WORTH OF SECONDARY DATA COMPARED TO PIEZOMETRIC DATA FOR THE PROBABILISTIC ASSESSMENT OF RADIONUCLIDE MIGRATION

A common approach for the performance assessment of radionuclide migra tion from a nuclear waste repository is by means of Monte-Carlo techni ques. Multiple realizations of the parameters controlling radionuclide transport are generated and each one of these realizations is used in a numerical model to provide a transport prediction. The statistical analysis of all transport predictions is then used in performance asse ssment. In order to reduce the uncertainty on the predictions is neces sary to incorporate as much information as possible in the generation of the parameter fields. In this regard, this paper focuses in the imp act that conditioning the transmissivity fields to geophysical data an d/or piezometric head data has on convective transport predictions in a two-dimensional heterogeneous formation. The Walker Lake data based is used to produce a heterogeneous log-transmissivity field with disti nct non-Gaussian characteristics and a secondary variable that represe nts some geophysical attribute. In addition, the piezometric head fiel d resulting from the steady-state solution of the groundwater flow equ ation is computed. These three reference fields are sampled to mimic a sampling campaign. Then, a series of Monte-Carlo exercises using diff erent combinations of sampled data shows the relative worth of seconda ry data with respect to piezometric head data for transport prediction s. The analysis shows that secondary data allows to reproduce the main spatial patterns of the reference transmissivity field and improves t he mass transport predictions with respect to the case in which only t ransmissivity data is used. However, a few piezometric head measuremen ts could be equally effective for the characterization of transport pr edictions.