Transient simulation and sensitivity analysis for transport of radionuclides in a saturated-unsaturated groundwater flow system

A transient, two-dimensional radionuclide transport code RADMIG, based upon the Fluid in Discrete Element (FLIDE) numerical formulation, was developed to simulate the radionuclide transport in a given groundwater flow system. The radionuclide transport code was employed to simulate tritium transport in a hypothetical saturated-unsaturated groundwater flow system.

The sensitivity analysis methodology for the radionuclide transport problem was developed based on generalized perturbation theory. . The adjoint transport equation was first derived using variational principle formulation. Sensitivity coefficients were formulated for four input parameters: longitudinal and transverse dispersivities; retardation factor; and radioactive decay constant.

Sensitivity coefficients were computed by combining the solved timedependent concentration and adjoint fields together. The computational procedure for performing the radionuclide transport sensitivity analysis is presented. Relative sensitivity coefficient fields of two responses to four input parameters are presented and discussed.

The radionuclide transport sensitivity theory was validated by comparing the relative sensitivity coefficients obtained from both the perturbation predictions and direct calculations. The agreement between the sensitivity results from both approaches is good. Linearity ranges of the parameters for two responses were estimated.

It is concluded that the generalized perturbation theory provides a feasible, economic means to perform sensitivity analysis on radionuclide transport problems.