Underground nuclear testing via deep vertical shafts was conducted at the Nevada Test Site (NTS) from 1951 until 1992. The Pahute Mesa area of the NTS was used for 27 years. The Underground Testing Area (UGTA) Project is currently conducting correction action investigations to ensure the protection of the public and the environment, and one of these activities is the construction and calibration of a flow model for the Pahute Mesa corrective action units (CAUs). As part of the overall flow model assessment required by regulatory agreement (and simply good modeling practice) a sensitivity and parameter uncertainty analysis was performed using a complementary suite of approaches.
The flow model for Pahute Mesa includes 47 hydrostratigraphic units and 25 faults with multiple parameters associated with each. In addition, different parameterization approaches were evaluated during calibration that gave very similar fits, and it was desired to understand the important features and develop better understanding of the flow system.
The local sensitivity analysis was conducted with PEST, and as such represents the model response very near the final calibrated values. The next phase of sensitivity analysis was perturbation, where an individual parameter was systematically varied over its range of uncertainty. Finally, 1,000 realizations of model parameters were created via Latin Hypercube sampling, flow models run and results collected, and analyzed with entropy statistics, and classification and regression tree analysis. This analysis – combining a conventional local sensitivity analysis with a global approach – provided important insights about model input/output relationships, with each technique adding information to the overall perspective.