[en] WAPPA-C is a waste package performance assessment code that predicts the temporal and spatial extent of the loss of containment capability of a given waste package design. This code was enhanced by the addition of the capability to calculate the sensitivity of model results to any parameter. The GRESS automated procedure was used to add this capability in only two man-months of effort. The verification analysis of the enhanced code, WAPPAG, showed that the sensitivities calculated using GRESS were accurate to within the precision of perturbation results against which the sensitivities were compared. Sensitivities of all summary table values to eight diverse data values were verified