The physics of nuclear fission will be briefly illustrated, from the basic mechanism behind this phenomenon to the relevant physical quantities like nuclear cross sections, neutron flux and reaction ...products, together with the accompanying phenomenon of neutron capture and its role in determining how the fuel transforms in a nuclear reactor. The basic concepts underlying the operation of different types of nuclear reactors will be illustrated, along with the concept of fuel cycle. The aspects of radioactive waste, fuel resources and safety will also be briefly illustrated.
This study aims at evaluating the optimal properties of friction pendulum bearings to be employed for the seismic protection of elastic isolated structural systems under earthquake excitations with ...different characteristics in terms of frequency content. A two-degree-of-freedom model is considered to describe the isolated system behavior while accounting for the superstructure flexibility and a non-dimensional formulation of the governing equations of motion is employed to relate the characteristic parameters describing the isolator and structure properties to the response parameters of interest for the performance assessment. Seismic excitations are modeled as time-modulated filtered Gaussian white noise random processes of different intensity within the power spectral density method. The filter parameters control the frequency content of the random excitations and are calibrated to describe stiff, medium and soft soil conditions, respectively. Finally, multi-variate regression expressions are obtained for the optimum values of the friction coefficient that minimize the superstructure displacements relative to the base mass as a function of the structural system properties, of the seismic input intensity and of the soil condition.
•Seismic performance of systems isolated by FPS for different soil conditions.•Artificial records with frequency contents related to different soils are used.•Influence of soft soils on the statistics of the response of isolated systems.•Optimal friction coefficient to minimize structure response for different soils.•Non-linear multivariate regression equations to design base-isolated systems.
Energy from nuclear fission Ripani, M.
EPJ Web of Conferences,
2020, Letnik:
246
Journal Article, Conference Proceeding
Recenzirano
Odprti dostop
The physics of nuclear fission will be briefly illustrated, from the basic mechanism behind this phenomenon to the relevant physical quantities like nuclear cross sections, neutron flux and reaction ...products, together with the accompanying phenomenon of neutron capture and its role in determining how the fuel transforms in a nuclear reactor. The basic concepts underlying the operation of different types of nuclear reactors will be illustrated, along with the concept of fuel cycle. After touching on the aspect of safety, the current situation of nuclear power in the world, with its costs, its role in reducing carbon emissions, the available resources and finally the issues of waste management and accidents will be briefly illustrated.
The aim of this work is to evaluate the optimal properties of friction pendulum system (FPS) bearings for the seismic protection of bridge piers under earthquake excitations having different ...frequency characteristics representative of different soil conditions in order to reduce the seismic vulnerability of infrastructures increasing their safety level. A two-degree-of-freedom model is adopted to describe, respectively, the response of the infinitely rigid deck isolated by the FPS devices and the elastic behaviour of the pier. The FPS isolator behaviour is modeled through a widespread velocity-dependent rule. By means of a non-dimensional formulation of the motion equations, proposed in this study, a wide parametric analysis considering several structural parameters is performed to investigate their influence on the response parameters relevant to the performance assessment. Seismic excitations, modeled as time-modulated filtered Gaussian white noise random processes having different intensities and frequency contents, are considered. Specifically, the filter parameters, which control the frequency contents, are properly calibrated to reproduce stiff, medium and soft soil conditions, respectively. Finally, multi-variate non-linear regression relationships are derived to estimate the optimum values of the sliding friction coefficient able to minimize a percentile of the pier displacements relative to the ground as a function of the structural properties, of the seismic input intensity and of the soil condition.
•Seismic performance of bridges isolated by FPS for different soil conditions.•Artificial records with frequency contents related to different soils are used.•Influence of soil conditions on the response statistics of isolated bridges.•Optimal friction coefficient to minimize bridge response for different soils.•Non-linear multivariate regression equations to design isolated bridges.
We measured the triple coincidence A(e,e^{'}np) and A(e,e^{'}pp) reactions on carbon, aluminum, iron, and lead targets at Q^{2}>1.5 (GeV/c)^{2}, x_{B}>1.1 and missing momentum >400 MeV/c. This was ...the first direct measurement of both proton-proton (pp) and neutron-proton (np) short-range correlated (SRC) pair knockout from heavy asymmetric nuclei. For all measured nuclei, the average proton-proton (pp) to neutron-proton (np) reduced cross-section ratio is about 6%, in agreement with previous indirect measurements. Correcting for single-charge exchange effects decreased the SRC pairs ratio to ∼3%, which is lower than previous results. Comparisons to theoretical generalized contact formalism (GCF) cross-section calculations show good agreement using both phenomenological and chiral nucleon-nucleon potentials, favoring a lower pp to np pair ratio. The ability of the GCF calculation to describe the experimental data using either phenomenological or chiral potentials suggests possible reduction of scale and scheme dependence in cross-section ratios. Our results also support the high-resolution description of high-momentum states being predominantly due to nucleons in SRC pairs.
