Performance of Passive Heat Removal System Under Accident Conditions: a Study Case

In many innovative LWR designs, passive safety systems are widely envisaged. In general, their behaviour involves natural and simple physical events as free convection, thus entailing an higher safety degree of the plant, inversely proportional to the necessity of human intervention or energized devices to prime the safety system. A thorough behaviour’s knowledge of this kind of systems re-quires experimental campaigns as well as numerical simulations: both the Regulatory Commissions and the Utilities are engaged to verify the effectiveness and reliability of these systems. From the simulation view point, a different choice in nodaliza-tion, heat transfer correlation and system dimensions could lead to different system performance during accident transients. This problem is pointed out in this paper: as test case, a Passive Heat Removal loop under Steam Line Break accident conditions is concerned. The results obtained show that thick circuit nodalization, con-sistent with a value of the Courant number close to unity, is a ne-cessity to find out the correct system’s behaviour. Besides they confirm the need of experimental facilities, in particular to inves-tigate the circuit’s activation, and of parametric studies for opti-mization design; for the last issue, computer codes that hold fast-running feature even with a large number of circuit nodes are very useful tools

Winds and Infalling Gas in Lyman Break Galaxies

A model for gas outflows is proposed which simultaneously explains the correlations between the (i) equivalent width of low ionization and Ly-alpha lines, (ii) outflow velocity, and (iii) star formation rate observed in Lyman Break Galaxies (LBGs). Our interpretation implies that LBGs host short-lived (30 +/- 5 Myr) starburst episodes observed at different evolutionary phases. Initially, the starburst powers a hot wind bound by a denser cold shell, which after about 5 Myr becomes dynamically unstable and fragments; afterwards the fragment evolution is approximately ballistic while the hot bubble continues to expand. As the fragments are gravitationally decelerated, their screening ability of the starlight decreases as the UV starburst luminosity progressively dims. LBG observations sample all these evolutionary phases. Finally, the fragments fall back onto the galaxy after approximately 60 Myr. This phase cannot be easily probed as it occurs when the starburst UV luminosity has already largely faded; however, galaxies dimmer in the UV than LBGs should show infalling gas.Comment: Accepted by MNRA

Constraints on the Dark Matter Particle Mass from the Number of Milky Way Satellites

We have conducted N-body simulations of the growth of Milky Way-sized halos in cold and warm dark matter cosmologies. The number of dark matter satellites in our simulated Milky Ways decreases with decreasing mass of the dark matter particle. Assuming that the number of dark matter satellites exceeds or equals the number of observed satellites of the Milky Way we derive lower limits on the dark matter particle mass. We find with 95% confidence m_s > 13.3 keV for a sterile neutrino produced by the Dodelson and Widrow mechanism, m_s > 8.9 keV for the Shi and Fuller mechanism, m_s > 3.0 keV for the Higgs decay mechanism, and m_{WDM} > 2.3 keV for a thermal dark matter particle. The recent discovery of many new dark matter dominated satellites of the Milky Way in the Sloan Digital Sky Survey allows us to set lower limits comparable to constraints from the complementary methods of Lyman-alpha forest modeling and X-ray observations of the unresolved cosmic X-ray background and of dark matter halos from dwarf galaxy to cluster scales. Future surveys like LSST, DES, PanSTARRS, and SkyMapper have the potential to discover many more satellites and further improve constraints on the dark matter particle mass.Comment: 17 pages, 13 figures, replaced with final version published in Physical Review

On Nonshearing Magnetic Configurations in Differentially Rotating Disks

A new class of disk MHD equilibrium solutions is described, which is valid within the standard local (``shearing sheet'') approximation scheme. These solutions have the following remarkable property: velocity streamlines and magnetic lines of force rotate rigidly, even in the presence of differential rotation. This situation comes about because the Lorentz forces acting upon modified epicycles compel fluid elements to follow magnetic lines of force. Field line (and streamline) configurations may be elliptical or hyperbolic, prograde or retrograde. These structures have previously known hydrodynamical analogs: the ``planet'' solutions described by Goodman, Narayan, & Goldreich. The primary focus of this investigation is configurations in the disk plane. A related family of solutions lying in a vertical plane is briefly discussed; other families of solutions may exist. Whether these MHD structures are stable is not yet known, but could readily be determined by three-dimensional simulations. If stable or quasi-stable, these simple structures may find important applications in both accretion and galactic disks

