Current status of NRPB work on the mathematical modelling of radionuclide migration

The current status of the generic model developed at the National Radiological Protection Board for the prediction of radionuclide migration from geologic disposal sites is described. The model is simple but extremely flexible. It can be used to simulate the migration of radionuclide chains of any length. It has recently been extended to predict migration along flow paths with heterogeneous sorption characteristics. This capability is illustrated by two simple examples. In the first it is assumed that sorption is high close to the geologic repository and decreases along the flow path. This corresponds to the case of a repository which has been back-filled with a material of high sorbent capacity. In the second example it is assumed that there is no sorption of radionuclides in the immediate vicinity of the repository but considerable sorption at greater distances. This provides a means of estimating the potential consequences of large scale fracturing around a repository. The model can also be used to predict the migration of a radionuclide chain in which one of the members exists in two valence states which are sorbed to different extents. This is illustrated for 237Np and its parent and daughter nuclides. Finally some planned extensions to the model are described