Technical Report NTB 96-05
Far-Field Sorption Data Bases for Performance Assessment of a L/ILW Repository in an Undisturbed Palfris Marl Host Rock
A Palfris marl formation at Wellenberg (Gemeinde Wolfenschiessen, NW) has been chosen by NAGRA as a potential repository site for low-and intermediate-level radioactive waste, L/ILW. In the coming years a series of performance assessment studies will be performed for this site. One set of key data required for such safety analysis calculations is sorption data bases (SDB) for safety relevant radionuclides in the far-field. The purpose of this report is to describe the procedures used to generate sorption data bases appropriate for the in situ conditions existing along the different potential flow paths in an undisturbed marl host rock formation. An important aim was to document the sources of sorption data used and, in particular, the processes by which data selections were made. The main guiding principles here were "transparency" and "traceabllity". Inherent within this whole process is also the justification for, and defensibility of, the selected values.
Much of the sorption data used to generate the SDS for marl came from the open literature. A major part of this report is concerned with describing the procedures whereby these initial literature values are modified so that they apply to the actual marl mineralogies and groundwater chemistries. The resulting "reference Rd values" are then further modified using so called Lab -> Field transfer factors to produce sorption values which are appropriate to the in situ bulk rock conditions. The Lab -> Field transfer factors attempt to correct for the differences in sorption site availability between the crushed rock state used in batch tests and the intact rock state existing in reality in the host rock.
There are two main groundwater chemistries and five characteristic mineralogical compositions which cover the three broad types of flow paths which have been identified in the Palfris marl formation. A sorption data base corresponding to the in situ conditions for each of these groundwater/ mineralogy combinations are given. In principle the methology described here to construct sorption data bases for marl is applicable to any type of host rock.