Implementation of nonlinear sorption approaches into transport codes MARNIE, NAMMU and TOUGH2 and their testing by comparative calculations

The assessment of the sorption of radionuclides and other contaminants on the transport path in near field and geosphere is an essential task of long-term safety analyses of repositories for nuclear wastes. The sorption parameters for these analyses are very often derived from linear empirical isotherms based on the HENRY model. This concept, however, has to be critically re-assessed with regard to the applicability to higher concentrations which might occur within the near field area. In these cases non-linear isotherm concepts are more adequate since a relative decrease of the sorbed mass during an increase of the solved concentration is taken into consideration. Therefore the transport codes MARNIE, NAMMU and TOUGH2 used by GRS were extended by adding the non-linear FREUNDLICH and LANGMUIR sorption models which were examined by means of one- and two-dimensional test calculations. A further objective was to demonstrate in how far the results of transport calculations are influenced by the choice of a certain isotherm model. Test calculations indicated, that the extended isotherm models implemented in the codes NAMMU, MARNIE and TOUGH2 are able to produce corresponding results. Furthermore it could be demonstrated that the selection of the sorption model may have a crucial influence on the results of transport calculations. (orig.)