Polynomial Chaos Expansion for Global Sensitivity Analysis applied to a model of radionuclide migration in randomly heterogeneous aquifers

V. Ciriello (1) , V. Di Federico (1) , M. Riva (2) , F. Cadini (3) , J. de Sanctis (3) , Enrico Zio (4) , Alberto Guadagnini (2)

1 Dipartimento di Ingegneria Civile, Ambientale e dei Materiali
2 Dipartimento di Ingegneria Idraulica, Ambientale, Infrastrutture Viarie, Rilevamento
3 Dipartimento di Energia [Milano]
4 SSEC - Chaire Sciences des Systèmes et Défis Energétiques EDF/ECP/Supélec

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, (c) the effective dispersivity at the scale of interest. These quantities are treated as random variables and a variance-based GSA is performed in a numerical Monte Carlo framework. 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 of the variable of interest. The 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 accuracy.

Domaines: Sciences de l'ingénieur [physics]

V. Ciriello, V. Di Federico, M. Riva, F. Cadini, J. de Sanctis, et al.. Polynomial Chaos Expansion for Global Sensitivity Analysis applied to a model of radionuclide migration in randomly heterogeneous aquifers. Stochastic Environmental Research and Risk Assessment, 2013, 27 (4), pp.945-954. ⟨10.1007/s00477-012-0616-7⟩. ⟨hal-00926341⟩