IAEA Coordinated Research Project (CRP) The use of selected safety indicators (concentrations, fluxes) in the assessment of radioactive waste disposal. Report 7: Site-specific natural geochemical concentrations and fluxes at four repository investigation sites in Finland for use as indicators of nuclear waste repository safety

This report concerns site-specific data achieved during the studies of four Finnish candidate sites for nuclear fuel repository. The aims are to examine the level of radioactive concentrations (U, Rn, K, Rb, Cs, Ra, Th), their sources and their fluxes at different sites and at different depths, and whether differences in chemical conditions result from the geology or hydrogeology.

The sites cover virtually the formation history of Finnish Precambrian bedrock and differ geochemically significantly. Romuvaara represents Archean basement gneisses with low K and U contents. Proterozoic migmatitic mica gneisses at Olkiluoto represent a very reducing geochemical environment with graphite and sulphides. Oxidising conditions have characterised the formation of early synkinematic granitoids at Kivetty and as well as anorogenic rapakivi granite Hästholmen. Particularly the rapakivi granite is enriched with incompatible elements such as K, Rb, U and Th. Hydrogeologically the sites differ as well. The inland sites, Romuvaara and Kivetty, have been above the highest shoreline since the retreat of the ice of the Weichselian glaciation, whereas the coastal sites have been below sea level. Hydraulic gradient is also higher inland than on shore.

The contents of radioactive elements vary significantly between the sites and between deep and shallow groundwaters. Uranium concentrations at each site decrease mainly with the increasing depth, and they correlate relatively with bedrock contents between the sites. However, the observed exceptionally low contents are considered to result from a short water-rock interaction time in shallow groundwaters at Kivetty and from actively reducing groundwater conditions in deep groundwaters at Olkiluoto. Radon contents correlate mainly with U-concentrations, suggesting that Rn derives predominantly from dissolved U. Potassium and rubidium concentrations correlate withsalinity, indicating that their main source is seawater or ancient brine. Therefore, U proves the best element to study rock derived radioactive fluxes in this study.

Groundwater fluxes control mainly the level of geochemical fluxes between the values through deep reference planes and shallow discharge planes. Geochemical conditions are indicated most clearly in the low fluxes of U and Rn deep in the bedrock at Romuvaara and Olkiluoto compared with Kivetty and Hästholmen. The low mass fluxes reflect the low U-content of the rock at Romuvaara and the strongly reducing groundwater conditions at Olkiluoto. The current results of groundwater chemistry do not indicate any direct links between deep reference planes and discharge areas. The flux from the repository level may be at its maximum per mille level in shallow discharge.