Swedish activities in the Oklo natural analogue project

Presently, many countries are concerned about the need for a safe method to dispose of high-level nuclear wastes. The various national regulatory agencies involved generally require quantitative predictions for at least the first 104 years after disposal, and qualitative predictions of repository safety performance for up to 106 years. Predicting what this future behaviour will be can be considerably facilitated by comparison with the evolution of parallel natural geochemical systems, particularly those rich in radioelements. Such natural systems, or natural analogues, therefore provide a unique opportunity to test, by observation and measurement, many of the geochemical processes that are expected to influence the predicted reliability of high-level radioactive waste containment deep in the bedrock for long periods of geological time, thus providing direct and indirect validation support for some of the models and data used in performance assessment. Oklo, by virtue of representing a fossil natural reactor which achieved criticality some 2 000 Ma ago, provides a unique opportunity to show whether the stable nuclear fission end-products have remained, in the near-vicinity of the reactor zone subsequent to fission. Furthermore, the materials found in the Oklo reactors resemble closely important waste forms such as spent nuclear fuel and bitumised residue and have been subject to extreme geochemical reactions and radiolysis when related to expected site disposal conditions, and for a much longer time. In order to study such effects, not only do the reactor zones require further detailed mineralogical, geochemical and radiochemical study, but equally the geological setting and hydrological environment of the reactor zones also need to be understood. For this reason SKB are interested to study the spatial distribution of the reactor zones at Oklo, Okelobondo and Bagombe in the context of the large-and small-scale groundwater hydraulics of the host sedimentary rocks. (author). 7 refs., 1 fig