1,721,009 research outputs found
A Review on Radiation Damage in Concrete for Nuclear Facilities: From Experiments to Modeling
Full text linkConcrete is a relatively cheap material and easy to be cast into variously shaped structures. Its good shielding properties against neutrons and gamma-rays, due to its intrinsic water content and relatively high-density, respectively, make it the most widely used material for radiation shielding also. Concrete is so chosen as biological barrier in nuclear reactors and other nuclear facilities where neutron sources are hosted. Theoretical formulas are available in nuclear engineering manuals for the optimum thickness of shielding for radioprotection purposes; however they are restricted to one-dimensional problems; besides the basic empirical constants do not consider radiation damage effects, while its long-term performance is crucial for the safe operation of such facilities. To understand the behaviour of concrete properties, it is necessary to examine concrete strength and stiffness, water behavior, volume change of cement paste, and aggregate under irradiated conditions. Radiation damage process is not well understood yet and there is not a unified approach to the practical and predictive assessment of irradiated concrete, which combines both physics and structural mechanics issues. This paper provides a collection of the most distinguished contributions on this topic in the past 50 years. At present a remarkable renewed interest in the subject is shown
Stochastic response spectrum determination of nonlinear systems endowed with fractional derivative elements
No full textThis paper considers the response to white noise excitation of nonlinear dynamic systems comprising fractional derivative elements. Specifically, it focuses on estimating the stationary power spectral densities of the response displacement and velocity for systems characterized by polynomial nonlinearities, and nonlinearities with memory effects. Various approximations for the spectral response are derived. The first approximation is based on the concept of the conditional spectrum. In doing this, a weighted averaging is performed over a set of surrogate spectral densities. Each surrogate density corresponds to the stationary random response of a linearized system, associated with the dynamics at specific response amplitude levels. These amplitude levels, assumed to vary slowly over time, are treated as constant within individual oscillation cycles. To refine the estimate, an enhanced formulation of the conditional spectrum is pursued. This improved approach introduces a corrective ter..
Studio analitico della stabilità dinamica di elementi strutturali mono- e bidimensionali piani smorzati
No full textA perspective on conditional spectrum-based determination of response statistics of nonlinear systems
Full text linkThis work focuses on determining the stochastic response properties, in the frequency domain, of a general class of nonlinear systems with polynomial nonlinearities. Specifically, the results are presented in terms of the stationary power spectral densities of the system's displacement and velocity. This is pursued by revisiting the conditional power spectrum concept, with the assumption that the response process is both ergodic and pseudo-harmonic and characterized by an amplitude, and a phase. A theoretical elucidation of an existing formula for the conditional spectrum is attempted. In particular, this concept is interpreted in conjunction with the time averaging approximation made in the definition of the stationary probability density function of a response amplitude quantity, associated with the original nonlinear system. It is shown that a proper definition of the stationary probability density of the response amplitude, along with a reasonable treatment of the distribution over..
Parametric resonance of fractional multiple-degree-of-freedom damped beam systems
No full textThis article investigates the problem of viscoelastic beams characterized by a fractional constitutive model and dynamically excited by longitudinal harmonic point loads. Damped systems of this kind undergo parametric resonance for specific values of the amplitude and frequency of the load. The study addresses the definition of the instability regions of the system when damping is related to a fractional-order derivative of transverse displacements. By using a Galerkin variational approach on multiple-degree-of-freedom beam systems, differently constrained, the problem is formulated as a set of fractional differential equations. The harmonic balance method is then extended to the fractional problem to determine the first region of instability of such systems. The results are presented in the form of stability regions in the parameter space. They are discussed for different constraint conditions, fractional order, and degree of damping. Particularly, the order of the fractional operator seems to affect the shape and the size of such regions. It is found that a fractional order in the range 0–0.5 shifts the regions of instability to high excitation frequencies, while it is conversely for a fractional order in the range 0.5–1, thus changing the resonant properties of the system. A comparison with the undamped conservative solution is made, which confirms that the presence of diffuse dissipation forces generally increases the stability margins of the system
Coupled THM analysis of a full-scale test for high-level nuclear waste and spent fuel disposal under actual repository conditions during 18 years of operation
No full textThe study focuses on the Thermo-Hydro-Mechanical (THM) analysis of the behavior of the Full-scale Engineered Barriers Experiment designed to study the long-term response of a disposal concept for High-Level radioactive Waste (HLW) and Spent Fuel (SF). Crucially, the test involved two dismantling operations, a partial one after 5 years of heating and a final one, after 18.3 years of operation, allowing the direct observation of the state of the barrier at two different times. The coupled THM modeling discussed in this paper mainly focuses on the last 13 years of test operation and also on the final dismantling of the barrier system. The numerical model was unchanged from the early stages of the test, so the results presented can be considered as blind predictions. The numerical model predicted reasonably well the state of the barrier and its evolution, especially in terms of the distribution of the bentonite dry density, confirming the predictive capability of the model to capture the transient THM processes. The combination of experimental and numerical investigations involving one of the most long-lasting full-scale in-situ experiment monitoring the behavior of the barrier components of an HLW/SF repository has led to a better understanding of the THM behavior of this complex system
FEBEX in-situ test final dismantling: THM model predictions and outlook post THM modelling
No full textTHM modelling of the cooling, excavation and final dismantling of the FEBEX in-situ test
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