Simplification process of safety assessment model for engineered barrier system by using adjoint sensitivity analysis method

Sensitivity analyses for mass transport model in porous media were performed by using adjoint method. The mass transport model employed is to evaluate the performance of engineered barrier of shallow land disposal, assuming that water flows through a cylinder packed with sand. In this model instantaneous sorption equilibrium between liquid and solid phases is assumed and two types of boundary conditions which represent the nuclide release from waste package, i.e. solubility-limited case and constant leaching case, are considered. From the sensitivity analysis, it was shown that the effect of longitudinal dispersion on performance measure is very small and calculated normalized sensitivity is in the order 10^-4∼10^-3 around the most probable value of longitudinal dispersion coefficient. This suggests that the term of longitudinal dispersion can be removed from the original model. In this case analytical solution is easily introduced for two boundary conditions respectively to evaluate the performance measure of the barrier system. These simplified models, in fact, gives larger estimate of the nuclide release from the engineered barrier system than that calculated from the model considering the longitudinal dispersion. They are acceptable from the standpoint of conservatism of safety assessment. (author)