TransPyREnd: a code for modelling the transport of radionuclides on geological timescales

The German site selection procedure for a high-level nuclear waste repository is entering a stage in which preliminary safety assessments have to be conducted and the release of radionuclides has to be estimated for a large number of potential sites. Here, we present TransPyREnd, a 1D finite-differences code for modeling the transport of radionuclides in the subsurface at geological timescales. The code simulates the processes advection, diffusion, equilibrium sorption, decay of radionuclides, and the build-up of daughter nuclides. We summarize the modeled physical processes, their mathematical description and our numerical approach to solve the governing equations. Finally, two simple tests are shown, one considering diffusion, sorption, and radioactive decay, the other involving diffusion and a radioactive decay chain. In both tests, the code shows good agreement with the reference solutions. Caveats of the model and future additions are discussed.