5,548 research outputs found
Pre-test analysis of protected loss of primary pump transients in CIRCE-HERO facility
In the frame of LEADER project (Lead-cooled European Advanced Demonstration Reactor), a new configuration of the steam generator for ALFRED (Advanced Lead Fast Reactor European Demonstrator) was proposed. The new concept is a super-heated steam generator, double wall bayonet tube type with leakage monitoring [1]. In order to support the new steam generator concept, in the framework of Horizon 2020 SESAME project (thermal hydraulics Simulations and Experiments for the Safety Assessment of MEtal cooled reactors), the ENEA CIRCE pool facility will be refurbished to host the HERO (Heavy liquid mEtal pRessurized water cOoled tubes) test section to investigate a bundle of seven full scale bayonet tubes in ALFRED-like thermal hydraulics conditions. The aim of this work is to verify thermofluid dynamic performance of HERO during the transition from nominal to natural circulation condition. The simulations have been performed with RELAP5-3D© by using the validated geometrical model of the previous CIRCE-ICE test section [2], in which the preceding heat exchanger has been replaced by the new bayonet bundle model. Several calculations have been carried out to identify thermal hydraulics performance in different steady state conditions. The previous calculations represent the starting points of transient tests aimed at investigating the operation in natural circulation. The transient tests consist of the protected loss of primary pump, obtained by reducing feed-water mass flow to simulate the activation of DHR (Decay
Heat Removal) system, and of the loss of DHR function in hot conditions, where feed-water mass flow rate is absent. According to simulations, in nominal conditions, HERO bayonet bundle offers excellent thermal hydraulic behavior and, moreover, it allows the operation in natural circulation
Post-test simulation of a PLOFA transient test in the CIRCE-HERO facility
CIRCE is a leadâbismuth eutectic alloy (LBE) pool facility aimed to simulate the primary system of a heavy liquid metal (HLM) cooled pool-type fast reactor. The experimental facility was implemented with a new test section, called HERO (Heavy liquid mEtal pRessurized water cOoled tubes), which consists of a steam generator composed of seven double-wall bayonet tubes (DWBT) with an active length of six meters. The experimental campaign aims to investigate HERO behavior, which is representative of the tubes that will compose ALFRED SG. In the framework of the Horizon 2020 SESAME project, a transient test was selected for the realization of a validation benchmark. The test consists of a protected loss of flow accident (PLOFA) simulating the shutdown of primary pumps, the reactor scram and the activation of the DHR system. A RELAP5-3D© nodalization scheme was developed in the pre-test phase at DIAEE of âSapienzaâ University of Rome, providing useful information to the experimentalists. The model consisted to a mono-dimensional scheme of the primary flow path and the SG secondary side, and a multi-dimensional component simulating the large LBE pool. The analysis of experimental data, provided by ENEA, has suggested to improve the thermalâhydraulic model with a more detailed nodalization scheme of the secondary loop, looking to reproduce the asymmetries observed on the DWBTs operation. The paper summarizes the post-test activity performed in the frame of the H2020 SESAME project as a contribution of the benchmark activity, highlighting a global agreement between simulations and experiment for all the primary circuit physical quantities monitored. Then, the attention is focused on the secondary system operation, where uncertainties related to the boundary conditions affect the computational results
Unveiling the inner morphology and gas kinematics of NGC 5135 with ALMA
The local Seyfert 2 galaxy NGC5135, thanks to its almost face-on appearance,
a bulge overdensity of stars, the presence of a large-scale bar, an AGN and a
Supernova Remnant, is an excellent target to investigate the dynamics of
inflows, outflows, star formation and AGN feedback. Here we present a
reconstruction of the gas morphology and kinematics in the inner regions of
this galaxy, based on the analysis of Atacama Large Millimeter Array (ALMA)
archival data. To our purpose, we combine the available 100 pc resolution
ALMA 1.3 and 0.45 mm observations of dust continuum emission, the spectroscopic
maps of two transitions of the CO molecule (tracer of molecular mass in star
forming and nuclear regions), and of the CS molecule (tracer of the dense star
forming regions) with the outcome of the SED decomposition. By applying the
BAROLO software (3D-Based Analysis of Rotating Object via Line
Observations), we have been able to fit the galaxy rotation curves
reconstructing a 3D tilted-ring model of the disk. Most of the observed
emitting features are described by our kinematic model. We also attempt an
interpretation for the emission in few regions that the axisymmetric model
fails to reproduce. The most relevant of these is a region at the northern edge
of the inner bar, where multiple velocity components overlap, as a possible
consequence of the expansion of a super-bubble.Comment: 15 pages, 13 figures, resubmitted to MNRAS after moderate revision
Floating-bending tensile-integrity structures
This is a conceptual work about the form-finding of a hybrid tensegrity structure. The structure was obtained from the combination of arch-supported membrane systems and diamond-type tensegrity systems. By combining these two types of structures, the resulting system features the âtensile-integrityâ property of cables and membrane together with what we call âfloating-bendingâ of the arches, a term which is intended to recall the words âfloating-compressionâ introduced by Kenneth Snelson, the father of tensegrities. Two approaches in the form-finding calculations were followed, the Matlab implementation of a simple model comprising standard constant-stress membrane/cable elements together with the so-called stick-andspring elements for the arches, and the analysis with the commercial software WinTess, used in conjunction with Rhino and Grasshopper. The case study of a T3 floatingbending tensile-integrity structure was explored, a structure that features a much larger enclosed volume in comparison to conventional tensegrity prisms. The structural design of an outdoor pavilion of 6 m in height was carried out considering ultimate and service limit states. This study shows that floating-bending structures are feasible, opening the way to the introduction of suitable analysis and optimization procedures for this type of structure
Physical properties of high-mass clumps in different stages of evolution
(Abridged) Aims. To investigate the first stages of the process of high-mass
star formation, we selected a sample of massive clumps previously observed with
the SEST at 1.2 mm and with the ATNF ATCA at 1.3 cm. We want to characterize
the physical conditions in such sources, and test whether their properties
depend on the evolutionary stage of the clump.
