We perform global sensitivity analysis (GSA) through polynomial chaos expansion (PCE) on a contaminant transport model for the assessment of radionuclide concentration at a given control location in a heterogeneous aquifer, following a release from a near surface repository of radioactive waste. The aquifer hydraulic conductivity is modeled as a stationary stochastic process in space. We examine the uncertainty in the first two (ensemble) moments of the peak concentration, as a consequence of incomplete knowledge of (a) the parameters characterizing the variogram of hydraulic conductivity, (b) the partition coefficient associated with the migrating radionuclide, and (c) dispersivity parameters at the scale of interest. These quantities are treated as random variables and a variance-based GSA is performed in a numerical Monte Carloframework. This entails solving groundwater flow and transport processes within an ensemble of hydraulic conductivity realizations generated upon sampling the space of the considered random variables. The Sobol indices are adopted as sensitivity measures to provide an estimate of the role of uncertain parameters on the (ensemble) target moments. Calculation of the indices is performed by employing PCE as a surrogate model of the migration process to reduce the computational burden. We show that the proposed methodology (a) allows identifying the influence of uncertain parameters on key statistical moments of the peak concentration (b) enables extending the number of Monte Carlo iterations to attain convergence of the (ensemble) target moments, and (c) leads to considerable saving of computational time while keeping acceptable accurac

Decay modes of excited nuclei are investigated in 78,82 Kr+ 40 Ca reactions at 5.5 MeV/nucleon. Charged products were measured by means of the 4π INDRA array. Kinetic-energy spectra and angular distributions of fragments with atomic number 3 Z 28 indicate a high degree of relaxation and are compatible with a fissionlike phenomenon. Persistence of structure effects is evidenced from elemental cross sections (σ Z ) as well as a strong odd-even staggering (o-e-s) of the light-fragment yields. The magnitude of the staggering does not significantly depend on the neutron content of the emitting system. Fragment-particle coincidences suggest that the light partners in very asymmetric fission are emitted either cold or at excitation energies below the particle emission thresholds. The evaporation residue cross section of the 78 Kr+ 40 Ca reaction is slightly higher than the one measured in the 82 Kr+ 40 Ca reaction. The fissionlike component is larger by ∼25% for the reaction having the lowest neutron-to-proton ratio. These experimental features are confronted to the predictions of theoretical models. The Hauser-Feshbach approach including the emission of fragments up to Z = 14 in their ground states as well as excited states does not account for the main features of σ Z . For both reactions, the transition-state formalism reasonably reproduces the Z distribution of the fragments with charge 12 Z 28. However, this model strongly overestimates the light-fragment cross sections and does not explain the o-e-s of the yields for 6 Z 10. The shape of the whole Z distribution and the o-e-s of the light-fragment yields are satisfactorily reproduced within the dinuclear system framework which treats the competition among evaporation, fusion-fission, and quasifission processes. The model suggests that heavy fragments come mainly from quasifission while light fragments are predominantly populated by fusion. An underestimation of the cross sections for 16 Z 22 could signal a mechanism in addition to the capture process.

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[1] The estimation of the extent and timing of solute migration in a fractured medium is a fundamental task for verifying the level of protection against contaminant releases (e.g., toxic chemicals or radionuclides) offered by the engineered and natural barriers of a waste repository. In this paper we present a novel approach for modeling solute transport in a fractured medium, based on an extension of the Kolmogorov-Dmitriev theory of stochastic branching processes. The model equations for the expected values of the solute concentration take a form similar to that of classical dual-continua models. On the other hand, the stochastic nature of the modeling approach lends itself to a new particle tracking scheme of resolution, which allows accounting for realistic features of the transport process. The proposed stochastic modeling framework and simulation solution approach are illustrated with reference to the experimental results from a case study of literature. Some of the model parameters are optimally identified by means of a genetic algorithm search aimed at best fitting the experimental data.Citation: Cadini, F., I. Bertoli, J. De Sanctis, and E. Zio (2012), A novel particle tracking scheme for modeling contaminant transport in a dual-continua fractured medium, Water Resour.

One of the problems in the analysis of nucleus-nucleus collisions is to get information on the value of the impact parameter b. This work consists in the application of pattern recognition techniques aimed at associating values of b to groups of events. To this end, a support vector machine (SVM) classifier is adopted to analyse multifragmentation reactions. This method allows us to backtrace the values of b through a particular multidimensional analysis. The SVM classification consists of two main phases. In the first one, known as the training phase, the classifier learns to discriminate events that are generated by a model. In this case we used a classical molecular dynamics (CMD) model for the reaction: 58 Ni + 48 Ca at 25 A MeV. To check the classification of events in the second one, known as the test phase, what has been learned is tested on new events generated by the same model. These new results have been compared to those obtained through others techniques of backtracing the impact parameter (estimate function of b and PCA analysis). This approach better classifies central and peripheral collisions with respect to other techniques. We have finally performed the SVM classification on the experimental data measured by the NUCL-EX Collaboration with CHIMERA apparatus for the previous reaction and we show some results of the method.