Numerical modelling for the interpretation of a laboratory mock-up experiment of bentonite/granite interface

摘要

Performance assessment of a deep geological repository requires understanding diffusion and determining diffusion parameters under real conditions because diffusion is a key transport mechanism in hosting geological formation. FEBEX (Full-scale Engineered Barrier Experiment) is a demonstration and research project dealing with the bentonite engineered barrier designed for sealing and containment of the high-level radioactive waste repository. To support field investigations of FEBEX in situ test, a large-scale laboratory mock-up experiment (MUE) is being performed at CIEMAT facilities to study tracer migration at the bentonite/granite interface. Numerical models of MUE are presented here for HTO, ~(36)Cl~- and ~(137)Cs~+. Experiments are modeled with 2-D axi-symmetric finite element grids and are solved with CORE2D V4. Model results indicate that numerical solutions with reference parameters reproduce measured data for HTO and ~(136)Cl~- but show large discrepancies for ~(137)Cs~+. Relevant diffusion and retention parameters are identified by sensitivity analysis for tracer concentrations in borehole, bentonite and granite, respectively. Interpretation of ~(137)Cs~+ data measured in the tracer chamber is perfomed by taking into account the uncertainties in initial activity C0 and initial time t0,. Optimum values of C0 and t0 are obtained. The best fit is obtained with De-filter equal to 2.03·10~(-10) m~2/s and Kd-bentonite equal to 5 m~3·Kg~(-1). Key words: