Palmottu Natural Analogue

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:

  • Understanding the natural groundwater flow system (comprehensive site hydrogeological studies)
  • Geochemical evolution of the water-rock system
  • Redox processes
  • Retardation and migration of radionuclides
  • Modeling of the system including modeling of speciation and solubility of dissolved elements, blind predictive modeling (BPM), comparative transport modeling using different conceptual approaches
  • Conclusions relevant to repository performance assessment (PA)

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.

Main results

The main important contributions for the performance assessment of repositories for spent nuclear fuel are summarized in the following: 

  • Four general bedrock groundwater types characterize the Palmottu site. Recently recharged, shallow Ca-HCO 3 type groundwater evolves in the upper part of the bedrock into a Na-HCO 3 type groundwater at increasing depth within a few decades, partly due to Ca to Na ion exchange processes (Figure 3).
  • Brackish Na-SO 4 and Na-Cl type groundwaters (some thousands of years old) prevail below the HCO 3 groundwater bodies at depths from 300 to 350 m;
  • Na-SO 4 type groundwater is present only around the mineralized zone whilst Na-Cl is found at greater depths in the surrounding area. The age of Na-Cl type groundwater is approximately 11 000 B.P., i.e. close to the retreat of the Weichselian ice sheet.
  • The persistence of old groundwater under low conductive hydraulic conditions underlines the hydrochemical stability of the system over long periods of geological time, at least since the last glaciation approximately 10 000 years ago. At greater depths, more applicable to repository depths, conditions would be expected to be even more stable.
  • The research at Palmottu gave important and new information about the effects of glaciation on bedrock groundwater. For example, there are indications in the oxygen isotope composition of the groundwater, which suggest that the site may have been subjected to deep (several hundred metres) penetration of glacial melt water (Figure 4).
  • Glacial melt water does not appear to have changed the reducing conditions prevailing below the upper hydraulically active zone. Another possible process, capable to generate the depleted oxygen isotope values is freezing front under permafrost conditions.
  • Modelling of the redox and pH controls in a flow system supported that the rock would have sufficient redox buffering capacity to counteract deep penetration of oxidising conditions. Moreover, measured present-day redox potentials demonstrate that if oxidising conditions had penetrated to depth, the conditions have been restored to reducing.
  • Based on the occurrence of young uranophanes (Figure 5) and uranium accumulations in association with fracture calcites, it can be concluded that a continuous alteration and oxygenating process is in progress at Palmottu. This process is also manifested by high concentrations of dissolved uranium (100-500 ppb) in the upper bicarbonate groundwater type at depths 0 to 150 m (Figure 6).
  • The dominant part of the uranium has remained in the ancient U(IV) phases, uraninite and coffinite, throughout its geological history for 1700–1800 Ma, demonstrating that reducing conditions have prevailed in the bedrock for most of its existence.
  • Although the mineralization has been subject to supergene weathering processes for millions of years, the dispersion of uranium has been very limited. This indicates that the bedrock itself has a very effective reducing capacity and that the dispersion of uranium has been limited over millions of years which increases confidence in the safety of spent fuel disposal in crystalline, igneous rocks
     

 
Figure 2. Hydrostructural model of Palmottu (Updated from Blomqvist et al. 1998).
Enlarge 
 
 
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 
 

 

Auranofaani Buranofaani
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).


Selected literature:

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].)

 

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Palmottu järvi 
Figure 1. Lake Palmottu. Photo: Juha Kaija, GTK

 
 

Useful links

TEM: www.tem.fi/energia
STUK: www.stuk.fi/ydinturvallisuus
POSIVA:  www.posiva.fi
SKB: www.skb.se
IAEA:  www.iaea.org  
NEA: www.nea.fr
EURATOM:  www.euratom.org
FENNOVOIMA:  www.fennovoima.fi