriers undergo ultrafast internal relaxation processes, nowadays pivotal in a variety of applications, from photodetection and sensing to the driving of photochemical reactions and ultrafast all-optical modulation of light. Despite the intense research activity, exploitation of hot carriers for real-world nanophotonic devices remains extremely challenging. This is due to the com- plexity inherent to hot carrier relaxation phenomena at the nanoscale, involving short-lived out-of-equilibrium electronic states over a very broad range of energies, in interaction with thermal electronic and phononic baths. These issues call for a comprehensive understanding of ultrafast hot electron dynamics in plasmonic nanostructures. This paper aims to review our contribution to the field: starting from the fundamental physics of plasmonic nanostructures, we first describe the experimental techniques used to probe hot electrons; we then introduce a numerical model of ultrafast nanoscale relaxation processes, and present examples in which experiments and modelling are combined, with the aim of designing novel optical functionalities enabled by ultrafast hot-electron dynamics
Parametric Nonlinear Optics with Layered Materials and Related Heterostructures
Nonlinear optics is of crucial importance in several fields of science and
technology with applications in frequency conversion, entangled-photon
generation, self-referencing of frequency combs, crystal characterization,
sensing, and ultra-short light pulse generation and characterization. In recent
years, layered materials and related heterostructures have attracted huge
attention in this field, due to their huge nonlinear optical susceptibilities,
their ease of integration on photonic platforms, and their 2D nature which
relaxes the phase-matching constraints and thus offers a practically unlimited
bandwidth for parametric nonlinear processes. In this review the most recent
advances in this field, highlighting their importance and impact both for
fundamental and technological aspects, are reported and explained, and an
outlook on future research directions for nonlinear optics with atomically thin
materials is provided
A Critical Review of the Recent Improvements in Minimizing Nuclear Waste by Innovative Gas-Cooled Reactors
This paper presents a critical review of the recent improvements in minimizing nuclear waste in terms of quantities, long-term activities, and radiotoxicities by innovative GCRs, with particular emphasis to the results obtained at the University of Pisa. Regarding these last items, in the frame of some EU projects (GCFR, PUMA, and RAPHAEL), we analyzed symbiotic fuel cycles coupling current LWRs with HTRs, finally closing the cycle by GCFRs. Particularly, we analyzed fertile-free and Pu-Th-based fuel in HTR: we improved plutonium exploitation also by optimizing Pu/Th ratios in the fuel loaded in an HTR. Then, we chose GCFRs to burn residual MA. We have started the calculations on simplified models, but we ended them using more "realistic" models of the reactors. In addition, we have added the GCFR multiple recycling option usingkeffcalculations for all the reactors. As a conclusion, we can state that, coupling HTR with GCFR, the geological disposal issues concerning high-level radiotoxicity of MA can be considerably reduced
Resonant optical control of the structural distortions that drive ultrafast demagnetization in CrO
We study how the color and polarization of ultrashort pulses of visible light
can be used to control the demagnetization processes of the antiferromagnetic
insulator CrO. We utilize time-resolved second harmonic generation
(SHG) to probe how changes in the magnetic and structural state evolve in time.
We show that, varying the pump photon-energy to excite either localized
transitions within the Cr or charge transfer states, leads to markedly
different dynamics. Through a full polarization analysis of the SHG signal,
symmetry considerations and density functional theory calculations, we show
that, in the non-equilibrium state, SHG is sensitive to {\em both} lattice
displacements and changes to the magnetic order, which allows us to conclude
that different excited states couple to phonon modes of different symmetries.
Furthermore, the spin-scattering rate depends on the induced distortion,
enabling us to control the timescale for the demagnetization process. Our
results suggest that selective photoexcitation of antiferromagnetic insulators
allows fast and efficient manipulation of their magnetic state.Comment: 7 pages, 5 figure
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