(GRS 297) Realistic Integration of Sorption Processes in Transport Codes for Long-Term Safety Assessments

In Germany, high-level radioactive waste is to be disposed of in deep geological formations. Long-term safety assessments have to be carried out as part of the postclosure safety case for radioactive waste repositories. According to the German safety requirements for heat-generating radioactive waste an assessment period of one million years has to be regarded /BMU 10/. Although the assessment is focussed on the proof of a safe contaiment of the waste in the so-called contaiment providing rock zone, scenarios, which lead to the mobilization of radionuclides from the waste and to their transport through the repository system, cannot be excluded and need to be adressed in long-term safety assessment. During transport through the repository system sorption on mineral surfaces of rocks and technical materials is an important retardation process for radionuclides. Besides dispersion and dilution, the increased travel time due to retardation leads to a reduction of their concentration and thus of the radiation exposure because of additional radioactive decay of many radionuclides during transport. On the other hand, retarding the transport of a mother nuclide could lead to an elevated generation of daughter nuclides and, in case the daughter nuclide is dose relevant, to an elevated radiation exposure. Describing the sorption as realistically as possible is therefore one important aspect in long-term safety assessment.