Methods. With ATCA we observed the selected sources in the NH3(1,1) and (2,2)
transitions and in the 22 GHz H2O maser line. Ammonia lines are a good
temperature probe that allow us to accurately determine the mass and the
column-, volume-, and surface densities of the clumps. We also collected all
data available to construct the spectral energy distribution of the individual
clumps and to determine if star formation is already occurring, through
observations of its most common signposts, thus putting constraints on the
evolutionary stage of the source. We fitted the spectral energy distribution
between 1.2 mm and 70 microns with a modified black body to derive the dust
temperature and independently determine the mass.
Results. The clumps are cold (T~10-30 K), massive (M~10^2-10^3 Mo), and dense
(n(H2)>~10^5 cm^-3) and they have high column densities (N(H2)~10^23 cm^-2).
All clumps appear to be potentially able to form high-mass stars. The most
massive clumps appear to be gravitationally unstable, if the only sources of
support against collapse are turbulence and thermal pressure, which possibly
indicates that the magnetic field is important in stabilizing them.
Conclusions. After investigating how the average properties depend on the
evolutionary phase of the source, we find that the temperature and central
density progressively increase with time. Sources likely hosting a ZAMS star
show a steeper radial dependence of the volume density and tend to be more
compact than starless clumps.Comment: Published in A&A, Vol. 556, A1
Post-test simulations for the NACIE-UP benchmark by STH codes
This paper illustrates the results obtained in the last phase of the NACIE-UP benchmark activity foreseen inside the EU SESAME Project. The purpose of this research activity, performed by system thermalâhydraulic (STH) codes, is finalized to the improvement, development and validation of existing STH codes for Heavy Liquid Metal (HLM) systems. All the participants improved their modelling of the NACIE-UP facility, respect to the initial blind simulation phase, adopting the actual experimental boundary conditions and reducing as much as possible sources of uncertainty in their numerical model. Four different STH codes were employed by the participants to the benchmark to model the NACIE-UP facility, namely: CATHARE for ENEA, ATHLET for GRS, RELAP5-3D© for the âSapienzaâ University of Rome and RELAP5/Mod3.3(modified) for the University of Pisa. Three reference tests foreseen in the NACIE-UP benchmark and carried out at ENEA Brasimone Research Centre were analysed from four participants. The data from the post-test analyses, performed independently by the participant using different STH codes, were compared together and with the available experimental results and critically discussed
A timeline for massive star-forming regions via combined observation of o-HD and ND
Context: In cold and dense gas prior to the formation of young stellar
objects, heavy molecular species (including CO) are accreted onto dust grains.
Under these conditions H and its deuterated isotopologues become more
abundant, enhancing the deuterium fraction of molecules such as NH that
are formed via ion-neutral reactions. Because this process is extremely
temperature sensitive, the abundance of these species is likely linked to the
evolutionary stage of the source.
Aims: We investigate how the abundances of o-HD and ND vary
with evolution in high-mass clumps.
Methods: We observed with APEX the ground-state transitions of o-HD
near 372 GHz, and ND(3-2) near 231 GHz for three massive clumps in
different evolutionary stages. The sources were selected within the
G351.77-0.51 complex to minimise the variation of initial chemical conditions,
and to remove distance effects. We modelled their dust continuum emission to
estimate their physical properties, and also modelled their spectra under the
assumption of local thermodynamic equilibrium to calculate beam-averaged
abundances.
