The Palmottu uranium occurrence at Nummi-Pusula, Southern Finland has been a target of extensive studies since 1987 (Figure 1 and 2). Investigations started as a national analogue research project 1987 – 1996 between GTK, Laboratory of Engineering Geology and Geophysics of Helsinki University of Technology and Department of Radiochemistry of the University of Helsinki. Later the team was complemented by the contribution of VTT-Technical Research Center. The study was mainly financed by the Ministry of Trade and Industry, and later by STUK.
Later on studies continued within the GTK coordinated European Commission financed international Palmottu Natural Analogue Project 1996 – 2000: “ Transport of radionuclides in a natural flow system at Palmottu”. After that, up to 2003, Palmottu data has been used in an IAEA coordinated research project “ The use of selected safety indicators in the assessment of radioactive waste disposal”.
The Palmottu uranium deposit has a number of features analogous to certain issues being considered in the performance assessment (PA) of spent nuclear fuel repositories in fractured crystalline bedrock. Furthermore, it includes some unique characteristic owing to its glacial and post-glacial history.
In fact, the Palmottu project was exceptional in the sense that it is still the only major international natural analogue project in which uranium mineralization in a crystalline bedrock has been exposed to cyclic glaciations. Thus, the research results from the site are of considerable benefit especially to the Nordic countries considering crystalline bedrock as a spent nuclear fuel disposal alternative. The geological evolution of Palmottu area also resembles the evolution of the Olkiluoto area where the repository for the Finnish spent nuclear fuel is being excavated.
Research at Palmottu consisted several subprojects:
Palmottu review report of research activities performed in the framework of Palmottu natural analogue studies is available at: http://tupa.gtk.fi/julkaisu/ydinjate/yst_121.pdf. This report gives a comprehensive summary of the research activities, emphasizing the methodological aspects and geosciences. An up-to-date list of over 200 publications and reports related to the studies of the Palmottu site is also given.
A wide variety of borehole geophysical and hydrogeological methods were used during the study, furthermore new downhole groundwater sampling methods of a fractured bedrock medium were developed. Extensive high quality geochemical data set from a relative small area was compiled. It includes analytical results from minerals, bedrock, soil, peat, lake sediments, precipitation, surface water and groundwater. For example, the hydrochemical data set includes chemical (major anions, cations, trace elements and rare earth elements) and isotopic analyses (3H, 13C, 14C, 18O, 2H, 87Sr/ 86Sr, 37Cl, 34S, 11B, 234U/ 238U) and physical measurements from 234 water samples. The number of analyses is over 7500. Uranium concentration data in different media is especially comprehensive. These data have been used e.g. for the repository PA calculations.
The main important contributions for the performance assessment of repositories for spent nuclear fuel are summarized in the following:
Figure 2. Hydrostructural model of Palmottu (Updated from Blomqvist et al. 1998).
Figure 3. Quaternary deposits around Palmottu. Retreating ice sheet is depicted in the upper left corner. Flow lines show the main routes of meltwater discharge. The insert at the lower right corner visualizes the position of the Palmottu site during retreat of the ice sheet (Ahonen et al. 2004). Enlarge
|Figure 4. Uranophane corona crystallised around calcite crystals. The surface below is fresh potassium feldspar. Drill core sample R384/41.70 m (Kaija et al. 2000).|
Ahonen, Lasse; Kaija, Juha; Paananen, Markku; Ruskeeniemi, Timo; Hakkarainen, Veikko 2004. Palmottu natural analogue : a summary of the studies. Geological Survey of Finland, Report YST-121, 39 p. + 1 app. [ Fulltext] )
Blomqvist, R.; Kaija, J.; Lampinen, P.; Paananen, M.; Ruskeeniemi, T.; Korkealaakso, J.; Pitkänen, P.; Ludvigson, J.-E.; Smellie, J.; Koskinen, L.; Floría, E.; Turrero, M. J.; Galarza, G.; Jakobsson, K.; Laaksoharju, M.; Casanova, J.; Grundfelt, B.; Hernan, P. 1998. The Palmottu natural analogue project. Phase I: Hydrogeological evaluation of the site. Final report - Phase I. European Commission Nuclear Science and Technology Series EUR 18202 EN. Luxembourg: Office for Official Publications of the European Communities. 98 p.
Blomqvist, R.; Ruskeeniemi, T.; Kaija, J.; Ahonen, L.; Paananen, M.; Smellie, J.; Grundfelt, B.; Pedersen, K.; Bruno, J.; Pérez del Villar, L.; Cera, E.; Rasilainen, K.; Pitkänen, P.; Suksi, J.; Casanova, J.; Read, D.; Frape, S. 2000. The Palmottu natural analogue project. Phase II: Transport of radionuclides in a natural flow system at Palmottu. Final report. European Commission Nuclear Science and Technology Series EUR 19611 EN. Luxembourg: Office for Official Publications of the European Communities. 174 p.
Kaija, Juha; Rasilainen, Kari; Blomqvist, Runar 2003. IAEA Coordinated Research Project (CRP) "The use of selected safety indicators (concentrations, fluxes) in the assessment of radioactive waste disposal". Report 6: Site-specific natural geochemical concentrations and fluxes at the Palmottu U-Th mineralisation (Finland) for use as indicators of nuclear waste repository safety. Geological Survey of Finland, Report YST-114, 45 p. [ Fulltext].)