The Waste Package Performance Assessment (WAPPA) code has recently been completed, verified, and documented within the SCEPTER program for ONWI. This is a systems model of the gradual degradation of physical and retardation barriers comprising a nuclear waste package, under repository isolation conditions, in various geologic media. The model describes and predicts the temporal and spatial extent of the loss of containment capability for each barrier and for the entire waste package, on timescales of hundreds of thousands of years. The output quantifies the residual state of integrity of waste package components as a function of time and calculates heat, and radionuclide fluxes to the repository.

The model includes five distinct degradation process models (Radiation, Thermal, Mechanical, Corrosion, and Leaching), which are driven both internally by waste decay and externally by repository stress and fluids. The process models are coupled at system level via state and coupling variables. The approach to degradation modeling adopted is barrier-integrated and process-sequential, allowing modification of the entire waste package sequentially by the five process models within each time step.

The code is intended to serve as a tool for evaluating both the relative and the absolute performance of waste package design concepts, for guiding or selecting a preliminary design among conceptual designs, and for support of licensing activities.