In the framework of evaluating the safety of a radioactive waste disposal in deep argillaceous formations and its assessment by the implementer, IRSN and MINES ParisTech built a hydrogeological model of the Paris Basin aquifer system with the aim of identifying radionuclide pathways, estimating transfer times associated with those pathways and locating potential outlets. Such modelling of fluid and mass transport through a sedimentary basin is a recurrent applied geology study. However, recent research shows that in a deep and saline environment, density effects induced by both temperature (to a small extent) and particularly salt concentration can play a major role in the predicted groundwater flow pathways. The present study highlights the relevance of building a hydrogeological model from the measured hydrogeological and hydrogeochemical parameters by combining the calibration of the piezometric levels and the salinity values simultaneously. The application of this strategy to the Paris Basin shows that the Keuper halite formation (Triassic), located in the eastern part of the basin, can be considered as the unique salt source, provided that the density effects are taken into account in the flow and transport modelling. The calibration requires, in particular, to also take into account some of the basin's major tectonic faults, which allow vertical hydraulic connections between aquifers and thus allow salt water fluxes toward the shallower formations. The results presented in this paper show that when considering "thermohaline" effects, the model effectively reproduces the observed hydraulic heads and salinity values throughout the entire Paris Basin, whilst at the same time outlining the role of faults and of the geometry of geological formations on salt transport. (C) 2012 Elsevier B.V. All rights reserved.

Performance assessment of a deep geological disposal requires the definition of a series of assumptions of very different natures. The particular features of the site studied, the amount and the characteristics of the waste to be disposed of, or the definition of the various assumptions and parameters used to build the transfer model must be specifically defined. The multiple sources of differences related to the assumptions made by the teams involved in this international exercise make the interpretation difficult when a broad comparison of results obtained is considered. The work performed and the results obtained within the recent SPA project (Spent fuel Performance Assessment) however enabled to draw some general conclusions on the main radionuclides and parameters that potentially govern the radiological impact of a spent fuel geological repository. The particular importance of 129 I was notably highlighted as well as the possible role of transuranic elements. For 129 I, matrix degradation rate and dilution were found to have a predominant influence on the calculated dose. For transuranic elements, disposal system was found to delay the arrival of activity into biosphere over a very long period of time. A relatively wide range of breakthrough times were however observed depending on sorption modelling in geosphere in particular. As for transuranic relative dose contribution, a determining influence was played by solubility limit values.

The main objective of the EVEREST project is the evaluation of the sensitivity of the radiological consequences associated with the geological disposal of radioactive waste to the different elements in the performance assessment. Three types of geological host formations are considered: clay, granite and salt. The sensitivity studies that have been carried out can be partitioned into three categories according to the type of uncertainty taken into account: uncertainty in the model parameters, uncertainty in the conceptual models and uncertainty in the considered scenarios. Deterministic as well as stochastic calculational approaches have been applied for the sensitivity analyses. For the analysis of the sensitivity to parameter values, the reference technique, which has been applied in many evaluations, is stochastic and consists of a Monte Carlo simulation followed by a linear regression. For the analysis of conceptual model uncertainty, deterministic and stochastic approaches have been used. For the analysis of uncertainty in the considered scenarios, mainly deterministic approaches have been applied.

A method is presented to estimate smoothly varying (as opposed to zoned) hydraulic parameters, transmissivity in particular, appearing in a time-dependent flow equation. A finite difference model based on a nested grid discretization reduces the computational effort while allowing local refinement. Structuring of the unknown parameter field, use of a priori information, and parameterization of the inverse problem are geostatistically based. Calibration is carried out by minimizing a quadratic objective function depending on head data. A primal-adjoint discrete-gradient method is used, where the unknowns are parameter values at a number of user-defined points, the “pilot points”. The key feature of this method consists of kriging together the pilot point values and the measured values, if any, in order to generate the parameter field needed at each iteration to solve the primal and adjoint systems. Minimization is performed by a BFGS algorithm. Two numerical examples are considered, where transmissivity is the unknown. The first one is adapted from Carrera and Neuman's[1] synthetic problem. The purpose is to compare kriged and zoned results obtained from different types of observation data sets e.g., stationary vs. transient head or drawdown. The second example is a case study of the Dijon (France) aquifer. Pilot point-based identification is applied to the same model (domain, equations, grid), which was manually calibrated in 1985. Only the measured data were made available. The results from manual calibration were kept unknown until the end of the inversion trials. Sensible use of pilot points and of a priori information appears to play a key role in yielding plausible results.