Direct alpha-recoil as a process to generate U-234/U-238 disequilibrium in groundwater
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Direct alpha-recoil as a process to generate U-234/U-238 disequilibrium in groundwater Kari Rasilainen1, Henrik Nordman1, Juhani Suksi2 and Nuria Marcos3 1 Technical Research Centre of Finland, P.O. Box 1608, FI-02044 VTT, Finland; 2University of Helsinki, P.O. Box 55, FI-00014 University of Helsinki, Finland; 3Helsinki University of Technology, P.O. Box 6200, FI-02015 TKK, Finland ABSTRACT We discuss quantitatively the capability of direct α-recoil to create observable radioactive disequilibrium (U-234/U-238 activity ratio > 1) in flowing groundwater. Coupled equations for radioactive decay chain are formulated for the solid U source and for the corresponding groundwater receiving the recoiling U-234 atoms. U-rich fracture coating material and the water flowing in the fracture are discussed in detail. The novel modelling approach worked well and provides a useful tool for more detailed site-specific studies with more detailed input data. The approach appears feasible because the equations are based on straightforward mass balance considerations. α-recoil -induced enrichment of U-234 in groundwater is a slow process: the increase of U234/U-238 activity ratio to notable values above unity will take hundreds to thousands of years. Strong groundwater flow prevents any local recoil-induced U-234 enrichment in the water. INTRODUCTION Safety studies of the geological disposal of nuclear waste have to cover long time spans into future. Paleohydrogeological studies are pursued to understand the past behaviour of bedrockgroundwater system. This understanding is subsequently used as the basis to estimate the future behaviour of the system. The uranium-series disequilibrium method (USD) is a good paleohydrogeological tool providing a redox-sensitive element and an absolute dating possibility of U mobilisations or immobilisations. Interpreting USD data from groundwater samples is a challenging task, as these are often mixtures of waters originating from several flow channels. Therefore, it is necessary to consider the stationary U source, i.e. the fracture coating and the rock matrix, from which the flowing groundwater obtains its USD signal. Uranium isotopes can enter groundwater from the U source either via physical or geochemical processes. In this context we are interested in the release of U-234 atoms that would cause U-234/U-238 activity ratio in groundwater to exceed the equilibrium value of unity. The physical process candidate is α-recoil, which is independent of geochemical conditions of the bedrock. It can not be prevented, slowed down or accelerated and it is present always wherever there is natural U. The geochemical process candidate is geochemically induced preferential release of U-234. It could be called indirect α-recoil as it is expressly α-recoil that has caused the increased susceptibility for selective release in U-234 as compared to U-238 isotope. As a difference to direct α-recoil geochemical release requires favourable conditions to occur. We have earlier discussed the role of α-recoil when analysing ra
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