344 research outputs found
Use of the Natural Circulation Flow Map for Natural Circulation Systems Evaluation
The aim of this paper is to collect and resume the work done to build and develop, at the University of Pisa, an engineering tool related to the natural circulation. After a brief description of the different loop flow regimes in single phase and two phase, the derivation of a suitable tool to judge the NC performance in a generic system is presented. Finally, an extensive comparison among the NC performance of various nuclear power plants having different design is done to show a practical application of the NC flow map
Thermal-Hydraulic Analysis of Coolant Flow Decrease in Fuel Channels of Smolensk-3 RBMK during GDH Blockage Event
One of the transients that have received considerable attention in the safety evaluation of RBMK reactors is the partial break of a group distribution header (GDH). The coolant flow rate blockage in one GDH might lead to excessive heat-up of the pressure tubes and can result in multiple fuel channels (FC) ruptures. In this work, the GDH flow blockage transient has been studied considering the Smolensk-3 RBMK NPP (nuclear power plant). In the RBMK, each GDH distributes coolant to 40–43 FC. To investigate the behavior of each FC belonging to the damaged GDH and to have a more realistic trend, one (affected) GDH has been schematized with its forty-two FC, one by one. The calculations were performed using the 0-D NK (neutron kinetic) model of the RELAP5-3.3 stand-alone code. The results show that, during the event, the mass flow rate is disturbed differently according to the power distribution established for each FC in the schematization. The start time of the oscillations in mass flow rate depends strongly on the attributed power to each FC. It was also observed that, during the event, the fuel channels at higher thermal power values tend to undergo first cladding rupture leaving the reactor to scram and safeguarding all the other FCs connected to the affected GDH
Magnetic Helicity Generation from the Cosmic Axion Field
The coupling between a primordial magnetic field and the cosmic axion field
generates a helical component of the magnetic field around the time in which
the axion starts to oscillate. If the energy density of the seed magnetic field
is comparable to the energy density of the universe at that time, then the
resulting magnetic helicity is about |H_B| \simeq (10^{-20} G)^2 kpc and
remains constant after its generation. As a corollary, we find that the
standard properties of the oscillating axion remain unchanged even in the
presence of very strong magnetic fields.Comment: 6 pages, 2 figures. Accepted for publication in Phys. Rev. D. Minor
revisions and new references adde
Production of Axions by Cosmic Magnetic Helicity
We investigate the effects of an external magnetic helicity production on the
evolution of the cosmic axion field. It is shown that a helicity larger than
(few \times 10^{-15} G)^2 Mpc, if produced at temperatures above a few GeV, is
in contradiction with the existence of the axion, since it would produce too
much of an axion relic abundance.Comment: 4 pages, 2 figures, major changes in the first part of the paper,
reference added, to be published in Phys. Rev. Let
A Note on the Cosmic Evolution of the Axion in a Strong Magnetic Field
It has been pointed out in the literature that in the presence of an external
magnetic field the axion mass receives an electromagnetic contribution. We show
that if a magnetic field with energy density larger than ~10^{-8} times the
energy density of the Universe existed at temperatures of a few GeV, that
contribution would be dominant and consequently the cosmic evolution of the
axion field would change substantially. In particular, the expected axion relic
abundance would be lowered, allowing a small relaxation of the present
cosmological bound on the Peccei-Quinn constant.Comment: 2 pages, no figures. Minor changes. References added. Accepted for
publication in JCA
Constraints on Axion-like Particles from a Hard -ray Observation of Betelgeuse
We use the first observation of Betelgeuse in hard -rays to perform a
novel search for axion-like particles (ALPs). Betelgeuse is not expected to be
a standard source of -rays, but light ALPs produced in the stellar core
could be converted back into photons in the Galactic magnetic field, producing
a detectable flux that peaks in the hard -ray band
(). Using a 50 ks observation of Betelgeuse by the
satellite telescope, we find no significant excess of events above the
expected background. Using models of the regular Galactic magnetic field in the
direction of Betelgeuse, we set a 95% C.L. upper limit on the ALP-photon
coupling of GeV (depending on
magnetic field model) for ALP masses eV
The supportive effects of IL-7 on eosinophil progenitors from human bone marrow cells can be blocked by anti-IL-5
Human rIL-7 was studied for its effects on myeloid and erythroid progenitors from human bone marrow cells. IL-7 did not support the granulocytic/monocytic or erythroid lineage but exclusively stimulated eosinophil colony formation (CFU-Eo) (4 ± 3 vs 48 ± 17 CFU-Eo/105 nonadherent fraction-non-T cell (NAF-NT) cells). This supportive effect was not mediated by T cells or monocytes because similar results were obtained with or without T cell or adherent depleted cell fractions. In addition, it was shown that CD34+ sorted cells could be stimulated by IL-7 (0 vs 15 ± 9 CFU-Eo/3 x 103 CD34+ cells). Furthermore studies with IL-3 or granulocyte- macrophage CSF (GM-CSF) demonstrated an additive effect on the IL-7 supported colony formation. Finally, experiments were performed with anti-IL-3, anti- GM-CSF, anti-IL-1, and anti-IL-5 to exclude the possibility that IL-7 indirectly stimulated the eosinophil progenitor cell. Anti-GM-CSF, anti-IL-1, or anti-IL-3 did not influence the supportive effects of IL-7. However, anti- IL-5 did abolish the effects of IL-7 on the eosinophil colony formation (69 ± 15 vs 3 ± 2 CFU-Eo/105 NAF-NT, n = 3). Similar results were obtained with CD34+ sorted cells. Moreover, IL-5 mRNA expression could be demonstrated in IL-7-stimulated NAF-NT cells. These data suggest that the supportive effects of IL-7 on eosinophil precursors are mediated by the endogenous release of IL-5.</p
On the Perturbative Stability of Quantum Field Theories in de Sitter Space
We use a field theoretic generalization of the Wigner-Weisskopf method to
study the stability of the Bunch-Davies vacuum state for a massless,
conformally coupled interacting test field in de Sitter space. We find that in
theory the vacuum does {\em not} decay, while in
non-conformally invariant models, the vacuum decays as a consequence of a
vacuum wave function renormalization that depends \emph{singularly} on
(conformal) time and is proportional to the spatial volume. In a particular
regularization scheme the vacuum wave function renormalization is the same as
in Minkowski spacetime, but in terms of the \emph{physical volume}, which leads
to an interpretation of the decay. A simple example of the impact of vacuum
decay upon a non-gaussian correlation is discussed. Single particle excitations
also decay into two particle states, leading to particle production that
hastens the exiting of modes from the de Sitter horizon resulting in the
production of \emph{entangled superhorizon pairs} with a population consistent
with unitary evolution. We find a non-perturbative, self-consistent "screening"
mechanism that shuts off vacuum decay asymptotically, leading to a stationary
vacuum state in a manner not unlike the approach to a fixed point in the space
of states.Comment: 36 pages, 4 figures. Version to appear in JHEP, more explanation
Third Yearly Activity Report
The calculation work performed during the 3rd project year in WP2 as well as the R&D activities carried out in WP3, WP4 and WP5 are described in this report. In addition, the work dedicated to the project management (WP1) as well as to WP6 regarding the dissemination/communication activities and the education/training program (e.g. the follow-up of the mobility program between different organizations in the consortium, training on simulation tools and activities accomplished by PhD/post-doctoral students) is also reported
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