Benchmark for reactive transport codes with application to concrete/clay interaction

International audienceNuclear waste repositories will use a significant quantity of cement: for the construction of accessdrifts, disposal cells and concrete plugs, and as containment material for low- to intermediate-levelwaste. Several European countries have chosen claystone formations as possible host rocks (Landais,2006). Numerous cement/clay interfaces will thus be present in a radioactive-waste repository.Due to contrasting geochemical conditions (including Eh, pH, solution composition), these interfacesare subjected to steep concentration gradients and are highly reactive. Predicting long-term changes(1,000 to 100,000 years) in these cementitious and clayey materials is thus crucial for assessing thebehaviour of such infrastructures. Experiments cannot provide sufficiently reliable information oversuch long time scale. Although natural and archaeological analogues can be very helpful, modelling isthe unique tool to analyse and test different evolution scenarios. In order to build a better confidence insuch calculations, it is of paramount importance to demonstrate that the results obtained are notdependent on the choice of the numerical reactive transport code to perform the simulation. In order toaddress this issue, a benchmark problem, divided into three steps with increasing geochemicalrefinement, has been set up (Marty et al., submitted).In all cases, the solutes transport across the interface between clayey host rock and concrete isdiffusion driven and a 1D radial geometry and isothermal conditions (25°C) have been bothconsidered. Both materials are full saturated. The first step of the benchmark only considers porewatersolutions and the clayed host rock which is only modelled by an exchanger. The second stepintroduces the full mineralogy for both the concrete and the claystone considering slow kinetics ratesfor mineral dissolution-precipitation reactions, whereas the third one focuses on fast reaction rates.Seven international teams have been involved in this benchmarking exercise. All reactive transportcodes used (TOUGHREACT, PHREEQC with two different ways of handling transport,CRUNCHFLOW, HYTEC, ORCHESTRA, MIN3P-THCm) gave very similar patterns in terms ofpredicted solute concentrations and of minerals distribution evolution (Fig. 1). The benchmarkingexercise demonstrates that reactive transport tools have reached such a level of maturity as to beconfidently used in support of long-term performance assessments