Dependence of the Inner DM Profile on the Halo Mass

I compare the density profile of dark matter (DM) halos in cold dark matter (CDM) N-body simulations with 1 Mpc, 32 Mpc, 256 Mpc and 1024 Mpc box sizes. In dimensionless units the simulations differ only for the initial power spectrum of density perturbations. I compare the profiles when the most massive halos are composed of about 10^5 DM particles. The DM density profiles of the halos in the 1 Mpc box show systematically shallower cores with respect to the corresponding halos in the 32 Mpc simulation that have masses, M_{dm}, typical of the Milky Way and are fitted by a NFW profile. The DM density profiles of the halos in the 256 Mpc box are consistent with having steeper cores than the corresponding halos in the 32 Mpc simulation, but higher mass resolution simulations are needed to strengthen this result. Combined, these results indicate that the density profile of DM halos is not universal, presenting shallower cores in dwarf galaxies and steeper cores in clusters. Physically the result sustains the hypothesis that the mass function of the accreting satellites determines the inner slope of the DM profile. In comoving coordinates, r, the profile \rho_{dm} \propto 1/(X^\alpha(1+X)^{3-\alpha}), with X=c_\Delta r/r_\Delta, r_\Delta is the virial radius and \alpha =\alpha(M_{dm}), provides a good fit to all the DM halos from dwarf galaxies to clusters at any redshift with the same concentration parameter c_\Delta ~ 7. The slope, \gamma, of the outer parts of the halo appears to depend on the acceleration of the universe: when the scale parameter is a=(1+z)^{-1} < 1, the slope is \gamma ~ 3 as in the NFW profile, but \gamma ~ 4 at a > 1 when \Omega_\Lambda ~ 1 and the universe is inflating.[abridged]Comment: Accepted for publication in MNRAS. 13 pages, including 11 figures and 2 tables. The revised version has an additional discussion section and work on the velocity dispersion anisotrop

Experimental Constraints on Self-consistent Reionization Models

A self-consistent formalism to jointly study cosmic reionization and thermal history of the IGM is presented. The model implements most of the relevant physics governing these processes, such as the inhomogeneous IGM density distribution, three different sources of ionizing photons (PopIII stars, PopII stars and QSOs), and radiative feedback. By constraining the free parameters with available data on redshift evolution of Lyman-limit systems, Gunn-Peterson and electron scattering optical depths, Near InfraRed Background (NIRB), and cosmic star formation history, we select a fiducial model, whose main predictions are: (i) H was completely reionized at z \approx 15, while HeII must have been reionized by z \approx 12. At z \approx 7, HeIII suffered an almost complete recombination as a result of the extinction of PopIII stars, as required by the interpretation of the NIRB. (ii) A QSO-induced complete HeII reionization occurs at z=3.5; a similar double H reionization does not take place due to the large number of photons above 1 Ryd from PopII stars and QSOs, even after PopIII stars have disappeared. (iii) Following reionization, the temperature of the IGM corresponding to the mean gas density is boosted to 15000 K. Observations of T_0 are consistent with the fact that He is singly ionized at z > 3.5, while they are consistent with He being doubly ionized at z < 3.5. This might be interpreted as a signature of (second) HeII reionization. (iv) Only 0.3% of the stars produced by z=2 need to be PopIII stars in order to achieve the hydrogen reionization. Such model not only relieves the tension between the Gunn-Peterson optical depth and WMAP observations, but also accounts self-consistently for all known observational constraints (abridged).Comment: Revised version. Accepted for publication in MNRA

A scheme of correlation for frictional pressure drop in steam–water two-phase flow in helicoidal tubes