Results: We find an anticorrelation between the abundance of o-HD and
that of ND, with the former decreasing and the latter increasing with
evolution. With the new observations we are also able to provide a qualitative
upper limit to the age of the youngest clump of about 10 yr, comparable to
its current free-fall time.
Conclusions: We can explain the evolution of the two tracers with simple
considerations on the chemical formation paths, depletion of heavy elements,
and evaporation from the grains. We therefore propose that the joint
observation and the relative abundance of o-HD and ND can act
as an efficient tracer of the evolutionary stages of the star-formation
process
Ab-initio calculation of all-optical time-resolved calorimetry of nanosized systems: Evidence of nanosecond-decoupling of electron and phonon temperatures
The thermal dynamics induced by ultrashort laser pulses in nanoscale systems,
i.e. all-optical time-resolved nanocalorimetry is theoretically investigated
from 300 to 1.5 K. We report ab-initio calculations describing the temperature
dependence of the electron-phonon interactions for Cu nanodisks supported on
Si. The electrons and phonons temperatures are found to decouple on the ns time
scale at 10 K, which is two orders of magnitude in excess with respect to that
found for standard low-temperature transport experiments. By accounting for the
physics behind our results we suggest an alternative route for overhauling the
present knowledge of the electron-phonon decoupling mechanism in nanoscale
systems by replacing the mK temperature requirements of conventional
experiments with experiments in the time-domain.Comment: 5 pages, 3 figures. Accepted on Physical Review B
Level-3 Calorimetric Resolution available for the Level-1 and Level-2 CDF Triggers
As the Tevatron luminosity increases sophisticated selections are required to
be efficient in selecting rare events among a very huge background. To cope
with this problem, CDF has pushed the offline calorimeter algorithm
reconstruction resolution up to Level 2 and, when possible, even up to Level 1,
increasing efficiency and, at the same time, keeping under control the rates.
The CDF Run II Level 2 calorimeter trigger is implemented in hardware and is
based on a simple algorithm that was used in Run I. This system has worked well
for Run II at low luminosity. As the Tevatron instantaneous luminosity
increases, the limitation due to this simple algorithm starts to become clear:
some of the most important jet and MET (Missing ET) related triggers have large
growth terms in cross section at higher luminosity. In this paper, we present
an upgrade of the Level 2 Calorimeter system which makes the calorimeter
trigger tower information available directly to a CPU allowing more
sophisticated algorithms to be implemented in software. Both Level 2 jets and
MET can be made nearly equivalent to offline quality, thus significantly
improving the performance and flexibility of the jet and MET related triggers.
However in order to fully take advantage of the new L2 triggering capabilities
having at Level 1 the same L2 MET resolution is necessary. The new Level-1 MET
resolution is calculated by dedicated hardware. This paper describes the
design, the hardware and software implementation and the performance of the
upgraded calorimeter trigger system both at Level 2 and Level 1.Comment: 5 pages, 5 figures,34th International Conference on High Energy
Physics, Philadelphia, 200
ATLASGAL-selected massive clumps in the inner Galaxy: I. CO depletion and isotopic ratios
In the low-mass regime, it is found that the gas-phase abundances of
C-bearing molecules in cold starless cores rapidly decrease with increasing
density, as the molecules form mantles on dust grains. We study CO depletion in
102 massive clumps selected from the ATLASGAL 870 micron survey, and
investigate its correlation with evolutionary stage and with the physical
parameters of the sources. Moreover, we study the gradients in [12C]/[13C] and
[18O]/[17O] isotopic ratios across the inner Galaxy, and the virial stability
of the clumps. We use low-J emission lines of CO isotopologues and the dust
continuum emission to infer the depletion factor fD. RATRAN one-dimensional
models were also used to determine fD and to investigate the presence of
depletion above a density threshold. The isotopic ratios and optical depth were
derived with a Bayesian approach. We find a significant number of clumps with a
large fD, up to ~20. Larger values are found for colder clumps, thus for
earlier evolutionary phases. For massive clumps in the earliest stages of
evolution we estimate the radius of the region where CO depletion is important
to be a few tenths of a pc. Clumps are found with total masses derived from
dust continuum emission up to ~20 times higher than the virial mass, especially
among the less evolved sources. These large values may in part be explained by
the presence of depletion: if the CO emission comes mainly from the low-density
outer layers, the molecules may be subthermally excited, leading to an
overestimate of the dust masses. CO depletion in high-mass clumps seems to
behave as in the low-mass regime, with less evolved clumps showing larger
values for the depletion than their more evolved counterparts, and increasing
for denser sources. The C and O isotopic ratios are consistent with previous
determinations, and show a large intrinsic scatter.Comment: 20 pages, 17 figures, 38 pages of online material (tables and
figures
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