The Office of Civilian Radioactive Waste Management (OCRWM) oj e U.S. Department of • Energy (DOE) and the Power Reactor and Nuclear Fuel Development Corporation of Japan (PNC) have supported the development of the Analytical Repository Source-Term (AREST) at Pacific Northwest Laboratory. The purpose of this report is to describe the mathematical models and logic of AREST.
eAREST is a computer model developed to evaluate radionuclide release from an underground geologic repository. The AREST code can be used to calculate/estimate the amount and rate of each radionuclide that is released from the engineered barrier system (EBS) of the repository. The EBS is the man-made or disrupted area of the repository. AREST was designed as a "system-level" models to simulate the behavior of the total repository by combining "process-level" models for the release from an individual waste package or container. AREST contains primarily analytical models for calculating the release/transport of radionuclides to the lost rock that surrounds each waste package. Analyt.,al models were used because of the small computational overhead that allows all the input parameters to be derived from a statistical distribution. Recently, a one-dimensional numerical model was also incorporated into AREST, to allow for more detailed modeling of the transport process with arbitrary length decay chains.The next step in modeling the EBS, is to develop a model that couples the probabilistic capabilities of AREST with a more detailed process model. This model will need to look at the reactive coupling of the processes that are involved with the release process. Such coupling would include: 1) the dissolution of the waste form, 2) the geochemical modeling of the groundwater, 3) the corrosion of the container overpacking, and 4) the backfill material, just to name a few. Several of these coupled processes are already incorporated in the current version of AREST.