In the nuclear field, helically coiled tube steam generators (SGs) are considered as a primary option for different nuclear reactor projects of Generation III+ and Generation IV. For their characteristics, in particular compactness of the component design, higher heat transfer rates and better capability to accommodate thermal expansion, they are especially attractive for small-medium modular reactors (SMRs) of Generation III+. In this paper, starting from two existing databases, a new correlation is developed for the determination of the two-phase frictional pressure drop. The experimental data cover the ranges 5–65 bar for the pressure, 200 to 800 kg/m2 s for the mass flux and 0 to 1 for the quality. Two coil diameters have been considered, namely 0.292 m and 1.0 m. The coil diameter in particular is crucial for a correct estimation of the two-phase frictional pressure drop. Actually, no general correlation reliable in a wide range of coil geometries is available at the moment. Starting from the noteworthy correlation of Lockhart and Martinelli, corrective parameters are included to account for the effect of the centrifugal force, introduced by the helical geometry, and the system pressure. The correlation is developed with the aim to obtain a form of general validity, while keeping as low as possible the number of empirical coefficients involved. The average relative deviation between the correlation and the experimental data is about 12.9% on the whole database, which results the best among numerous literature correlations. In addition, the new correlation is characterized by an extended range of validity, in particular for the diameter of the coil

CFD study of an air–water flow inside helically coiled pipes

CFD is used to study an air–water mixture flowing inside helically coiled pipes, being at the moment considered for the Steam Generators (SGs) of different nuclear reactor projects of Generation III+ and Generation IV. The two-phase mixture is described through the Eulerian–Eulerian model and the adiabatic flow is simulated through the ANSYS FLUENT code. A twofold objective is pursued. On the one hand, obtaining an accurate estimation of physical quantities such as the frictional pressure drop and the void fraction. In this regard, CFD simulations can provide accurate predictions without being limited to a particular range of system parameters, which often constricts the application of empirical correlations. On the other hand, a better understanding of the role of the centrifugal force field and its effect on the two-phase flow field and the phase distributions is pursued. The effect of the centrifugal force field introduced by the geometry is characterized. Water is pushed by the centrifugal force towards the outer pipe wall, whereas air accumulates in the inner region of the pipe. The maximum of the mixture velocity is therefore shifted towards the inner pipe wall, as the air flows much faster than the water, having a considerably lower density. The flow field, as for the single-phase flow, is characterized by flow recirculation and vortices. Quantitatively, the simulation results are validated against the experimental data of Akagawa et al. (1971) for the void fraction and the frictional pressure drop. The relatively simple model of momentum interfacial transfer allows obtaining a very good agreement for the average void fraction and a satisfactory estimation of the frictional pressure drop and, at the same time, limits the computational cost of the simulations. Effects of changes in the diameter of the dispersed phase are described, as its value strongly affects the degree of interaction between the phases. In addition, a more precise treatment of the near wall region other than wall function results in a better definition of the liquid film at the wall, although an overestimation of the frictional pressure drop is obtained

Cosmic Microwave Background Polarization and reionization: constraining models with a double reionization

Neutral hydrogen around high-z QSO and an optical depth tau ~ 0.17 can be reconciled if reionization is more complex than a single transition at z ~ 6-8. Tracing its details could shed a new light on the first sources of radiation. Here we discuss how far such details can be inspected through planned experiments on CMB large-scale anisotropy and polarization, by simulating an actual data analysis. By considering a set of double reionization histories of Cen (2003) type, a relevant class of models not yet considered by previous works, we confirm that large angle experiments rival high resolution ones in reconstructing the reionization history. We also confirm that reionization histories, studied with the prior of a single and sharp reionization, yield a biased tau, showing that this bias is generic. We further find a monotonic trend in the bias for the models that we consider, and propose an explanation of the trend, as well as the overall bias. We also show that in long-lived experiments such a trend can be used to discriminate between single and double reionization patterns.Comment: 8 pages, 11 figures. Substantial rewriting, replaced with accepted version. To be published in A&