Energy from nuclear fission Ripani, M.
EPJ Web of Conferences,
01/2018, Letnik:
189
Journal Article, Conference Proceeding
Recenzirano
Odprti dostop
The main features of nuclear fission as a physical phenomenon will be revisited, emphasizing its peculiarities with respect to other nuclear reactions. Some basic concepts underlying the operation of ...nuclear reactors and the main types of reactors will be illustrated, including fast reactors, showing the most important differences among them. The nuclear cycle and radioactive nuclear waste production will be also discussed, along with the perspectives offered by next-generation nuclear assemblies being proposed. The current situation of nuclear power in the world, its role in reducing carbon emission and the available resources will be briefly illustrated.
There is a significant discrepancy between the values of the proton electric form factor, G(E)(p), extracted using unpolarized and polarized electron scattering. Calculations predict that small ...two-photon exchange (TPE) contributions can significantly affect the extraction of G(E)(p) from the unpolarized electron-proton cross sections. We determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections using a simultaneous, tertiary electron-positron beam incident on a liquid hydrogen target and detecting the scattered particles in the Jefferson Lab CLAS detector. This novel technique allowed us to cover a wide range in virtual photon polarization (ϵ) and momentum transfer (Q(2)) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing ϵ at Q(2)=1.45 GeV(2). This measurement is consistent with the size of the form factor discrepancy at Q(2)≈1.75 GeV(2) and with hadronic calculations including nucleon and Δ intermediate states, which have been shown to resolve the discrepancy up to 2-3 GeV(2).
A multidimensional extraction of the structure function ratio σLT′/σ0 from the hard exclusive e→p→e′nπ+ reaction above the resonance region has been performed. The study was done based on beam-spin ...asymmetry measurements using a 10.6 GeV incident electron beam on a liquid-hydrogen target and the CLAS12 spectrometer at Jefferson Lab. The measurements focus on the very forward regime (t/Q2 ≪ 1) with a wide kinematic range of xB in the valence regime (0.17 <xB < 0.55), and virtualities Q2 ranging from 1.5 GeV2 up to 6 GeV2. The results and their comparison to theoretical models based on Generalized Parton Distributions demonstrate the sensitivity to chiral-odd GPDs and the directly related tensor charge of the nucleon. In addition, the data is compared to an extension of a Regge formalism at high photon virtualities. It was found that the Regge model provides a better description at low Q2, while the GPD model is more appropriate at high Q2.
We present results on the electroexcitation of the low mass resonances {delta}(1232)P{sub 33}, N(1440)P{sub 11}, N(1520)D{sub 13}, and N(1535)S{sub 11} in a wide range of Q{sup 2}. The results were ...obtained in the comprehensive analysis of data from the Continuous Electron Beam Accelerator Facility (CEBAF) large acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility (JLab) on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for {pi} electroproduction off the proton. The data were analyzed using two conceptually different approaches--fixed-t dispersion relations and a unitary isobar model--allowing us to draw conclusions on the model sensitivity of the obtained electrocoupling amplitudes. The amplitudes for the {delta}(1232)P{sub 33} show the importance of a meson-cloud contribution to quantitatively explain the magnetic dipole strength, as well as the electric and scalar quadrupole transitions. They do not show any tendency of approaching the pQCD regime for Q{sup 2}{<=}6 GeV{sup 2}. For the Roper resonance, N(1440)P{sub 11}, the data provide strong evidence that this state is a predominantly radial excitation of a three-quark (3q) ground state. Measured in pion electroproduction, the transverse helicity amplitude for the N(1535)S{sub 11} allowed us to obtain the branching ratios of this state to the {pi}N and {eta}N channels via comparison with the results extracted from {eta} electroproduction. The extensive CLAS data also enabled the extraction of the {gamma}*p{yields}N(1520)D{sub 13} and N(1535)S{sub 11} longitudinal helicity amplitudes with good precision. For the N(1535)S{sub 11}, these results became a challenge for quark models and may be indicative of large meson-cloud contributions or of representations of this state that differ from a 3q excitation. The transverse amplitudes for the N(1520)D{sub 13} clearly show the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q{sup 2}>1 GeV{sup 2}, confirming a long-standing prediction of the constituent quark model.
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