3. DESCRIPTION OF PROGRAM OR FUNCTION
WAPPA the Waste Package Performance Assessment program, 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. WAPPA is a barrier degradation code for a nuclear waste package in a geologic repository. The WAPPA model includes five distinct degradation process models (radiation, thermal, mechanical, corrosion, and leaching), which are driven internally by waste decay and externally by repository stress and fluids. The modeling approach is barrier-integrated and process-sequential, allowing modification of the entire waste package by the sequential operation of five process models within each time-step. The process models are coupled at the system level via state and coupling variables. The penetration of water into the waste package and its effects on the barriers it contacts are tracked radially inwards. The WAPPA 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 any time scale up to one million years after isolation.
Input includes the radial geometry and material properties of each component, degradation rates for each process modeled, and time-dependent boundary conditions (fluid flux, mechanical stress, and temperature) as the waste package/repository interface. The output quantifies the residual state of integrity of waste package components as a function of time and calculates the heat and radionuclide fluxes output to the repository. Other output at each calculation time point includes: the temperature and nuclide concentration profiles internal to the waste package, residual mass fractions of radionuclides in the wasteform and residual waste mass fraction, and various barrier status degradation indices andparameters. In addition, summary output includes the total mass for each nuclide output to the repository and the barrier's status history used to pinpoint times of failure.